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The Demographic Transition: Are We Any Closer to an Evolutionary Explanation
Abstract
The radical shift in human reproduction in the late 19th century, known as the demographic transition, constitutes a major challenge to evolutionary approaches to human behaviour. Why would people ever choose to limit their reproduction voluntarily when, at the peak of the Industrial Revolution, resources were apparently so plentiful? Can the transition be attributed to standard life history tradeoffs, is it a consequence of cultural evolutionary processes, or is it simply a maladaptive outcome of novel and environmental social conditions? Empirical analyses and new models suggest that reproductive decision making might be driven by a human psychology designed by natural selection to maximize material wealth. If this is the case, the mechanisms governing fertility and parental investment are likely to respond to modern conditions with a fertility level much lower than that that would maximize fitness.
... generate even more resources rather than produce more offspring (Borgerhoff Mulder, 1998), and the degree to which additional investments in resource generation translate into long-term offspring lineage success (Hill & Reeve, 2004). However, absent is a discussion that examines the psychological experience of individuals residing in modern settings whilst accounting for the fundamental motives that underlie reproductive decisions (cf., Kenrick et al., 2002;Yong, Li, Jonason, & Tan, 2019). ...
... Indeed, fertility varies as a function of environmental harshness as indexed by GDP (Weil, 2004), economic development (Hafner & Mayer-Foulkes, 2013), pathogen prevalence (Rotella, Varnum, Sng, & Grossmann, 2021), violent crime rates Rotella et al., 2021), and mortality risk (Wilson & Daly, 1997), with populations exhibiting lower fertility as harshness decreases. As modern environments are more developed and possess abundant resources, social welfare, and stable infrastructure-all of which reduce environmental harshness and mortality risk-people may focus more on quality by building their individual capacities and producing fewer children in whom they invest greater time and resources (Borgerhoff Mulder, 1998). ...
... First, we elucidate the evolutionary factors that drive the regulation of reproductive behavior, from which more precise insights may be gleaned on how fertility rates can be better managed. This approach complements and extends other evolutionarily guided efforts to unpack the paradox surrounding why having more resources does not necessarily translate into having more offspring (e.g., Borgerhoff Mulder, 1998;Hill & Reeve. 2004). ...
Low fertility is a growing concern in modern societies. While economic and structural explanations of reproductive hindrances have been informative to some extent, they do not address the fundamental motives that underlie reproductive decisions and are inadequate to explain why East Asian countries, in particular, have such low fertility rates. The current paper advances a novel account of low fertility in modern contexts by describing how modern environments produce a mismatch between our evolved mechanisms and the inputs they were designed to process, leading to preoccupations with social status that get in the way of mating and reproductive outcomes. We also utilize developed East Asian countries as a case study to further highlight how culture may interact with modern features to produce ultralow fertility, sometimes to the extent that people may give up on parenthood or even mating altogether. Through our analysis, we integrate several lines of separate research, elucidate the fundamental dynamics that drive trade-offs between social status and reproductive effort, add to the growing literature on evolutionary mismatch, and provide an improved account of low fertility in modern contexts.
... In low and middle-income countries where populations are at different stages of this transition, women in wealthier households have fewer children on average (Hruschka & Burger, 2016;Hruschka, Sear, Hackman, & Drake, 2019;Lutz & KC, 2011;Myrskylä, Kohler, & Billari, 2009;Pérusse, 1993). Over the course of the fertility transition, wealthier families also reduce their fertility earlier and more dramatically than the rest of the population (Borgerhoff Mulder, 1998;Livi-Bacci, 1986;Skirbekk, 2008). Finally, studies using historical samples have identified a switch in the relationship between socioeconomic status and fertility, whereby highstatus individuals move to low-fertility strategies while low-status individuals move to having relatively higher fertility (Skirbekk, 2008). ...
