Chapter

Human sex ratios: adaptations and mechanisms, problems and prospects

Authors:
To read the full-text of this research, you can request a copy directly from the author.

Abstract

Covering sex allocation, sex determination and operational sex ratios, this multi-author volume provides both a conceptual context and an instruction in methods for many aspects of sex ratio research. Theory, statistical analysis and genetics are each explained and discussed in the first three sections. The remaining chapters each focus on research in one of a wide spectrum of animal, plant and microbial taxa, including sex ratio distorting bacteria in invertebrates, malarial parasites, birds, human and other mammals, giving critical appraisals of such research. Sex Ratios: Concepts and Research Methods is primarily intended for graduate and professional behavioural and evolutionary ecologists in this field, but it will also be useful to biologists building evolutionary models, and researchers analysing data involving proportions or comparisons across phylogenetically related species.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the author.

... A second possibility is that the differences in subsistence and geography (i.e. the local environmental ecology) explain the observed relationship. Such a view is consistent with predictions resulting from evolutionary hypotheses that envision offspring sex preferences as strategies that allow parents to maximize reproductive benefits accruing from the investments in their children, which vary with ecological context [41][42][43]. Several evolutionary models offer explanations for why parents might prefer a given sex (e.g. ...
... Several evolutionary models offer explanations for why parents might prefer a given sex (e.g. [41,42,44]) and we do not attempt to adjudicate among them here (but see, e.g. [45,46] for empirical tests of alternatives). ...
... [45,46] for empirical tests of alternatives). We suspect, however, that a generalized Trivers-Willard effect [13,42,47,48] may lie at the root of the differences revealed by the data. This is supported by previous results [13], which showed an association between wealth in land and reproductive success in the matrilineal Mosuo that was consistent with higher marginal returns to parental fitness associated with investing land in daughters. ...
... Human sex ratio research must be interdisciplinary if it is to be successful (Lazarus, 2002). ...
... Full conformity to Fisherian assumptions is probably a biological rarity (Bull & Charnov, 1988). In particular, human sex allocation may be affected by sexually differential fitness returns (Bereczkei & Dunbar, 1997;Lazarus, 2002;Mace & Jordan, 2005;Almond & Edlund, 2007;James, 2012James, , 2013: predicting how these might influence the sex ratio response of unconstrained parents to the presence of individuals reproducing via ART would not be straightforward (West, 2009) and key information on parental investment is currently lacking (Orzack et al., 2016). Further, human population sex ratios at conception may adhere to the 'baseline' expectation of 0.5 (Orzack et al. 2016) but at birth are typically slightly male biased, ca. ...
... Theory developed to complement the Düsing-Fisher approach indicates that sex ratios of local sub-populations should influence sex ratio evolution (Argasinski, 2013). Moreover, human reproductive behaviour has been reported to respond to local sex ratio bias in a range of ways (Chipman & Morrison, 2013) including overproduction of the rarer sex (Lummaa et al., 1998;Ranta et al., 2000;Lazarus, 2002;Helle et al., 2008;but see James, 2000), overproduction of the rarer sex particularly supporting the notion that sex ratios of untreated members of the population could be affected indirectly by the practice of ART. These reports derive from studies within the framework of evolutionary ecology but analogous frequency-dependent responses to sex ratio bias are also reported by social scientists whose discipline encompasses the complex array of behavioural and social processes that shape human reproductive decisions (Bhattacharya, 2013;Zhou et al., 2013). ...
Article
Infertility affects around 15% of human couples and in many countries approximately 1-4% of babies are born following Assisted Reproductive Technologies (ART). Several ART techniques are used and these differentially affect the sex ratio of offspring successfully produced. These direct effects on sex ratio also have the potential to influence, indirectly, the sex ratios of offspring born to untreated couples. This is of concern because human sex ratio bias may adversely affect public health. Here the extent of indirect effects of ART that could operate, via Fisherian frequency-dependent natural selection, on the progeny sex ratio of unassisted members of a population is heuristically modelled. Given the degrees to which ART techniques bias sex ratios directly, it is predicted that well over 20% of couples would have to reproduce via ART for there to be any discernible effect on the sex ratios produced, in response, by the remainder of the population. This value is greater than the estimated prevalence of infertility problems among human couples. It is concluded that providing ART to couples with fertility problems does not currently generate significant ethical issues or public health concern in terms of indirect effects on the offspring sex ratios of untreated couples.
... Species where the theory has demonstrated less applicability include sea lions and chickens, with inconsistent support evident across studies on bison, lion tamarins, certain species of ungulates, pigs, mice, rhesus monkeys, and baboons. 1 Its applicability to humans is just as open to debate. While several studies on humans have found support for the Trivers-Willard hypothesis, some have provided 1 For good reviews of studies addressing sex ratio variation and differential parental investment across various species (including humans) refer to any of the following: Anderson & Crawford (1993); Brown (2001); Clutton-Brock & Iason (1986); Caley & Nudds (1987); Cameron (2004); Carranza (2002); Cockburn, Legge, & Double (2002); Hamilton (1967); Hardy (1997); Hewison & Gaillard (1999) ;Hrdy, (1988); James (1987); Lazarus (2002); Pedersen (1991); Rosenfeld & Roberts (2004); Sheldon & West (2004);and van Hooff (1997). inconclusive results on account of differences in measuring evidence of sex ratio biasing and parental resource allocation biasing (Keller, Nesse, & Hofferth, 2001). ...
... Hrdy, 1999) or the reproductive success of men in male-biased countries is so low that women favor producing daughters (e.g., cost of reproduction hypothesis; Gomendio et al., 1990;Myers, 1978). Better yet, status and reproductive success are not related in humans the same way that they seem to impact other animals (Freese & Powell, 1999;Lazarus, 2002). ...
... To test for sex of offspring alone, in the absence of the predicted covariates, actually omits significant components of the theory. Indeed, with the exception of a study by Borgerhoff Mulder (1998), an interactive effect between status and differential reproductive success between human siblings has not been thoroughly established nor even examined (Lazarus, 2002). ...
... It was observed that parents invested more in sons because, sons, in good condition, have a reproductive advantage over daughters. The most successful female may be limited to fewer offspring when compared to a successful male, particularly in a polygamous setting, where males have access to more female partners (Lazarus, 2002). However, an unsuccessful female may still reproduce at least once as compared to an unsuccessful male that may never mate either due to competition or poor condition (Cameron, 2004). ...
... Generalized linear model analysis (Binary logistic option) was performed to determine the odds of offspring sex from their mother's digit ratios (Wilson & Hardy, 2002). Due to the possible effects of maternal sociodemographic variables on offspring sex at birth, the maternal age, educational status employment status, cultural group and body mass index were included in the models as covariates to produce partial coefficients (Lazarus, 2002). Since lifetime offspring were not used, it was not possible to calculate the traditional sex ratio at birth, rather, mothers were put into groups or cohorts by their digit ratios, and the CSR was then calculated as the proportion of sons [sons/(sons + daughters)] (Orzack et al., 2015). ...
... In light of the effect differences in method on study outcomes, it should be noted that there are differences between the current study and previous ones; the current study examined CSRs from first-time mothers as opposed to the lifetime offspring sex ratios (Helle & Lilley, 2008;Kim et al., 2015;Manning et al., 2002). The choice of the study population was to reduce the effect of previous births on the offspring sex of subsequent births due to changes in maternal hormonal environment from previous pregnancies (Lazarus, 2002). ...
Article
Full-text available
Objectives: The 2D:4D ratio is the putative marker of prenatal androgen exposure. Low maternal 2D:4D ratio has been associated with high or male-biased secondary sex ratio. Hitherto, there has not been any study in Ghana regarding the maternal 2D:4D ratio and sex ratio at birth. This study sought to investigate this observation in a Ghanaian population. Method: The study was cross-sectional from December 2020 to April 2021 involving 272 first-time mothers. The mean ± SD age of the mothers was 23.9 ± 3.67 years. The right (2D:4DR), the left (2D:4DL), the mean (M2D:4D) digit ratios and the right–left difference (Dr-l) of the mothers were measured using computer-assisted analysis. The mothers were stratified by their digit ratios, and the cohort sex ratio (CSR) for each stratum was then calculated as the proportion of sons. Results: The mean ± SD of the 2D:4DR of mothers-with-daughters and mothers-with-sons were 0.941 ± 0.032 and 0.933 ± 0.037, respectively. The mean ± SD of the 2D:4DL of mothers-with-daughters was 0.934 ± 0.034, while that of mothers-with-sons was 0.931 ± 0.039. The offspring sex at birth and the CSR was not associated with either the mother's right, left, mean 2D:4D ratio or their difference (Dr-l). However, mothers-with-daughters showed rightward asymmetry in their 2D:4D ratios as the Dr-l significantly and positively deviated from zero (p < .010). Conclusion: The maternal 2D:4D ratio may not be associated with their offspring sex at birth among first-time mothers. This study adds to the limited data on studies regarding offspring sex at birth and the 2D:4D ratio in Ghana and Sub-Saharan Africa.
... A second possibility is that the differences in subsistence and geography (i.e. the local environmental ecology) explain the observed relationship. Such a view is consistent with predictions resulting from evolutionary hypotheses that envision offspring sex preferences as strategies that allow parents to maximize reproductive benefits accruing from the investments in their children, which vary with ecological context [41][42][43]. Several evolutionary models offer explanations for why parents might prefer a given sex (e.g. ...
... Several evolutionary models offer explanations for why parents might prefer a given sex (e.g. [41,42,44]) and we do not attempt to adjudicate among them here (but see, e.g. [45,46] for empirical tests of alternatives). ...
... [45,46] for empirical tests of alternatives). We suspect, however, that a generalized Trivers-Willard effect [13,42,47,48] may lie at the root of the differences revealed by the data. This is supported by previous results [13], which showed an association between wealth in land and reproductive success in the matrilineal Mosuo that was consistent with higher marginal returns to parental fitness associated with investing land in daughters. ...
Article
Full-text available
Son preference predominates in China, yet there are patterned exceptions to this rule. In this paper, we test whether lineality (patrilineal versus matrilineal inheritance and descent) is associated with son versus daughter preference among the ethnic Mosuo (Na) of Southwest China. Our results show (i) an increased probability of continued fertility among matrilineal women after having a son compared with a daughter and (ii) an increased probability of continued fertility among patrilineal women after having a daughter compared with a son. These results are consistent with son preference among patrilineal Mosuo and more muted daughter preference among the matrilineal Mosuo. Furthermore, we show (iii) the lowest probability of continued fertility at parity 2 once women have one daughter and one son across both systems, suggesting that preferences for at least one of each sex exist alongside preferences for the lineal sex. The Mosuo are the only known small-scale society in which two kinship systems distinguish subgroups with many otherwise shared cultural characteristics. We discuss why this, in conjunction with differences in subsistence, may shed light on the evolutionary underpinnings of offspring sex preferences.
... It was observed that parents invested more in sons because, sons, in good condition, have a reproductive advantage over daughters. The most successful female may be limited to fewer offspring when compared to a successful male, particularly in a polygamous setting, where males have access to more female partners (Lazarus, 2002). However, an unsuccessful female may still reproduce at least once as compared to an unsuccessful male that may never mate either due to competition or poor condition (Cameron, 2004). ...
... Generalized linear model analysis (Binary logistic option) was performed to determine the odds of offspring sex from their mother's digit ratios (Wilson & Hardy, 2002). Due to the possible effects of maternal sociodemographic variables on offspring sex at birth, the maternal age, educational status employment status, cultural group and body mass index were included in the models as covariates to produce partial coefficients (Lazarus, 2002). Since lifetime offspring were not used, it was not possible to calculate the traditional sex ratio at birth, rather, mothers were put into groups or cohorts by their digit ratios, and the CSR was then calculated as the proportion of sons [sons/(sons + daughters)] (Orzack et al., 2015). ...
... In light of the effect differences in method on study outcomes, it should be noted that there are differences between the current study and previous ones; the current study examined CSRs from first-time mothers as opposed to the lifetime offspring sex ratios (Helle & Lilley, 2008;Kim et al., 2015;Manning et al., 2002). The choice of the study population was to reduce the effect of previous births on the offspring sex of subsequent births due to changes in maternal hormonal environment from previous pregnancies (Lazarus, 2002). ...