... The demographic transition and associated changes in the wealth and fertility relationship have been discussed in great detail in evolutionary social sciences (Borgerhoff Mulder, 1998;Irons, 1983;Mace, 1996;Sear et al., 2016;Vining, 1986). The causes of the fertility decline and changing relationships between socioeconomic status and fertility have been attributed to increasing costs and benefits of status competition (Boone & Kessler, 1999;Borgerhoff Mulder, 1998;Hill & Reeve, 2005;Low, Simon, & Anderson, 2002;Mace, 2008), the increasing costs and benefits of parental investments in novel market economies (Becker, Murphy, & Tamura, 1990;Kaplan, 1996), women's education (Low et al., 2002), changing payoffs to human capital investments (Kaplan, Hill, Lancaster, & Hurtado, 2000), the breakdown of kinship networks (Newson, Postmes, Lea, & Webley, 2005;Turke, 1989), cultural evolution (Boyd & Richerson, 1985;Richerson & Boyd, 2005), or the costs and benefits of fertility reduction as a social mobility strategy in a stratified society (Rogers, 1990). ...
... The demographic transition and associated changes in the wealth and fertility relationship have been discussed in great detail in evolutionary social sciences (Borgerhoff Mulder, 1998;Irons, 1983;Mace, 1996;Sear et al., 2016;Vining, 1986). The causes of the fertility decline and changing relationships between socioeconomic status and fertility have been attributed to increasing costs and benefits of status competition (Boone & Kessler, 1999;Borgerhoff Mulder, 1998;Hill & Reeve, 2005;Low, Simon, & Anderson, 2002;Mace, 2008), the increasing costs and benefits of parental investments in novel market economies (Becker, Murphy, & Tamura, 1990;Kaplan, 1996), women's education (Low et al., 2002), changing payoffs to human capital investments (Kaplan, Hill, Lancaster, & Hurtado, 2000), the breakdown of kinship networks (Newson, Postmes, Lea, & Webley, 2005;Turke, 1989), cultural evolution (Boyd & Richerson, 1985;Richerson & Boyd, 2005), or the costs and benefits of fertility reduction as a social mobility strategy in a stratified society (Rogers, 1990). ...
Studies have shown mixed associations between wealth and fertility, a finding that has posed ongoing puzzles for evolutionary theories of human reproduction. However, measures of wealth do not simply capture economic capacity, which is expected to increase fertility. They can also serve as a proxy for market opportunities available to a household, which may reduce fertility. The multifaceted meaning of many wealth measures obscures our ability to draw inferences about the relationship between wealth and fertility. Here, we disentangle economic capacity and market opportunities using wealth measures that do not carry the same market-oriented biases as commonly used asset-based measures. Using measures of agricultural and market-based wealth for 562,324 women across 111,724 sampling clusters from 151 DHS surveys in 64 countries, we employ a latent variable structural equation model to estimate (a) latent variables capturing economic capacity and market opportunity and (b) their effects on completed fertility. Market opportunities had a consistent negative effect on fertility, while economic capacity had a weaker but generally positive effect on fertility. The results show that the confusion between operational measures of wealth and the concepts of economic capacity can impede our understanding of how material resources and market contexts shape reproduction.
... Cultural evolutionary theory and human behavioural ecology have led a parallel existence for most of their development because of a range of different starting assumptions and overlapping conceptual categories (Borgerhoff Mulder 1998;Colleran 2016). Cultural evolutionists have also had their disagreements with evolutionary psychologists, who have tended to consider that culture is "evoked" by fitness-relevant environmental experiences (Barkow and others 1992), and not "transmitted" through learning and interaction and therefore separable from fitness constraints. ...
... It is much harder when they conflict. The most obvious example is the global transition to low fertility, which does not appear to optimize reproductive success (Colleran 2016;Borgerhoff Mulder 1998). Because of its global reach and seemingly law-like patterning, fertility decline is finally drawing the focus of evolutionary demographers to proximate mechanisms (Sear and others 2016b;Colleran 2016). ...
Human evolutionary demography is an emerging field blending natural science with social science. This edited volume provides a much-needed, interdisciplinary introduction to the field and highlights cutting-edge research for interested readers and researchers in demography, the evolutionary behavioural sciences, biology, and related disciplines.
By bridging the boundaries between social and biological sciences, the volume stresses the importance of a unified understanding of both in order to grasp past and current demographic patterns. Demographic traits, and traits related to demographic outcomes, including fertility and mortality rates, marriage, parental care, menopause, and cooperative behavior are subject to evolutionary processes. Bringing an understanding of evolution into demography therefore incorporates valuable insights into this field; just as knowledge of demography is key to understanding evolutionary processes. By asking questions about old patterns from a new perspective, the volume—composed of contributions from established and early-career academics—demonstrates that a combination of social science research and evolutionary theory offers holistic understandings and approaches that benefit both fields.