Article
Full-text available
Objectives The 2D:4D ratio is influenced by prenatal testosterone (PT) and estrogen (PE) exposure in utero. This study sought to determine whether evidence of Manning's hypothesis can still be observed even in the postpartum period. We hypothesize that the offspring 2D:4D ratios will be inversely correlated with maternal postpartum circulating testosterone but positively correlated with estradiol. Methods This study was conducted between December 2020 and April 2021 and was cross-sectional in nature. There were 272 mother-offspring pairs; the mothers were aged between 18 and 36 years while the median (IQR) age of their offspring was 111 (44–210) days. Offspring right (2D:4DR) and left (2D:4DL) digit ratios were measured using computer-assisted analysis. Sampling was done at 111 (44–210) days postpartum and blood was analyzed for total testosterone (TT), estradiol (E2) and sex hormone-binding globulins using the enzyme-linked immunosorbent assay technique. Results The 2D:4DR of sons was significantly lower compared to daughters (p = .031). Mothers with sons had significantly increased levels of serum TT (p = .001) while mothers with daughters had significantly increased levels of E2 (p = .000). As hypothesized, the maternal serum free testosterone (FT%) was inversely correlated with their daughters' (r = −0.320, p = .003), and also with their sons' (r = −0.213, p = .047), 2D:4DL. Unexpectedly, daughters' 2D:4DL was inversely correlated with maternal circulating free E2 (r = −0.255, p = .015). Conclusions In humans, evidence of the relationship between maternal testosterone levels and their offspring's 2D:4D ratio may persist even into the postpartum period.
... Therefore, it can be expected that low status parents direct their resources toward their female children, whereas high status parents should favor male children. While the Trivers-Willard (T-W) effect has been demonstrated in many species (for a review see Cameron, 2004), evidence in human societies is inconclusive (Lazarus, 2002). Dickemann (1979), Boone (1986), Voland et al. (1997) found such effects in data from historical India, China and Europe: Low class families tended to favor daughters more than upper class families. ...
... Researchers agreed that parental support for educational attainment is an effective and costly form of investment and thus lends itself to investigating the T-W effect. In view of this, it is surprising that only 48% of studies report a T-W effect in modern human societies (Lazarus, 2002). Two main interpretations emerged why the T-W effect remains elusive. ...
Article
Full-text available
From an evolutionary point of view, sex differences in intergenerational transmission of income may be influenced by the Trivers-Willard (T-W) effect: Low status parents should invest more in daughters, whereas high status parents are expected to invest more in sons. This bias in parental investment may result in status-dependent sex biased parental support for higher education and educational attainment and should therefore affect the level of intergenerational income transmission for the sons and daughters. We used the data from the Wisconsin Longitudinal Study (WLS) to model the effect of parental financial investment on the child's income and educational attainment controlling for the number of siblings. The observed sex differences in intergenerational income transmission demonstrate that sons profited more from parental income and education in terms of their own income than daughters. Furthermore, we showed that fathers with a high socioeconomic index (SEI) invest more in their sons' education in terms of completed years of education and financial support during college. In contrast daughters of low SEI fathers completed more years of education and received more financial support than sons of low SEI fathers. However, the pattern in intergenerational income transmission might be better explained as a product of sociological factors and reproductive trade-offs in later life rather than as a consequence of the T-W effect.
... Whereas conditions (a) and (b) find support from a variety of sources even for humans in contemporary societies[3][4][5][6][7], it has been argued by evolutionary psychologists that the existence of a link between socioeconomic status and fertility is not a necessary precondition for a Trivers-Willard (TW) effect to occur in contemporary societies – if it held for long enough in our evolutionary, ancestral environment[8,9]. In human populations, a sex ratio of about 105–106 boys for every 100 girls is considered 'natural'[10]. Whereas in a number of historical populations prior to the demographic transition, sex ratios differed markedly from this natural sex ratio for status-based subgroups[7,11,12], empirical evidence in support of the TW hypothesis for humans in contemporary developed societies remains inconclusive[13]. ...
... However, recent advances on the timing of a possible TW mechanism may explain at least part of the mixed empirical pattern. From a theoretical perspective, a potential TW mechanism may operate at various stages, e.g., by affecting survival or fertilization probabilities of Xand Y-chromosome-carrying spermatozoa, by differential vulnerability of male or female fetuses during pregnancy, or by means of sex-selective parental investment post-partum[10]. From an evolutionary perspective, costs associated with the respective mechanism should be minimized in order to maximize reproductive fitness. ...
Article
Full-text available
On the Forbes 400: Based on evolutionary theory, Trivers & Willard (TW) predicted the existence of mechanisms that lead parents with high levels of resources to bias offspring sex composition to favor sons and parents with low levels of resources to favor daughters. This hypothesis has been tested in samples of wealthy individuals but with mixed results. Here, I argue that both sample selection due to a high number of missing cases and a lacking specification of the timing of wealth accumulation contribute to this equivocal pattern. This study improves on both issues: First, analyses are based on a data set of U.S. billionaires with near-complete information on the sex of offspring. Second, subgroups of billionaires are distinguished according to the timing when they acquired their wealth. Informed by recent insights on the timing of a potential TW effect in animal studies, I state two hypotheses. First, billionaires have a higher share of male offspring than the general population. Second, this effect is larger for heirs and heiresses who are wealthy at the time of conception of all of their children than for self-made billionaires who acquired their wealth during their adult lives, that is, after some or all of their children have already been conceived. Results do not support the first hypothesis for all subgroups of billionaires. But for males, results are weakly consistent with the second hypothesis: Heirs but not self-made billionaires have a higher share of male offspring than the U.S. population. Heiresses, on the other hand, have a much lower share of male offspring than the U.S. average. This hints to a possible interplay of at least two mechanisms affecting sex composition. Implications for future research that would allow disentangling the distinct mechanisms are discussed.
... In addition to the issues mentioned above, recent theoretical work suggests that the investment 97 variant of the TWE cannot be treated as a simple extension of the birth-ratio variant, because the 98 evolutionary value of parental investment is not measured by the absolute reproductive values of 99 male and female offspring, but by marginal reproductive value per unit of investment (Carranza, 100 2002;Keller et al., 2001). Lazarus (2002) even suggests that the TWH should be reformulated, such 101 that it refers to the difference in reproductive value before and after parental investment. In the 102 context of pre-partal investment the reproductive value before investment equals zero, yielding the 103 classical TWH as a special case. ...
... These results 373 are in line with the theoretical analysis of Veller et al. (2016). Furthermore, they support the 374 hypothesis that parental investment allocation will evolve to match the expected change in fitness 375 per unit of investment (Keller et al., 2001;Lazarus, 2002). 376 Some limitations concerning the scope of the model have to be mentioned here. ...
Article
Full-text available
The Trivers-Willard hypothesis (TWH) states that parents in good condition preferentially produce the sex with a higher variation in reproductive success, whereas parents in bad condition favour the opposite sex. Theorists distinguish two variants of the TWH: (a) a biased sex-ratio at birth and (b) biased parental investment after birth. It has been argued before that the conditions stated by Trivers and Willard (good condition is inherited and affects reproductive success more strongly for one of the sexes) are sufficient for the sex-ratio version but insufficient for the investment version of the TWH. However, it has not yet been investigated how these conditions affect parental investment in high and low quality parents, depending on the life-cycle of a species. The present study aims to fill this gap by introducing a multi-stage matrix population model with nonlinear mating to describe the effects of parental investment after birth on the reproductive values of male and female individuals. Using methods from adaptive dynamics and evolutionary invasion analysis, evolutionary trajectories and evolutionarily stable strategies are derived for different parameterizations of the model. Simulation results demonstrate that the conditions given by Trivers and Willard produce a general bias of parental investment towards the sex with higher variance in reproductive value. This bias is stronger for low-quality parents than for high-quality parents and matches the expected marginal offspring reproductive values for parental investment.
... For human populations, a sex ratio at birth (SRB) of about 105 boys to 100 girls is seen as "natural" [1]. But a number of studies have found that the SRB in a population may be affected by individual-level stressors and macro-social shocks that lead to short-term deviations from the time trend [2,3]. ...
... This allowed us to check whether the SRB in 1991 deviated from the trend in East but not in West Germany. In a second step, we added additional control variables on the level of the individual birth: maternal characteristics at the time of birth (employment status: employed or not employed, age, marital status: married or unmarried, and nationality: German or foreign), and month of birth to adjust for seasonality effects [1,55]. Although the age of both parents at the time of birth has been linked to sex composition [56], we could not control for paternal age, as this information was not available for all births c . ...
Article
Full-text available
The economic stress hypothesis (ESH) predicts decreases in the sex ratio at birth (SRB) following economic decline. However, as many factors influence the SRB, this hypothesis is difficult to test empirically. Thus, researchers make use of quasi-experiments such as German reunification: The economy in East, but not in West Germany, underwent a rapid decline in 1991. A co-occurrence of a decline in the East German SRB in 1991 has been interpreted by some as support for the ESH. However, another explanation might be that the low SRB in 1991 stems from increased random variation in the East German SRB due to a drastically reduced number of births during the crisis. We look into this alternative random variation hypothesis (RVH) by re-examining the German case with more detailed data. Our analysis has two parts. First, using aggregate-level birth register data for all births in the period between 1946 and 2011, we plot the quantum and variance of the SRB and the number of births and unemployment rates, separately for East and West Germany, and conduct a time-series analysis on the East German SRB over time. Second, we model the odds for a male birth at the individual level in a multiple logistic regression (1991-2010, ~13.9 million births). Explanatory variables are related to the level of the individual birth, the mother of the child born, and the regional economic context. The aggregate-level analysis reveals a higher degree of variation of the SRB in East Germany. Deviations from the time trend occur in several years, seemingly unrelated to economic development, and the deviation in 1991 is not statistically significant. The individual-level analysis confirms that the 1991-drop in the East German SRB cannot directly be attributed to economic development and that there is no statistically significant effect of economic development on sex determination in East or West Germany. Outcomes support the RVH but not the ESH. Furthermore, our results speak against a statistically significant effect of the reunification event itself on the East German SRB. We discuss the relative importance of behavioral and physiological responses to macro-level stressors, a distinction that may help integrate previously mixed findings.
... Enfin la théorie des traits d'histoire de vie prédit que l'effort de reproduction devrait augmenter avec l'âge à mesure que la valeur reproductive diminue (Stearns, 1992), et donc que la sélection pourrait favoriser lajustement du sexe ratio de la progéniture (SRS) selon l'âge des parents et en particulier chez les espèces où il existe un coût délevage différentiel selon le sexe (Lazarus, 2002 ;Weimerskirch et al., 2005). Nous avons donc examiné les variations du sexe ratio secondaire en fonction de lexpérience moyenne des parents. ...
... Concernant lexpérience, étant donné que lexpérience des reproducteurs montre un effet net sur la taille de la nichée (cf. IV.4 ;Martin, 2008 ;Nevoux et al., 2008b), et que la théorie des traits dhistoire de vie prédit que leffort de reproduction devrait augmenter avec lâge à mesure que la valeur reproductive diminue (Stearns, 1992), on aurait pu sattendre à ce que la sélection favorise lajustement du sexe ratio de la progéniture (SRS) en fonction de lâge des parents (Lazarus, 2002 ;Weimerskirch et al., 2005). Ceci ne semble pas être le cas ici, car lexpérience reproductive des parents na aucun effet sur le sexe ratio secondaire. ...
Thesis
Full-text available
Understanding adaptations that allow species to live in temporally and spatially variable environments is crucial to predict how species may respond to current and future global changes. Long-term studies have shown that climatic variations affect the dynamic of populations. However, the relative influence of habitat selection and density-dependence processes is still poorly understood and explained. The aim of this thesis is, firstly by a correlative approach, to investigate the density-dependent habitat selection at varying spatial scales in an increasing white stork (Ciconia ciconia) population. Secondly, by a mechanistic approach, we studied the relative effects of climate, habitat and breeder experience in terms of reproductive fitness (fledging success, body condition and sex ratio) in this population monitored for 30 years in a sensitive wetland in Charente-Maritime, western France. This work helped identifying the mechanisms that lead to nesting habitat selection. This work supports research showing that habitat degradation by human activities may affect habitat selection decisions. In addition, these results provide crucial information to understand the adaptations of this population in a changing environment, which allows predicting more efficiently the response of the population to future environmental changes.
... Given the added nutritional burden that is associated with childbirth, women who can conceive and give birth during famine tend to be stronger and better nourished on average and so are their children. Second, the sex of newly born infant follows a binomial distribution [31]. At the population-level, sex ratio at birth is expected to show significant cross-temporal variations, a mixture of long-term secular trend, seasonal variations, and other short-term fluctuations [32][33][34]. ...
Article
This article reviews the growing interdisciplinary literature on the effect of privation and stress on human sex ratio at birth. Borrowing strength from the potential outcomes causal analysis framework, the discussion focuses on the issues of study design and identification strategy and how they have influenced the current state of the field. The review suggests that much of the inconsistency in the literature regarding the effect of privation and stress on human sex ratio at birth is due to the weak designs and over-simplistic identification strategies used in previous studies. Studies based on natural experimental designs and well-thought-out identification strategies, on the other hand, have produced rather compelling and consistent evidence suggesting that maternal privation and stress during pregnancy reduce male births.
... This investment can be biological investment by the mother before birth, and this kind of investment is shown in the sex ratio at birth (for a review of evidence for all societies, see Lazarus 2002). In the U.S., Almond and Edlund (2007) found evidence of a Trivers-Willard effect in the sex ratio of births and infant deaths to white mothers between 1983 and 2001. ...
Chapter
Full-text available
Sociobiology predicts that high-status, dominant individuals will out reproduce low-status individuals in a population. This prediction has been seemingly contradicted in modern societies, where women in high income households have fewer children than women in low income households. Yet if status is measured as personal income, in the U.S. and a variety of European countries there is a great deal of evidence that high-status males out reproduce low-status males (while the reverse is true for women). In this paper, I show how these findings are consistent with trends in pre-industrial societies. I further review studies of modern societies that support another important sociobiological prediction, as given by the Trivers-Willard hypothesis. I suggest that all these results are evidence that sociobiology (and associated evolutionary psychology) are relevant to modern populations.
... Genetic analysis should prove edifying, with special attention given to sexually antagonistic genetic regions (Rice, 1984;Saifl & Chandra, 1999;Stearns & Koella, 2007). Also, forthcoming studies should investigate the significance of extreme 2D:4D digit ratios (Manning, Stewart, Bundred, & Trivers, 2004) and extreme male-biased sex ratios (Garenne, 2002;Lazarus, 2002;Ruder, 1986) among Asian populations. Finally, in addition to simply strengthening the physical and physiological data as advised above, research efforts should be directed toward fundamentally explaining the cause of such data. ...
Article
Full-text available
The signs of mating competition are written into the physiology of the human male, but they are not written equally into the physiology of all racial groupings of human males. It seems that Asian males are different, different in that they are more fully dissimilar from the gorilla than are other races, showing less sexual dimorphism, muscularity, and less marked secondary sexual characteristics, and different in that they are more fully dissimilar from the chimpanzee than are other races, showing less sexual drive and activity as well as smaller testicles and lower sperm counts. It is presently argued that such anatomical differences are a testament to a more peaceably monogamous mating history. In turn, it is then argued that such physiological markers are directly associated with the collectivist ethos that has been historically, anthropologically, and sociologically observed among the Asian people.
... Importantly, we find that 48.5% of all the CEO children are daughters. In human populations, a sex ratio of about 105-106 sons for every 100 daughters is considered natural (e.g., Lazarus (2002)). That is, the sex ratio in our data set of CEOs is comparable to what would be expected in human populations. ...
Article
Full-text available
We show that large U.S. companies' policies regarding society at large and stakeholders other than their shareholders are systematically impacted by the firm's top decision-maker parenting a daughter. A CEO who has a female child increases a firm's corporate social responsibility (CSR) rating by about 11.9% compared to a median firm, the effect being about one third of the effect of an executive herself being female. The impact is strongest for diversity ratings, but also significant for broader pro-social policies related to the environment and employee relations. The evidence is consistent with a social preferences model in which male executives partially internalize their daughters' experiences and values. The results are robust to considering several sources of endo-geneity, e.g., examining only first-born CEO daughters, Trivers-Willard effects, and child gender preferences. The study contributes to several research areas, including female socialization studies, the origins of firms' CSR policies, and exogenous determinants of executives' styles.
... Similar inconsistencies have been found in humans. Lazarus (2002) revealed that only twenty-six of fifty-four studies (48%) of status and birth sex ratio among humans have reported results consistent with the Trivers-Willard hypothesis. Although these inconsistencies do not appear to bode well for sex-ratio biasing, they may be due to differences in the way maternal condition is measured across studies. ...
Chapter
Mothers play an important role in helping their children achieve maximal reproductive success. We explore how mothers across species manipulate birth sex ratios favoring the sex that will be best suited to their environments and how maternal competition affects offspring reproductive success in nonhuman mammals as well as humans. The Trivers-Willard hypothesis, resource competition hypothesis, resource enhancement hypothesis, and maternal dominance hypothesis are considered with respect to maternal birth sex ratio manipulation. Next, the primate literature is reviewed as inspiration for hypotheses on maternal competition for positive offspring outcomes. Nonhuman primates as well as humans are argued to compete for status, breeding opportunities, and allomothers (i.e., caregivers apart from the mother), and these factors have an impact on their reproductive success. Status is passed on from mother to offspring, amplifying the effects of competition for status. Future directions are delineated to fill in gaps in the existing literature.
... (James, 2012). The typical observed sex ratio in human populations is about 105-106 boys for every 100 girls (Lazarus, 2002). A majority of studies on historical populations prior to the demographic transition and on contemporary low-income populations show that sex ratios differ markedly from this natural sex ratio in subgroups with different socioeconomic status (Bereczkei and Dunbar, 1997;Gibson and Mace, 2003;Mealey and Mackey, 1990;Pollet et al., 2009;Scott and Duncan, 1999; for an exception, see Zald ıvar et al., 1991). ...
Article
Available to download at : https://goo.gl/uFxrwq This study examines if there exists a positive association between socioeconomic status and the proportion of male births in humans, as proposed by Trivers and Willard in 1973, using individual-level data drawn from the complete population of Sweden. We examine more than 3,000,000 births between 1960 and 2007 using administrative register data with comprehensive information on various dimensions of socioeconomic status. We use six different operationalizations of socioeconomic status, including earnings, post-transfer income (including government allowances), wealth, parental wealth, educational level, and occupational class. We apply regression models that compare both changes in status for the same woman over time and differences in status across different women. We also measure socioeconomic status both at the year of child birth and the year of conception. Our results show the absence of any relationship between socioeconomic status and sex ratios, using a large number of different operationalizations of status. We conclude that no substantive relationship between socioeconomic status and sex ratios exists for the population and period of our study.
... Finally, there can exist combinations of these first three evolutionary explanations of sex differences, such as obligate sex differences that are emergently-moderated in certain contexts (e.g., height differences suppressed by poor nutrition), facultatively-mediated adaptations that are emergently-moderated in certain contexts (e.g., mate preference differences suppressed by religion), and so forth. For instance, although the variation appearing in local sex ratios is strongly associated with shifts in men's and women's sexuality (Guttentag and Secord 1983), it remains unclear which sex differences in sexuality are facultatively versus emergently responding to local sex ratios (Hudson and Den Boer 2004;Lazarus 2002;Marlowe and Berbesque 2012;Pedersen 1991). Only by clearly specifying the precise mechanisms-obligate, facultative, and emergent-will researchers be able to fully explain the connections between human sex ratio variation and psychological sex differences. ...
Chapter
Full-text available
Psychologists have uncovered dozens of ways men and women differ in affect, behavior, and cognition. Social role theorists assume that men’s and women’s psychological differences solely result from sex role socialization processes and sociopolitical power differentials, and, as a consequence, social role theorists further assume psychological sex differences will be smaller in cultures with more egalitarian sex role socialization and greater sociopolitical gender equity. In this chapter, evidence is marshaled across 21 data sources that directly challenge this foundational assumption of social role theory. Empirically, sex differences in most psychological traits—in personality, sexuality, attitudes, emotions, behaviors, and cognitive abilities—are conspicuously larger in cultures with more egalitarian sex role socialization and greater sociopolitical gender equity. Even sex differences in many physical traits such as height, obesity, and blood pressure are larger in cultures with more egalitarian sex role socialization and greater sociopolitical gender equity. Three alternative evolutionary perspectives on psychological sex differences—obligate sex differences, facultatively mediated sex differences, and emergently-moderated sex differences—appear to better explain the universal and culturally-variable sex differences reliably observed across human cultures.
... Accordingly, when the maternal condition declines, mothers tend to "produce a lower ratio of males to females" (Trivers and Willard 1973: 90;also Cronk 2007). This hypothesis, also referred to as adaptive sex ratio adjustment hypothesis, has been tested by several studies of human populations in recent years, although the results have been mixed (Keller et al. 2001;Lazarus 2002;Gibson and Mace 2003;Stein et al. 2004;Cronk 2007). In a recent paper published by Proceedings of the Royal Society B, which is entitled "Does famine influence sex ratio at birth? ...
Article
Full-text available
One of the largest famines in human history took place in China half a century ago. This disaster, lasting from 1958 to 1961 in many areas, resulted in a huge number of excess deaths. While the causes, magnitude and profound impacts of this catastrophe have been brought to light in recent decades, many issues about the famine remain to be adequately examined. This paper aims to fill some gaps in our knowledge about the demography of China's great famine. It concentrates on the demographic consequences of the famine and individual demographic responses in some of the most severely affected provinces. By analysing demographic data collected by China's 1982 and 1988 national fertility sample surveys, the study provides further insights on changes in marriage, mortality, fertility and pregnancy outcomes during the famine period. The study reveals a dramatic increase in mortality and a decrease in marriage and fertility during the famine period, which had a significant impact on Chinese population. There were remarkable differences, however, in the demographic consequences of the famine between urban and rural areas and in demographic responses between people with different social and demographic characteristics. These findings are very important in improving our understanding of past demographic behaviour.
... Over the past century, a huge number of studies has documented sex ratios skewed in response to a variety of environmental and social changes, including, for example, marital status (Norberg 2004), social class (Lazarus 2002), natural disasters (Fukuda et al. 1998), and other stressful events such as wars (James 2009) and psychological stress (Obel et al. 2007). Because the sex ratios for human offspring are most often collected at birth, it is difficult to pinpoint when the influences take place. ...
Article
The ability to adjust sex ratios at the individual level exists among all vertebrate groups studied to date. In many cases, there is evidence for facultative adjustment of sex ratios in response to environmental and/or social cues. Because environmental and social information must be first transduced into a physiological signal to influence sex ratios, hormones likely play a role in the adjustment of sex ratio in vertebrates, because the endocrine system acts as a prime communicator that directs physiological activities in response to changing external conditions. This symposium was developed to bring together investigators whose work on adjustment of sex ratio represents a variety of vertebrate groups in an effort to draw comparisons between species in which the sex-determination process is well-established and those in which more work is needed to understand how adjustments in sex ratio are occurring. This review summarizes potential hormone targets that may underlie the mechanisms of adjustment of sex ratio in humans, non-human mammals, birds, reptiles, and fishes.
... Overall evidence for the TWH is somewhat mixed and has been confined to a limited number of human populations (see Lazarus (2002)). Most closely related to our paper, studies by Denny (2008) on British data, as well as Pollet and Nettle (2010) on British and Guatemalan anthropometric data of parents have rejected the gTWH. ...
Article
Full-text available
This paper tests the generalized Trivers Willard hypothesis in the spirit of Kanazawa (2005), which predicts that parents with heritable traits that increase the relative reproductive success of males compared to females will have relatively more male than female offspring. We test whether taller mothers are more likely to have a male first-born using data on 400,302 mothers in a sample of Demographic Health and Surveys (DHS) from 46 developing countries. Despite using a plethora of statistical models that take into account the multi-level structure of the data, we find no strong evidence in favor of the hypothesis between and within communities, as well as on a country-by-country basis. Conversely, Andrews (1989)'s inverse power calculations suggest that the absence of a statistically small effect cannot be rejected.
... Reviews have shown that 89 tests of the hypothesis on primates (including man) have only supported it half of the time. Furthermore, some test results went contrary to this hypothesis (Brown and Silk, 2002;Lazarus, 2002). Male vulnerability is manifest in premature births, as well as in term babies, with higher morbidity and mortality rates that persevere into early childhood. ...
Article
Xjenza is the Journal of the Malta Chamber of Scientists and is published by the Chamber in in electronic format. Xjenza is a peer-reviewed, open access international journal. The scope of the journal encompasses research articles, original research reports, reviews, short communications and scientific commentaries in the fields of mathematics, statistics, geology, engineering, computer science, social sciences, natural and earth sciences, technological sciences, linguistics, industrial, nanotechnology, biology, chemistry, physics, zoology, medical studies, electronics and all other applied and theoretical aspect of sciences. The first issue of the journal was published in 1996 and the last volume (No. 12) was published in 2007. We are now planning to restart publishing this journal regularly and will have a new number in March 2013. The new editorial board has been formed with internationally recognised scientists and we will produce 2 issues per year. One of the aims of Xjenza, besides highlighting the exciting research being done nationally and internationally by Maltese scholars, is to provide training for undergraduate and graduate students and young researchers in the art of scientific publishing in a peer-reviewed environment. Xjenza, therefore, might become an essential tool in the scientific development of early-stage researchers, and represent a learning platform where they can learn not only how to write a scientific paper, but also how to get it published. Initially Xjenza was mainly open to Maltese researchers. Submit to Xjenza! Manuscripts and supporting information must be submitted electronically (via e-mail) to the Editor on xjenza@mcs.org.mt. Please review the Author Guidelines and Submission Checklist thoroughly before beginning your submission.
... Once again, the mechanism by which this result comes about is unclear. It may be that high-status men experience less stress and therefore are more likely to father boys, perhaps because of changes in sperm motility (Fukuda et al. 1998) or changes in the frequency and timing of coitus (Lazarus 2002). Alternatively, the female partners of high-SES men may be less likely to experience stress and are thus more likely to conceive a male and/or carry a male fetus to term. ...
Article
Full-text available
This paper tests the Trivers-Willard hypothesis that high-status individuals will invest more in sons and low-status individuals will invest more in daughters using data from the 2000 to 2010 General Social Survey and the 1979 National Longitudinal Survey of Youth. We argue that the primary investment U.S. parents make in their children is in their children's education, and this investment is facilitated by a diverse market of educational choices at every educational level. We examine two measures of this investment: children's years of education and the highest degree attained. Results show that sons of high-status fathers receive more years of education and higher degrees than daughters, whereas daughters of low-status fathers receive more years of education and higher degrees than sons. Further analyses of possible mechanisms for these findings yield null results. We also find that males are more likely to have high-status fathers than females.
... This prediction has been experimentally validated in a number of animal models including ruminants (for review see [2]) but also in humans. A systematic review found 26 of 54 human studies reported population sex ratios that supported the Trivers-Willard hypothesis [3], with only one study reporting evidence against and the remainder observing no effect [4]. A similar analysis of experimental studies in mammals found that 312 of 422 studies supported the hypothesis when maternal body condition was factored into the analysis [5]; that is, mothers with higher plasma fatty acid or glucose concentration (reflective of mothers in ''better body condition'') tended to produce more males. ...
Article
Full-text available
Maternal diet can significantly skew the secondary sex ratio away from the expected 0.5 (proportion males) but the details of how diet may do this are unclear. Here, we have altered dietary levels of salt (4% salt in the feed) and/or fructose (10% in the drinking water) of pregnant rats to model potential effects that consumption of a 'Western' diet may have on materno-fetal growth, development and sex ratio. We demonstrate that excess fructose consumption before and during pregnancy led to a marked skew in the secondary sex ratio (proportion males, 0.60; P < 0.006). The effect was not mediated by selective developmental arrest of female embryos, or influenced by fetal position in the uterine horn or sex-specific effects on sperm motility, suggesting a direct effect of glycolysable monosaccharides on the maternal ovary and/or ovulated oocyte. Furthermore, combined excess maternal consumption of salt and fructose-sweetened beverage significantly reduced fertility, reflected as a 50% reduction in preimplantation and term litter size. In addition, we also noted birth order effects in the rat with sequential implantation sites tending to be occupied by the same sex.
... One potential contributor to these differences is the sex ratio at birth. For many populations, the sex ratio at birth in non-crisis situations is at around 105 males per 100 females (Markle 1974;Lazarus 2002). To see how this can affect fertility levels, consider a simple hypothetical scenario that ignores mortality, and in which there is only one cohort of males and females having children with partners of the same cohort. ...
Article
Full-text available
Obtaining cross-country comparative perspectives on male fertility has long been difficult, as male fertility is usually less well registered than female fertility. Recent methodological advancements in imputing missing paternal ages at childbirth enable us to provide a new database on male fertility. This new resource covers more than 330 million live births and is based on a consistent and well-tested set of methods. These methods allow us to handle missing information on the paternal age, which is missing for roughly 10% of births. The data resource is made available in the Human Fertility Collection and allows for the first time a comparative perspective on male fertility in high-income countries using high-quality birth register data. We analyze trends in male–female fertility quantum and tempo differentials across 17 high-income countries, dating as back as far as the late 1960s for some countries, and with data available for the majority of countries from the 1980s onward. Using descriptive and counterfactual analysis methods, we find substantial variation both across countries and over time. Related to the quantum we demonstrate that disparities between male and female period fertility rates are driven to a large degree by the interplay of parental age and cohort size differences. For parental age differences at childbirth, we observe a development toward smaller disparities, except in Eastern Europe. This observation fits with expectations based on gender theories. However, variation across countries also seems to be driven by factors other than gender equality.
... [43]) and sex-balancing of families (e.g. [44]) that can obscure the influence of natural selection, plus ethical constraints on experimentation [45][46][47][48][49]. ...
... Our study confirms previous findings on negative effects of high parity [3,29] and positive influences of a monogamous mating system on SRB [26]. According to parity, both groups -owner and non-owner -follow the same distribution patterns, while dwelling owners have always increased SRBs. ...
Article
Full-text available
Socio-economic conditions can affect the secondary sex ratio in humans. Mothers under good environmental conditions are predicted to increase the birth rates of sons according to the Trivers-Willard hypothesis (TWH). This study analyzed the effects of ownership and non-ownership of dwellings on the sex ratio at birth (SRB) on a Ugandan sample. Our investigation included 438,640 mothers aged between 12 and 54 years. The overall average SRB was 0.5008. Mothers who live in owned dwellings gave increased births to sons (0.5019) compared to those who live in non-owned dwellings (0.458). Multivariate statistics revealed the strongest effects of dwelling ownership when controlling for demographic and social variables such as marital status, type of marriage, mothers' age, mothers' education, parity and others. The results are discussed in the framework of recent plausible models dealing with the adjustment of the sex ratio. We conclude that the aspect of dwelling status could represent an important socio-economic parameter in relation to SRB variations in humans if further studies are able to analyze it between different countries in a comparative way.
... The human offspring sex ratio is around 1 : 1 and is understood primarily in terms of Fisher's principle [3]. Fisher's principle does not necessarily predict an even offspring sex ratio, but it will do so if parental investment after birth is equal for the average female and male. ...
Article
The ratio of males to females among an individual's offspring at birth (offspring sex ratio) has long been of great interest to evolutionary biologists. The human offspring sex ratio is around 1 : 1 and is understood primarily in terms of Fisher's principle (R. A. Fisher, The genetical theory of natural selection, 1930), which is based on the insight that in a population with an unequal sex ratio, each individual of the rarer sex will on average have greater reproductive value than each individual of the more common sex. Accordingly, individuals genetically predisposed to produce the rarer sex will tend to have greater fitness and thus genes predisposing to bearing that sex will increase in frequency until the population sex ratio approaches 1 : 1. An assumption of this perspective is that individuals' offspring sex ratio is heritable. However, the heritability in humans remains remarkably uncertain, with inconsistent findings and important power limitations of existing studies. To address this persistent uncertainty, we used data from the entire Swedish-born population born 1932 or later, including 3 543 243 individuals and their 4 753 269 children. To investigate whether offspring sex ratio is influenced by genetic variation, we tested the association between individuals' offspring's sex and their siblings' offspring's sex (n pairs = 14 015 421). We estimated that the heritability for offspring sex ratio was zero, with an upper 95% confidence interval of 0.002, rendering Fisher's principle and several other existing hypotheses untenable as frameworks for understanding human offspring sex ratio.
... Nevertheless, most of the studies provide evidence rather in favor of the TWH (Barthold et al. 2012;Betzig 2012;Bereczkei and Dunbar 1997;Cronk 2000;Fieder and Huber 2007;Hopcroft 2005;Hopcroft and Martin 2014;Kanazawa 2005;Luo et al. 2016;Nettle and Pollet, 2008;Pollet et al. 2009;Voland et al. 1997;Wallner et al. 2012;Weeden et al. 2006), while Lazarus (2002) analysis revealed that 48% (out of 54) studies support the TWH, similarly as in James (2006) analysis, in which the percentage was slightly higher (50%, out of 89 studies). On the other hand, the prevalence of such disproportion may not be surprising due to the publication bias, as results without significant findings tend to be published less often than those reporting significant links (Koricheva 2003;Kotze et al. 2004), which was also shown in the context of the TWH studies (Festa-Bianchet 1996). ...
Article
Full-text available
The Trivers-Willard Hypothesis (TWH), frequently investigated by evolutionary psychologists, states that human beings may have evolved to produce a greater number of sons when having a high status, and a greater number of daughters when having a low status. To test this hypothesis, we examined the sex of children of Polish high status: kings, dukes, magnates families; and of low status: peasants, burghers and gentry. Our findings do not provide evidence for the Trivers-Willard Hypothesis (TWH), as there were no differences between offspring’s sex ratio among any of the investigated social classes (with the exception of magnates families). We draw our conclusions with caution, as historical data carry many limitations.
... One potential contributor to these differences is the sex ratio at birth. For many populations, the sex ratio at birth in non-crisis situations is at around 105 males per 100 females (Markle 1974;Lazarus 2002). 2 To see how this can affect fertility levels, consider a simple hypothetical scenario that ignores mortality, and in which there is only one cohort of males and females having children with partners of the same cohort. ...
Preprint
Full-text available
Obtaining cross-country comparative perspectives on male fertility has long been difficult, as male fertility is usually less well registered than female fertility. This paper presents analyses based on a new male fertility database providing data on more than 330 million live births. This new resource, made available in the Human Fertility Collection, allows for the first time a comparative perspective on male fertility in high-income countries using high-quality birth register data. Contrasting male and female fertility trends across 17 countries, we show that trends in disparities between male and female period fertility rates are driven to a large degree by the interplay of parental age and cohort size differences. For parental age differences at childbirth, we observe a tendency toward smaller disparities, except in Eastern Europe. This observation fits with expectations based on gender theories. However, variation across countries also seems to be driven by factors other than gender equality.
... In humans, the Trivers-Willard effect has been tested a number of times, and it has been proposed, for example, to explain a higher than expected number of sons born to billionaires (Cameron and Dalerum 2009) and more sons born to married, better educated and younger mothers in US linked birth statistics (Almond and Edlund 2007). Nonetheless, evidence for the effect in humans is broadly inconclusive, with approximately half of published studies reporting null results (Lazarus 2002). ...
Article
Full-text available
Throughout the world, there is a male-bias in the sex ratio at birth (SRB). It is not known whether this phenomenon has a genetic basis, though there is tentative evidence from genealogical and genomic studies that it may have. It has been proposed that the higher rate of male childhood mortality in humans is linked to the male-bias in SRB through parental investment, but this may only apply to facultative not genetic sex ratio adjustment. In previous population genetic models, elevated mortality in one sex (prior to breeding) has been shown not to affect the SRB, but these models did not consider the role of replacement births (i.e. births that only occur because a sibling died prematurely). In a set of population genetic modelling simulations, in which sex ratio is controlled by an autosomal gene expressed in the male line, this study shows that when there is replace-ment of dead offspring, this leads to a sustained bias in the SRB in the direction of the sex suffering the highest mortality. In the example of higher male mortality, this occurs, because replacement offspring are disproportionately drawn from fathers who were genetically predisposed to have initially had sons (because sons were more likely to die prematurely), and more likely to pass on male-biasing alleles to replacement offspring. To test the empirical basis for replacement births, an analysis of birth data from the Demographic and Health Survey program was conducted, which shows that parents do indeed tend to replace children who die prematurely.
... Strangely, empirical evidence for sex differences in parent-offspring similarity, or the extent to which parents transmit their traits to their offspring, is rarely the focus of investigation, or is simply assumed to ensue from differential parental allocations. This is surprising, insofar as a fundamental assumption underlying tests for adaptive variation in parental investment turns on how effective parents can be in transmitting their traits to their offspring of each sex [25,53,63]. ...
Article
Persistent interest lies in gender inequality, especially with regard to the favouring of sons over daughters. Economists are concerned with how privilege is transmitted across generations, and anthropologists have long studied sex-biased inheritance norms. There has, however, been no focused cross-cultural investigation of how parent–offspring correlations in wealth vary by offspring sex. We estimate these correlations for 38 wealth measures, including somatic and relational wealth, from 15 populations ranging from hunter–gatherers to small-scale farmers. Although small sample sizes limit our statistical power, we find no evidence of ubiquitous male bias, at least as inferred from comparing parent–son and parent–daughter correlations. Rather we find wide variation in signatures of sex bias, with evidence of both son and daughter-biased transmission. Further, we introduce a model that helps pinpoint the conditions under which simple mid-point parent–offspring wealth correlations can reveal information about sex-biased parental investment. Our findings are relevant to the study of female-biased kinship by revealing just how little normative descriptors of kinship systems, such as patrilineal inheritance, capture intergenerational correlations in wealth, and how variable parent–son and parent–daughter correlations can be. This article is part of the theme issue ‘The evolution of female-biased kinship in humans and other mammals'.
... Furthermore, the non-human literature also indicates an evolutionary or functional rationale for sex ratio manipulation which should also apply to humans; previous tests, largely in WEIRD (Western, educated, industrialised, rich and democratic) populations replete with potential confounds (Beckerman et al. 2017), however, ‡These authors contributed equally to the paper. have found mixed support for this (Cronk 2007;Lazarus 2002). Here we set out to explore the underlying explanationin terms of both evolutionary function and potential mechanismfor the malebiased sex ratio in the Palanan Agta, a foraging population from the Philippines. ...
Preprint
Full-text available
Male-biased sex ratios have been observed in multiple small-scale societies. Although intentional and systematic female-biased mortality has been posited as an explanation, there is often a lack of ethnographic evidence of systematic female neglect and/or infanticide. The Agta, a foraging population from the Philippines, have a skewed sex ratio of 1.29 (129 males per 100 females) aged 15 years or under. We hypothesised that this skew was not caused by greater female deaths, but due to an adaptive response, where more males were produced at birth in reaction to high male-biased extrinsic mortality. To test this hypothesis we utilise census, childcare and mortality data from 915 Agta. The Agta's sex ratio is significantly male-biased in the <1 (n = 48, 2:1) and 1-5 year (n = 170, 1.39:1) age cohorts; however, we find no evidence of systematic female neglect in patterns of childcare. Furthermore, the sex ratio decreases over cohorts, becoming balanced by the end of the juvenile period, owing to significantly higher male mortality. Taken together, these results are not supportive of female infanticide or neglect, and instead suggest an adaptive mechanism, acting in utero as a response to male-biased juvenile mortality, following Fisherian principles of equalising parental investment.
... Our focus on a specific historical determinant of the sex-ratio is not meant to imply that other factors are irrelevant in determining the sex ratio. The sex ratio can be affected by a host of factors at conception (primary sex ratio), during pregnancy (secondary sex ratio), and after birth (tertiary sex ratio) [18,19]. A large literature in biology and medicine has studied the determinants of primary and secondary sex ratios [11]. ...
Article
Full-text available
We study the historical origins of cross-country differences in the male-to-female sex ratio. Our analysis focuses on the use of the plough in traditional agriculture. In societies that did not use the plough, women tended to participate in agriculture as actively as men. By contrast, in societies that used the plough, men specialized in agricultural work, due to the physical strength needed to pull the plough or control the animal that pulls it. We hypothesize that this difference caused plough-using societies to value boys more than girls. Today, this belief is reflected in male-biased sex ratios, which arise due to sex-selective abortion or infanticide, or gender-differences in access to family resources, which results in higher mortality rates for girls. Testing this hypothesis, we show that descendants of societies that traditionally practiced plough agriculture today have higher average male-to-female sex ratios. We find that this effect systematically increases in magnitude and statistical significance as one looks at older cohorts. Estimates using instrumental variables confirm our findings from multivariate OLS analysis.
... Interestingly, there is a previous link between mercury and offspring sex ratio in humans; a decrease in the number of male births was observed among mothers exposed to mercury at Minamata, Japan (Sakamoto et al. 2001). Either of the above hypotheses could explain these results (Sakamoto et al. 2001), as adaptive sex ratio variation has been observed in humans and other mammals (Lazarus 2002, Rosenfeld and Roberts 2004, Sheldon and West 2004, Almond and Edlund 2007, and maternal testosterone levels have been implicated in human sex determination (Grant 1996, Harrison andMansfield 1980). The pathway for the production of steroid hormones is largely conserved across vertebrates, but more must be learned about mechanisms of sex ratio bias before hypothesizing about a link between mercury exposure and sex ratio bias in humans. ...
Article
Full-text available
Mercury is a ubiquitous environmental pollutant that can negatively impact physiology and behavior of vertebrates, causing sub-lethal changes in condition and reducing fitness. Here we examine its effect on offspring sex ratio. Previous studies demonstrate the ability of environmental contaminants to skew sex ratios in wild populations toward the production of females, and research in humans has demonstrated a decrease in male births following mercury exposure. We therefore hypothesized that female birds inhabiting the floodplain of a mercury-contaminated river would produce broods more biased towards the production of females relative to birds from uncontaminated areas. We examined complete broods of three species: the aquatic-feeding belted kingfisher Megaceryle alcyon, the terrestrial-feeding eastern bluebird Sialia sialis, and the tree swallow Tachycineta bicolor, which feeds from both aquatic and terrestrial sources. Nestling sex ratios were shifted toward the production of females in all three species on mercury-contaminated sites when compared to uncontaminated reference sites. These results may be explained by endocrine disruption or the Trivers—Willard theory of sex allocation. Our study is the first to examine the impact of mercury on offspring sex ratios in birds, and therefore contributes to our understanding of the potential for this persistent biomagnifying contaminant to alter fitness and effective population size in wildlife.
... Importantly, we find that 48.1% of all the CEO children are daughters. In human populations, a sex composition ratio of about 105-106 sons for every 100 daughters is considered natural (e.g., Lazarus, 2002). That is, the sex ratio in our data set of CEOs is comparable to what would be expected in human populations. ...
Article
Full-text available
Corporate executives managing some of the largest public companies in the U.S. are shaped by their daughters. When a firm's chief executive officer (CEO) has a daughter, the corporate social responsibility rating (CSR) is about 9.1% higher, compared to a median firm. The results are robust to confronting several sources of endogeneity, e.g., examining first-born CEO daughters and CEO changes. The relation is strongest for diversity, but significant also for broader pro-social practices related to the environment and employee relations. Our study contributes to research on female socialization, heterogeneity in CSR policies, and plausibly exogenous determinants of CEOs' styles.
... In theoretical biology, the Trivers-Willard hypothesis (see below) posits a connection between the sex ratio at birth (secondary sex ratio) and the condition (health, nutritional status, and so on) of the mother. Since its publication (Trivers and Willard 1973), it has been repeatedly tested against human data (reviews in Cronk 2007;Hrdy 1987;Lazarus 2002;Sieff 1990), with mixed results. Most of these human studies use data from complex societies with modern medicine and effective methods of birth control, whereas the original hypothesis refers to conditions where those evolutionarily novel potential confounders are absent. ...
Article
Full-text available
Partible paternity, the belief that a child can have more than one biological father, is widespread in lowland South America. An analysis of demographic data sets from four lowland tribes (Aché, Barí, Ese Eja, and Surui) reveals a systematic variation in the sex ratios of live births with respect to the number of fathers to whom the births are attributed. Births attributed to only one father show a sex ratio that is unexceptional for South America; births with two fathers are highly male biased, while children with three or more fathers are female biased. This pattern may be a manifestation of a phenomenon predicted by the Trivers-Willard hypothesis, which proposes that, in many circumstances, females in good condition might bias their offspring toward males, whereas those in poor condition would produce a preponderance of females. If, as suggested below, a woman with a husband and a single extramarital lover tends to be better cared for before and during a pregnancy than other women, this difference might result in the improved maternal condition required by the Trivers-Willard hypothesis for excess males, whereas women who accept two or more lovers might be preponderantly those who are already in distress, thus tending to produce female-biased offspring.
... The present study adds to an earlier study by Luo, Zhao & Weng (2016) which also provides support for the TWH in a contemporary rural Chinese context. In addition, both studies discover a significantly inverse relationship between offspring sex ratio and parity, as is consistent with past findings (Chahnazarian, 1988;Lazarus, 2002). That sex ratio tends to decrease as parity increases should be a general pattern in human populations, as a cross-country comparative study by Mace & Jordan (2005) discovered that sex ratio at birth was negatively correlated with fertility rates. ...
Article
Full-text available
According to the logic of the Trivers–Willard hypothesis, in a human population, if socioeconomic status is transmitted across generations to some extent, and if sons of high-status parents tend to have higher reproductive success than daughters, while daughters of low-status parents tend to have higher reproductive success than sons, then we should expect that offspring sex ratio is positively associated with socioeconomic status. This study examines whether the assumptions and prediction of this hypothesis apply to a rural population in northern China. Results show that (1) current family socioeconomic status is positively related to family head’s father’s socioeconomic status in around 1950, (2) low-status family heads have more grandchildren through their daughters than their sons, whereas high- or middle-status family heads have more grandchildren through sons, and (3) as family heads’ status increases, they tend to produce a higher offspring sex ratio. Therefore, the assumptions and prediction of the hypothesis are met in the study population. These results are discussed in reference to past studies on sex ratio manipulation among humans.
... Measurement of the primary sex ratio however is highly invasive and ethically prohibitive and sperm sex ratio, the proportion of XX and XY bearing chromosomes in ejaculated spermatozoa, is a frequently referred to as a possible proxy. Some studies have shown differences in proportion of XX and XY bearing sperm according to father's siring of only sons or only daughters, as well as reduced sexual abstinence (Lazarus 2002). Extrapolating from these and animal studies one hypothesis portends that it is the balance of hormones testosterone and gonadotrophin for couples which determine the sex ratio at conception and fertilisation, and this is further linked to the frequency of coitus and timing of it in the menstrual cycle (James 2008). ...
Thesis
Full-text available
Background: The human secondary sex ratio has been the subject of long-standing medical, environmental and social scientific curiosity and research. A decline in male birth proportion in some industrialised countries is linked to endocrine disruption and is validated by some empirical studies. Increasing parental age and population stress and associated decreases in sex ratio have also been demonstrated. A thorough literature review of 123 relevant and diverse studies provides context for these assessments. Methods: A spatial-temporal investigation of birth sex ratio in Scotland and potential determinants of endocrine disruptor pollution, socio-economic factors including neighbourhood stress, deprivation, smoking, and maternal age, was conducted. This involved review of national and regional sex ratio time trends, and stratified/spatial analysis of such factors, including the use of GIS tools. Secondary data were sourced from Scottish Government web portals including Scottish Neighbourhood Statistics and the Scottish Environmental Protection Agency. Results: Regional differences in sex ratio between 1973 and 2010 are observed which likely lever the national male birth proportion downwards, with the region of poorest air quality from industrial emissions, the Forth Valley, displaying the greatest sex ratio reduction. Further analysis shows significant upwards skewing in sex ratio for the population cohort experiencing the least and 2nd most deprivation. Localised reductions in sex ratio for areas of high modelled endocrine disruptor pollution within the Central Region in Scotland are also displayed.
... The physiological mechanisms of this adaptive selection are still unknown (Williams, 1979;Brown and Silk, 2002), but this sex ratio adjustment (alteration of the male-to-female ratio at birth in response to exogenous environmental changes) has been demonstrated in animal models (Huck et al., 1988;Meikle and Thornton, 1995;Greeff and Ferguson, 1999;Kruuk et al., 1999). However, recent studies on human populations have provided mixed results (Keller et al., 2001;Lazarus, 2002;Gibson and Mace, 2003;Stein et al., 2004;Cronk, 2007). Trivers and Willard (1973) also proposed that in the human population maternal condition could impact the secondary sex ratio, and this effect could be measured by using socioeconomic status. ...
Article
Full-text available
Variations in sex ratios at birth is still an active research field and several studies in the last decades have focused on this topic. In this article, studies on the main determinant of long- and short-term trends are briefly reviewed, taking into account findings and results from different kinds of disciplines. In his early studies, Corrado Gini concluded that the human sex ratio at birth was universally stable, without significant fluctuations across time and space. However, in the last decades several authors have directly challenged these conclusions. Therefore, after summarizing the results of Gini’s research on the historical trends of the sex ratio at birth, a brief review focuses on the analyses of contemporary trends. The main determinants of the variations of the sex ratio at birth in time and space mentioned in the literature and the corresponding theoretical explanations are summarized. Special attention is paid to the recent studies on the impact of the environment, pollution and climate on the levels of sex ratio at birth.
... The two genomic studies that have looked for genetic variation related to sex ratio have found conflicting results [18,19]. The evidence for correlation of sex ratio with social and financial status is mixed and is probably not significant when taking into account publication bias [20]. The hormonal system of sex ratio control, argued for by William James in publications spanning five decades, has not been tested in a randomised controlled way that would draw a line under questions about sample sizes, publication bias and confounding variables. ...
Article
At face value, questions about the sex ratio have always seemed to have straightforward answers, which on closer inspection turn out to be fiendishly complex. The familial distribution of male and female births is no exception.
... Accordingly, when the maternal condition declines, mothers tend to "produce a lower ratio of males to females" (Trivers and Willard 1973: 90;also Cronk 2007). This hypothesis, also referred to as adaptive sex ratio adjustment hypothesis, has been tested by several studies of human populations in recent years, although the results have been mixed (Keller et al. 2001;Lazarus 2002;Gibson and Mace 2003;Stein et al. 2004;Cronk 2007). In a recent paper published by Proceedings of the Royal Society B, which is entitled "Does famine influence sex ratio at birth? ...
Article
Full-text available
Background: The adaptive sex ratio adjustment hypothesis suggests that when mothers are in poor conditions the sex ratio of their offspring will be biased towards females. Major famines provide opportunities for testing this hypothesis because they lead to the widespread deterioration of living conditions in the affected population. Objective: This study examines changes in sex ratio at birth before, during, and after China's 1958-1961 famine, to see whether they provide any support for the adaptive sex ratio adjustment hypothesis. Methods: We use descriptive statistics to analyse data collected by both China's 1982 and 1988 fertility sample surveys and examine changes in sex ratio at birth in recent history. In addition, we examine the effectiveness of using different methods to model changes in sex ratio at birth and compare their differences. Results: During China's 1958-1961 famine, reported sex ratio at birth remained notably higher than that observed in most countries in the world. The timing of the decline in sex ratio at birth did not coincide with the timing of the famine. After the famine, although living conditions were considerably improved, the sex ratio at birth was not higher but lower than that recorded during the famine. Conclusions: The analysis of the data collected by the two fertility surveys has found no evidence that changes in sex ratio at birth during China's 1958-1961 famine and the post-famine period supported the adaptive sex ratio adjustment hypothesis.
Article
Brood sex ratios (BSRs) have often been found to be nonrandom in respect of parental and environmental quality, and many hypotheses suggest that nonrandom sex ratios can be adaptive. To specifically test the adaptive value of biased BSRs, it is crucial to disentangle the consequences of BSR and maternal effects. In multiparous species, this requires cross-fostering experiments where foster parents rear offspring originating from multiple broods, and where the interactive effect of original and manipulated BSR on fitness components is tested. To our knowledge, our study on collared flycatchers (Ficedula albicollis) is the first that meets these requirements. In this species, where BSRs had previously been shown to be related to parental characteristics, we altered the original BSR of the parents shortly after hatching by cross-fostering nestlings among trios of broods and examined the effects on growth, mortality and recruitment of the nestlings. We found that original and experimental BSR, as well as the interaction of the two, were unrelated to the fitness components considered. Nestling growth was related only to background variables, namely brood size and hatching rank. Nestling mortality was related only to hatching asynchrony. Our results therefore do not support that the observed BSRs are adaptive in our study population. However, we cannot exclude the possibility of direct effects of experimentally altered BSRs on parental fitness, which should be evaluated in the future. In addition, studies similar to ours are required on various species to get a clearer picture of the adaptive value of nonrandom BSRs.
Article
A sizeable economics literature explores the effect of prenatal shocks on later health or socioeconomic status. Work in other disciplines, following the seminal contribution of Trivers and Willard (1973), suggests that prenatal shocks may increase fetal loss and reduce the number of boys relative to girls at birth. This has been largely ignored in the economics literature and could affect the interpretation of estimates of the effect of prenatal shocks and that of gender in other applied economics contexts. This paper analyzes the effect of in utero exposure to a shock - civil conflict in Nepal - on (i) fetal loss, and (ii) gender and (iii) health at birth. Maternal fixed effects estimates show that exposed pregnancies are more likely to result in a miscarriage and in a female birth, but exposed newborns are neither smaller nor more subject to neonatal mortality.
Chapter
Full-text available
In this paper, we use evolutionary models of sex ratio variation to examine offspring sex ratios in Matlab, Bangladesh, from the 1960s to 2010, during which time sex ratios have shown decreasing male-bias. Evolutionary models lead us to examine particular aspects of family ecology, yielding predictions both unique from and similar to those proposed in the demographic literature. We examine three evolutionary models—the Costs of Reproduction model, the Trivers-Willard hypothesis, and the Local Resource Competition and Enhancement models. Our results support both the Trivers-Willard and Local Resource Competition/Enhancement model, but results for the Costs of Reproduction model are weak. In general, we find that variables associated with higher wealth, status, and engagement in agriculture are linked to more male-biased offspring sex ratios, while higher fertility, older age at marriage, and higher women’s education are associated with less male-biased sex ratios. We also examine how Bangladesh fits into the larger cultural area of South Asia known in the press and policy circles for its high sex ratios. We compare the correlates of sex ratio in Matlab to those in other parts of the region and discuss why sex ratios in Bangladesh are less male-biased despite shared cultural characteristics such as son preference and dowry some argue ‘cause’ high sex ratios in parts of India and Pakistan. We conclude with a discussion of the utility of evolutionary models and offer policy recommendations for the region.
Thesis
Full-text available
I am concerned with maternal investment in avian eggs. Firstly using the domestic goose, a seasonal breeder, changes in egg weight over the laying season, and changes in steroid hormone (androstenedione, testosterone, progesterone) concentrations in the outer layer of the egg yolk over the season are examined. Secondly, I am concerned with the phenomenon of ‘twinning’ in double-yolked eggs (multiple ovulations) in terms of a) its frequency, causation, and b) constraints on its evolution, and c) insights it may reveal about the incubation process itself. These studies were conducted with domestic ducks in a domestic environment when they lay all year round. Geese have limited endogenous reserves to invest in reproduction over a laying season, but will continue laying especially when feed supplemented and eggs are removed from the nest regularly. Here, even in feed supplemented adult geese the egg weight declined (weeks 1-8) to baseline levels and then – as was found in this study – the baseline weight was maintained until the end of the 19 week laying season. This weight loss is attributed to the female’s resource depletion and was accompanied with an increase in testosterone concentration over the laying season. The increase in testosterone is consistent with a trade-off for the seasonal decline in egg weight and viewed as a mechanism to compensate for small egg size affecting development, growth and / or behaviour of the offspring to enhance its survival and the mother’s fitness. Double-yolked (DY) eggs are produced via multiple ovulations, when higher numbers of yellow follicles are recruited to the hierarchy. Here, evidence shows the increased prevalence of small DY eggs at the onset of laying in several batches of new layers. Further, differences in the physical appearance of DY and single-yolked (SY) eggs are presented and discussed. This thesis shows that the size and order of yolks in DY eggs influence albumen secretion and fertilization, ie. larger yolks stimulate the secretion of more albumen and have higher chance of achieving fertilization than smaller ones. Moreover, yolk position in DY eggs is related to fertility. The lower hatchability of DY eggs is associated with incubational weight loss, mortality and embryo malpositioning.
Article
Full-text available
In the U.S. there is evidence of a Trivers-Willard (T-W) effect in educational attainment, such that the sons of high status fathers attain more education than the daughters, and the daughters of low status fathers attain more education than the sons. This paper seeks to uncover the mechanisms by which this T-W effect occurs. Data are from the High School and Beyond Study by the U.S. National Center for Education Statistics. Results show that the T-W effect is not a result of the fact that fathers are less likely to be present in the homes of children of low status fathers. Other results show that the sons of high status fathers are more likely than daughters to be sent to private high school, while the daughters of low status fathers are more likely than sons to be sent to private high school. These parental investments pay off, as the sex gap in academic GPAS (favoring women) is narrower for the children of high status fathers than the children of low status fathers. In turn, academic GPA in high school helps explain the T-W effect in educational attainment. Parental non-financial investment as measured by student academic expectations as sophomores in high school also helps explain why the sons of high status fathers obtain higher degrees than the daughters. Together, these two factors: student’s academic GPA in high school and his /her expectations of educational attainment, fully explain the T-W effect in educational attainment in the U.S.
Article
Historians have assumed that early modern Europeans did not practice neo-naticide similar to the great Asian civilizations, but sex-ratio studies are only now entering the demographic literature. This article passes in review both published and unpublished research on sex ratios at baptism in Italy, France, England and colonial Acadia, together with juvenile sex ratios drawn from censuses in Germany, France and Italy. Both endemic and conjunctural imbalances appear everywhere, but they could target females or males depending upon the context.
Article
Full-text available
Survivorship of children is dependent upon numerous variables, including the role that preferential treatment may play in biasing the birth and survival of sons and daughters across cultures. This study draws upon an evolutionary approach by examining a theory referred to as the "Trivers-Willard hypothesis" concerning condition-dependent sex allocation and differential parental investment. Previous research on humans concerning this hypothesis tends to be restricted to one cultural group and thereby limited in sample size. For this study, nationally representative household survey data collected by the Demographic and Health Surveys (DHS+) program across 35 countries was used to test biological, resource-oriented, and behavioral aspects affecting maternal condition, sex allocation, and parental investment in humans. The units of analysis for this study were the mothers and their lastborn child (N = 128,039 woman-child pairs). A series of hierarchical regressions were executed to empirically investigate the TW hypothesis in humans. Scales were developed for maternal socioeconomic resources (MSR), maternal biological condition (MBC), prenatal care for the lastborn child (PCL), and health-seeking for the lastborn child (HSL). MSR was measured by relative household economic status, woman's and partner's education, and residence in an urban/rural setting. MBC was defined by body mass index, pregnancy status and duration, and breast-feeding status. PCL was an index for type of prenatal care received, number of prenatal visits, and assistance during delivery of the lastborn child. Lastly, HSL measured indicators of treatment for diarrhea and immunizations received by the lastborn child. Across the 35 countries, the analyses did not support the Trivers-Willard hypothesis. However, there is evidence of regional and country level differences.
Article
Full-text available
To further the understanding of the relationship between social class and sexual attitudes and behavior, we present data from a study of undergraduate students. We look at the education of students' fathers and how it relates to students' sexual profiles. Among the men, some traditional social class differences are found, indicating that class differences persist among some upwardly mobile men. For the women, fewer social class differences appear. Further, we compare our 1992 sample of 554 college students, 19–22 years old, with a university sample of 904 similar age students from 1967, and find our sample more coitally experienced. College students today are following norms that in the past were associated with a lower educational level Implications of our findings for class convergence theory are addressed. Reliable birth control, gains in equality by women, and the sexual images of television and other media are discussed as major factors contributing to the increased sexual permissiveness among university students of the 1990s.
Article
Full-text available
Female-biased parental investment is unusual but not unknown in human societies. Relevant explanatory models include Fisher's principle, the Trivers-Willard model, local mate and resource competition and enhancement, and economic rational actor models. Possible evidence of female-biased parental investment includes sex ratios, mortality rates, parents' stated preferences for offspring of one sex, and direct and indirect measurements of actual parental behavior. Possible examples of female-biased parental investment include the Mukogodo of Kenya, the Ifalukese of Micronesia, the Cheyenne of North America, the Herero of southern Africa, the Kanjar of south Asia, the Mundugumor of New Guinea, contemporary North America, and historical Germany, Portugal, and the United States.
Article
Full-text available
A historical survey of the inheritance practices of farming families in North America and elsewhere indicates that resource allocations among children differed through time and space with regard to sex bias and equality. Tensions between provisioning all children and maintaining a productive economic entity (the farm) were resolved in various ways, depending on population pressures, the family’s relative resource level, and the number and sex of children. Against a backdrop of generalized son preference, parents responded to ecological circumstances by investing in offspring differentially within and between the sexes. Vesting the preponderance of family resources in one heir increased the likelihood of at least one line surviving across several generations, whereas varying degrees of parental investment in emigrating sons or out-marrying daughters might yield boom or bust harvests of grandchildren according to circumstances in more remote locales. Primogeniture (eldest son as primary heir) allowed early identification of heirs and appropriate socialization, as well as more time for parents to contribute to the heir’s reproductive success. Son bias and unigeniture decreased as numbers of children per family declined, as land became less critical to economic success, and as legal changes improved the resource-holding potential of females. We suggest that changing ecological conditions affected parental decisions regarding resource allocation among children at least as much as did changing ideologies of parent-child relations.
Article
Full-text available
Although theories of parental investment and sex ratio generally assume that a single resource limits reproduction, many organisms invest two or more qualitatively different types of resources in the production of offspring. We examine the consequences of multifaceted parental investment for offspring provisioning and sex allocation, building our argument around a study of the nest-building Hymenoptera (wasps, bees, and ants). We review empirical studies that demonstrate that lifetime reproductive success may be constrained not only by resources used to provision offspring but also by the supply of mature oocytes or, in some cases, by the availability of space within nest sites or the time required to defend nests. Under multifaceted parental investment, the factor limiting parental fitness determines the currency of the optimization problem; parents are predicted to adjust reproductive behavior to maximize fitness returns per unit of the limiting resource. We develop simple models that predict that a greater availability of resources used for provisions will lead to an increase in the amount provisioned per offspring and an increase in the numerical or biomass proportion of females produced. These predictions explain widely observed patterns of variation in offspring provisioning and sex allocation in the nest-building Hymenoptera.
Article
Full-text available
In most social species, position in the male social hierarchy and reproductive success are positively correlated; in humans, however, this relationship is less clear, with studies of traditional societies yielding mixed results. In the most economically advanced human populations, the adaptiveness of status vanishes altogether; social status and fertility are uncorrelated. These findings have been interpreted to suggest that evolutionary principles may not be appropriate for the explanation of human behavior, especially in modern environments. The present study tests the adaptiveness of social status with actual mating and reproductive data in a representative sample of males from an industrial society. Reproductive success, even when assessed by a more reliable measure of actual male fertility than the one commonly used, fails to correlate with social status. In striking contrast, however, status is found to be highly correlated with potential fertility, as estimated from copulation frequency. Status thus accounts for as much as 62% of the variance in this proximate component of fitness. This pattern is remarkably similar to what is found in many traditional societies and would result in a substantial positive relationship between cultural and reproductive success in industrial populations were it not for the novel conditions imposed by contraception and monogamy. Various underlying mechanisms are suggested for these findings, illustrating the value of current behavioral and reproductive data in the study of adaptation. It is concluded that evolutionary explanations of human behavior remain entirely relevant in modern societies.
Article
Full-text available
When parent-offspring relations in sexually reproducing species are viewed from the standpoint of the offspring as well as the parent, conflict is seen to be an expected feature of such relations. In particular, parent and offspring are expected to disagree over how long the period of parental investment should last, over the amount of parental investment that should be given, and over the altruistic and egoistic tendencies of the offspring as these tendencies affect other relatives. In addition, under certain conditions parents and offspring are expected to disagree over the preferred sex of the potential offspring. In general, parent-offspring conflict is expected to increase during the period of parental care, and offspring are expected to employ psychological weapons in order to compete with their parents. Detailed data on mother-offspring relations in mammals are consistent with the arguments presented. Conflict in some species, including the human species, is expected to extend to the adult reproductive role of the offspring: under certain conditions parents are expected to attempt to mold an offspring, against its better interests, into a permanent nonreproductive.
Chapter
Full-text available
The reduction in fertility accompanying modernisation poses a scientific puzzle that has yet to be solved. Despite the fact the problem has received a great deal of attention from economists, sociologists, demographers, anthropologists and biologists, no discipline in the social or biological sciences has offered a fully developed and coherent theory of fertility reduction that explains the timing and pattern of fertility reduction in the developed or developing world. The inability to offer an adequate theory raises fundamental questions about the theoretical foundations of those disciplines. For example, although economics has made great strides in explaining consumer behaviour, time allocation and labour force participation through the recognition that the household is a fundamental organisational unit of human action, there is no adequate explanation of why households are mostly composed of men and women who marry and have children. There is no economic theory of why reproductive partnerships form such a fundamental organisational principle in human societies nor of why people have and want children in the first place. The very modest progress of economists in explaining long-, medium- and short-term trends in fertility highlights this weakness.
Article
Full-text available
Hypothesized that men prefer women around their own age, but that as they grow older, men develop a preference for women who, although not absolutely younger, are progressively younger than themselves and that women begin with a preference for older men, and compared with men, show less variation in that preference over their life span. Six studies support this gender-differentiated prediction in age preferences expressed in 218 personal advertisements, 1,189 marriages from 2 US cities, 100 marriages in 1923, matrimonial advertisements from 2 European countries and India, 1,789 marriages recorded from 1913–1939 on a small island in the Philippines, and 213 singles advertisements placed by financially successful American women and men. Limitations of normative and evolutionary explanations of age preferences are considered. 30 commentaries and an author response follow. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
Article
Full-text available
Contemporary mate preferences can provide important clues to human reproductive history. Little is known about which characteristics people value in potential mates. Five predictions were made about sex differences in human mate preferences based on evolutionary conceptions of parental investment, sexual selection, human reproductive capacity, and sexual asymmetries regarding certainty of paternity versus maternity. The predictions centered on how each sex valued earning capacity, ambition— industriousness, youth, physical attractiveness, and chastity. Predictions were tested in data from 37 samples drawn from 33 countries located on six continents and five islands (total N = 10,047). For 27 countries, demographic data on actual age at marriage provided a validity check on questionnaire data. Females were found to value cues to resource acquisition in potential mates more highly than males. Characteristics signaling reproductive capacity were valued more by males than by females. These sex differences may reflect different evolutionary selection pressures on human males and females; they provide powerful cross-cultural evidence of current sex differences in reproductive strategies. Discussion focuses on proximate mechanisms underlying mate preferences, consequences for human intrasexual competition, and the limitations of this study.
Article
Full-text available
Biologically based behaviors are viewed as highly situation dependent, constrained, and shaped by local history and environments. This perspective is illustrated by tracing through time changes in the patterning of parental behavior toward sons vs. daughters. Although it is assumed that decisions about parental investment are rooted in evolved predispositions, parental behaviors change in response to changing conditions affecting the productive and reproductive contributions of sons vs. daughters.
Article
Full-text available
Because theoryp redictse qual productiono f the two sexes,a ny deviation frome quality invitesi nterpretationan, d prods the imaginationso f theoreticians. Triversa nd Willard( 1973) have putf orwarda hypothesitso explainw hy food or othere nvironmentaslt ressm ights elect fort he productiono f female offspringIn. this paper I will criticallyr eviewt he Trivers-Willarhdy pothesis and propose an alternativhe ypothesios fw hys ex ratioa djustmentm ighto ccur in populationsf orw hicht he resourcesn ecessaryf ort he productiono f young are limite
Article
Full-text available
Alternative evolutionary hypotheses concerning a female's offspring sex ratio predict opposing deviations from unity. I suggest that researchers are more likely to attempt to publish sex ratio data when post-hoc analyses reveal significant deviations from unity, and that published studies reporting deviations from unity are more widely read than studies reporting no deviations. As a result, the scientific literature represents a biased sample of the occurrence of skewed offspring sex ratios in nature. The merits of sex ratio studies should be evaluated independently of the direction of the results or the presence of significant deviations from unity. Investigators must be encouraged to publish sex ratio data from long-term studies.
Article
Full-text available
The present study evaluated the proportions of X-bearing and Y-bearing sperm within the semen of donors who were the declared fathers of three or more sons or daughters. The proportions of sperm were determined using dual-color fluorescence in situ hybridization to identify the X and Y chromosomes. The only difference observed was in semen volume. There was no increase in the proportion of Y-bearing sperm for men with only sons (49.7 +/- 1.3%) or of X-bearing sperm for men with only daughters (44.8 +/- 2.6%). A preponderance of either sons or daughters in a family cannot be explained simply by an altered ratio of X-bearing and Y-bearing sperm in the father's semen.
Article
Full-text available
Hierarchies of wealth and ethnic prestige among East African herders present an opportunity to test the Trivers-Willard hypothesis that low socioeconomic status should correlate with female biases in parental investment. The Mukogodo are at the bottom of such a regional hierarchy due to their poverty and low status as former hunters. As a result of these factors, Mukogodo men have lower polygyny rates than their neighbors, and Mukogodo women have higher mean reproductive success than Mukogodo men. The data fulfill the prediction that there should be a bias in parental investment in favor of daughters. The sex ratio of the 0–4 age group and the reported sex ratio at birth are both female-biased. Although there is no evidence of infanticide, sons may be neglected in favor of daughters. Evidence from a dispensary and from a clinic run by a Catholic mission both show that the Mukogodo take daughters for treatment more often than they take sons. Also, daughters may be nursed longer than sons.
Article
Full-text available
Demographic data from 848 Gabbra households are used to examine the relationships between herd size and reproductive success in relation to sex, in a traditional, pastoralist population. The number of camels in the household herd has a significant positive effect on the reproductive success of both men and women, although the effect of wealth is greater for men, as predicted from evolutionary theory. The greater the number of elder brothers a man has, the lower his reproductive success, as a result of a smaller initial herd and a later age at marriage. This is not true for women –number of elder sisters does not have a measurable effect on a woman’s fertility, although it does have a small, negative effect on the size of her dowry. These results are interpreted as competition between same-sex siblings for parental investment, in the form of their father’s herd, which is more intense between sons than daughters as parental investments are greatest in males.
Article
The world literature on manipulations purported to alter the gender of offspring is a quagmire full of reports presenting inadequate data and contradictory conclusions. Many are due to inappropriate statistical design or analysis. The authors present simple, nontechnical reference tables for determination of the minimum sample size necessary to detect a change in sex ratio (1) from a theoretical value, and (2) between two experimental populations. Simple methods for analyzing the results of gender manipulation experiments are described. Technical details are included in a separate section.
Article
Fluorescent (F) body distribution was determined in a group of men who did not have a fertility problem, but rather had fathered exclusively female offspring. The study was designed to analyze spermatozoa for the frequency of zero F-body (X-bearing) and one F-body (Y-bearing) spermatozoa. Semen samples were separated (processed) for Y-bearing spermatozoa enrichment and reanalyzed for fluorescent body distribution. The study consisted of 50 control (10 males) samples (unprocessed), 35 preseparation (35 patients) samples (unprocessed), and 18 postseparation (18 patients) samples (processed). A significantly higher frequency ( P
Article
Data from the Kipsigis of Kenya are used to test two models for how parents invest in offspring, the Trivers-Willard and local resource competition/enhancement hypotheses. Investment is measured as age-specific survival, educational success, marital arrangements, and some components of property inheritance, permitting an evaluation of how biases persist or alter over the period of dependence. Changes through time in such biases are also examined. Despite stronger effects of wealth on the reproductive success of men than women, the survival of sons and daughters is not related to parental wealth. However, a Trivers-Willard effect characterizes educational investment: poor families show a greater concern for daughters' (vis-à-vis sons') schooling than do rich families, a trend that has increased over time. In regard to models of local resource competition and enhancement, men's reproductive success decreases with number of brothers and increases with number of sisters; this pattern of competition with same-sex sibs and cooperation with opposite-sex sibs is not found among women. As predicted from these observations, parents show reduced investment in sons with a large number of brothers, and increased investment in sons with a large number of sisters. By contrast, investment in daughters is entirely unaffected by number of sisters and is influenced only in subtle ways by number of brothers. Levels of investment in relation to sibship size (irrespective of siblings' sex) are highest for younger children of large sib sets. Discussion of the results in relation to those from other studies leads to three conclusions. First, predictive models for how investment biases vary across societies must consider a broad range of socioecological factors constraining parental options and payoffs. Second, the timing of investment biases within societies will be affected by the value of children and the costs of parental investment. Third, measures of investment appropriate for between-sex and between-class comparisons need careful attention. Each of these issues is brought to bear on the question of why, in contrast to so many other parts of the world, sex preferences are so muted in Africa.
Article
Success, in evolutionary terms, means contributing more surviving offspring to the next generation than competing individuals of the same species in the same population. Human conception is a probabilistic event occurring against a background of frequent, usually infertile sex, which helps bond parents together. Humans have an innate drive for sex and for nurturing their children as they arrive, but they have no biological predisposition for a specific number of children. In preliterate societies, in the absence of artificial means of fertility regulation, pregnancies are spaced several years apart by unconscious physiological mechanisms based on breast-feeding. In preliterate and in preindustrial urban societies, socially successful individuals commonly had larger than average families. Once people have unconstrained access to a range of fertility-regulation options (including safe abortion), family size falls in all groups and in all societies. In such a context, social success tends to be associated with the accumulation of material wealth, rather than with having more children. The argument that development causes fertility decline is flawed because people cannot make choices about family size without realistic access to fertility-regulation technologies, and such access is historically recent and remains geographically limited. Where access to fertility regulation is constrained, the richer and more educated are usually better able than the less privileged to surmount the barriers between them and the needed technologies, hence the common inverse relationship between income and family size. Policies derived from this perspective are discussed.
Article
Independent samples of women were surveyed to test Trivers and Willard's hypothesis that the mother's condition and her ability to invest in her offspring affect the (secondary) sex ratio of her offspring. Patterns of sex ratios (number of males per 100 females) were analyzed in conjunction with four attributes of a mother's microenvironment: level of health in her community, family structure, relative access to resources, and her birthing history. The results inferentially support the hypothesis that the microenvironment of the woman would act to bias the sex ratio of her offspring. These specific data lend support to Trivers and Willard's general hypothesis.
Article
We present a quantitative model of sex allocation to investigate whether the simple "rules of thumb" suggested by Trivers and Willard (1973) would really maximize numbers of grandchildren in human populations. Using demographic data from the !Kung of southern Africa and the basic assumptions of the Trivers-Willard hypothesis, we calculate expected numbers of grandchildren based on age- and sex-specific reproductive value. Patterns of parental investment that would maximize numbers of expected grandchildren often differ from the Trivers-Willard rules. In particular, the optimum parental behavior is sensitive to population dynamics, type of parental investment, and, most important, relative ages of sons and daughters. It is doubtful whether a parent blindly following the simple Trivers-Willard rules would maximize numbers of expected grandchildren, on average. In addition, we show that sex-specific infanticide will almost never achieve the goal of maximizing expected numbers of grandchildren.
Article
There is some evidence that testosterone levels in women are correlated with a tendency to produce sons. We show that in a sample of 102 women, aged from 35–55 years, components of female size i.e., weight, waist circumference, hip circumference and waist:hip ratio are positively correlated to the proportion of sons in their family. Multiple regression analysis shows that of these variables it is waist circumference which is the strongest predictor of the proportion of sons. It is argued that in many traditional societies, where rank is inherited by males and females often marry upwards, high ranking males should prefer heavy women with thick waists and hips. By this means they increase their probability of having sons to inherit their rank. In less stratified societies other negative correlates of female size such as symmetry, health and fertility will result in a preference for light women with small waists and hips.
Article
n a recent Animal Behaviour review, Daly & Wilson (1999, hereafter D&W) present a stimulating, but in important ways misleading, account of the history and present state of evolutionary analyses of human behaviour. Concerned that those not familiar with this research will be misled by D&W's account, we offer the following commentary. We wish to make three main points: (1) D&W's account is heavily biased toward the theoretical and methodological preferences of one approach to the study of human behaviour and evol- ution, and underplays or misrepresents other approaches; (2) the approach D&W advocate, evolutionary psy- chology (EP), suffers from several methodological and conceptual limitations; (3) human behavioural ecology (HBE) provides a complementary approach that avoids these limitations.
Article
A rebuttal is offered to an article by Whiting (1993) who reported that polygynously married women are more likely to produce girls than their monogamous counterparts. He supported his case with data drawn from seven cultural groups in Kenya showing lower sex ratios among the children of polygynously married mothers than among the children of monogamously married mothers. The results were attributed to two possible mechanisms: polygynously married women time their intercourse more closely to the day of ovulation when in conception girls are favored or have less frequent intercourse than monogamously married women. There is corollary evidence that high coital rates increase the sex ratio. While the individual samples used in Whitings analysis ranged from 110 Kipsigis to 1340 Kikuyu children the overall effect was significant (monogamous 0.53 vs. polygynous 0.47). One of Whitings samples was drawn from the Kipsigis a Nilotic Kenyan group on the basis of data collected by Robert Daniels on the Itembe Settlement Scheme. The writers own Kipsigis sample in 1989 from Abosi failed to support Whitings result. Polygynously married women had a birth sex ratio of 0.521 and monogamously married mothers one of 0.526 compared with Itembes 0.458 and 0.534. This discrepancy may have been due to the common pattern in the Abosi area of the coresidence of co-wives in a single homestead. This suggests that the pattern Whiting observed in seven Kenyan populations may be contingent on co-wives maintaining separate and distant residences. In the Abosi sample children born to a mans older polygynously married wives showed lower sex ratios 0.475 than those born to his youngest wives or single wife (0.533). The analyses presented here suggest that more precise hypotheses pertaining to co-wives residence patterns relative age and conformity to customary ideals can be formulated that may explain why polygynous status is associated with low sex ratios in some populations and not others.
Article
While some dismiss sociobiological theories as untestable, post hoc explanations, this article argues that sociologists should instead increase their efforts to identify and engage those theories that have novel empirical implications. Regarding parental investment, Trivers and Willard use Darwinian reasoning to hypothesize that high-status parents favor sons over daughters and that low-status parents favor daughters over sons. The application of this hypothesis to contemporary societies has been widely accepted by sociobiologists, although it has received little actual empirical scrutiny. The Trivers-Willard hypothesis is tested in this study using two nationally representative surveys of American adolescents and their parents. Across several different measures of investment, little evidence of the predicted parental investment behaviors is found. This article seeks not only to contribute to settling the empirical point at issue but also to encourage a renewed and empirically focused dialogue between sociologists and sociobiologists.
Article
Data from the history of Havasupai fertility and fertility data on other populations indicate that variables other than stochastic flux and differential mortality may affect sex ratios in human populations. They suggest that sex ratios at birth may vary with birth order in a conjugal union, maternal age, form of marriage, and political relations between the sexes. The behavioral variable which appears to tie all these factors to sex ratio at birth is coital frequency. The bases of this connection remain obscure, but its consequences are evident and important. Not only do social practices through their impact on coital rates lead to variation, but that variation produces more variation. When sex ratio at birth is correlated with birth order and maternal age, a population's sex ratio at birth will reflect its age/sex-specific fertility and mortality rates. Further, because the process which ties the population sex ratio at birth to those rates is recursive, the process may generate systemic historic oscillations in the population's age/sex structure even when vital rates are stable.
Article
Gene-culture coevolutionary theory has been developed specifically to explore the interaction between genetic and cultural processes. The theory builds on standard population genetics models by formally incorporating cultural transmission into the analysis. Here we present a case study which exploits this theory to explore the demographic and evolutionary consequences of cultural processes differentially affecting the mortality of the sexes such as female infanticide, sex-biased abortion, and sex selection. The example is designed to illustrate the logic and methods of this approach and demonstrate its advantages over more traditional methods. We show that gene-culture coevolutionary theory can be usefully employed to describe, analyze, and predict the diffusion of cultural traits and genetic variation through populations and to explore the interaction between these levels. We go on to defend this approach against a number of criticisms that have been leveled at it. The models have already been successfully applied to several problems in the human sciences, and there are rich possibilities for their utilization by anthropologists, human scientists, and biologists.
Article
The fundamental postulate of sociobiology is that individuals exploit favorable environments to increase their genetic representation in the next generation. The data on fertility differentials among contemporary humans are not cotvietent with this postulate. Given the importance of Homo sapiens as an animal species in the natural world today, these data constitute particularly challenging and interesting problem for both human sociobiology and sociobiology as a whole.The first part of this paper reviews the evidence showing an inverse relationship between reproductive fitness and “endowment” (i.e. wealth, success, and measured aptitudes) in contemporary, urbanized societies. It is shown that a positive relationship is observed only for those cohorts who bore their children during a unique period of rising fertility, 1935–1960, and that these cohorts are most often cited by sociobiologists as supporting the central postulate of sociobiology. Cohorts preceding and following these show the characteristic inverse relationship between endowment and fertility. The second section reviews the existing so-ciobiological models of this inverse relationship, namely, those of Barkow, Burley, and Irons, as well as more informal responses among sociobiologists to the persistent violation of sociobiology's central postulate, such as those of Alexander and Dawkins. The third section asks whether the goals of sociobiology, given the violation of its fundamental postulate by contemporary human societies, might not be better thought of as applied rather than descriptive, with respect to these societies. A proper answer to this question begins with the measurement of the pace and direction of natural selection within modern human populations, as compared to other sources of change. The vast preponderance of the shifts in human trait distributions, including the IQ distribution, appears to be due to environmental rather than genetic change. However, there remains the question of just how elastic these distributions are in the absence of reinforcing genetic change.
Article
ABSTRACT Considerable progress has been made in behavioral genetics toward providing theoretical accounts of individual differences One theoretical task, however, has been largely neglected—that of constructing evolutionary accounts of behaviorally relevant genetic variance We attempt to address this task with respect to the genetic variance underlying sociosexuality, that is, the differences in the implicit prerequisites (in terms of time, attachment, commitment, etc) to entering a sexual relationship Specifically, we argue that genetic variance on this trait for females could have been maintained through frequency-dependent selection In our evolutionary past, restricted females-those who require relatively more time, attachment, and commitment-could have benefited through paternal investment in their offspring Unrestricted females—those who require relatively less time, attachment, and commitment—could have benefited through the quality of their mate's genes passed on to their sons Moreover, the value of these alternate „strategies” could have been frequency-dependent One prediction that follows from this evolutionary history is tested and supported in three studies Those females genetically predisposed to be unrestricted are found to produce relatively more sons than females predisposed to be restricted Additional predictions are offered and alternative accounts are discussed