Human Evolutionary Demography introduces an emerging field in an accessible style. It is suitable for graduate courses in demography, as well as upper-level undergraduates. Its range of research is sure to be of interest to academics working on demographic topics (anthropologists, sociologists, demographers), natural scientists working on evolutionary processes, and disciplines which cross-cut natural and social science, such as evolutionary psychology, human behavioral ecology, cultural evolution, and evolutionary medicine. As an accessible introduction, it should interest readers whether or not they are currently familiar with human evolutionary demography.
... A central difficulty for biological explanations of human striving for a large quantity of offspring (reproductive success) is that humans do not seem to maximise their number of offspring and that fertility has been declining considerably in the last two centuries (Hopcroft, 2019;Sear et al., 2016). Especially in industrialised societies, relatively many people deliberately remain childless (with rates of childlessness increasing since the late 19th, and even more the mid-20th, century, Borgerhoff Mulder, 1998;Rowland, 2007). The fact that not all people seem to be trying to maximise the number of offspring may be explicable by people focussing not only on quantity but also the quality of offspring (e.g., survival, chances of own reproduction; Kenrick et al., 2010), in an attempt to optimise long-term reproductive success (Borgerhoff Mulder, 2000). ...
... Factorial structure of capital facets The facets of capital can be grouped into two factors-abstract (e.g., social status, received attention in social interactions, popularity, prestige) and concrete (e.g., mating success, number of children, monetary income and fortune, cultural/ political roles, territory, embodied capital traits such as physical attractiveness, intelligence, physical strength, knowledge, general health including immune function) factors (e.g., using exploratory factor analysis) Qualitative vs. quantitative reproductive success Capital facets (especially facets like social status, cultural/political roles, holding power, monetary income and fortune, embodied capital traits) are more strongly associated with qualitative measures (e.g., children's and grand-children's social status, health, intelligence, physical attractiveness) than with quantitative measures (number of children and grand-children) of reproductive success Barthold et al. (2012), Borgerhoff Mulder (1998Mulder ( , 2000, and Kenrick et al. (2010) Effects of partner's and offspring's capital facets A focal person's partner's and offspring's social status (especially for male partners), physical attractiveness (especially for female partners), health, and intelligence predict the focal person's reproductive success Arnocky and Vaillancourt (2017) and Barber (1995) Motivation to attain and attained capital Self-and acquaintance-reported motivation to attain capital (facets) positively predicts attained capital Motivation to attain capital and reproductive success Self-and acquaintance-reported motivation to attain capital (facets) positively predicts reproductive success, mediated by attained capital Sex differences Physical attractiveness is positively and more strongly related to overall capital and reproductive success in women than in men Buss (1989) and Kanazawa and Savage (2009) Sex differences Social status is positively and more strongly related to overall capital and reproductive success in men than in women Buss (1989) and Kanazawa and Savage (2009) Sex differences Income is positively related to overall capital and reproductive success in men, and negatively in women Barthold et al. (2012) Cross-cultural differences There is a positive association between men's capital (in particular facets like social status) and some measures of reproductive success (such as (inversely) offspring mortality) in polygynous but not in monogamous societies von Rueden and Jaeggi (2016) Cross-cultural differences There is a positive association between men's capital (in particular facets like social status) and some measures of reproductive success (such as wife quality) ...
According to evolutionary theory, human cognition and behaviour are based on adaptations selected for their contribution to reproduction in the past, which in the present may result in differential reproductive success and inclusive fitness. Because this depiction is broad and human behaviour often separated from this ultimate outcome (e.g., increasing childlessness), evolutionary theory can only incompletely account for human everyday behaviour. Moreover, effects of most studied traits and characteristics on mating and reproductive success turned out not to be robust. In this article, an abstract descriptive level for evaluating human characteristics, behaviour, and outcomes is proposed, as a predictor of long-term reproductive success and fitness. Characteristics, behaviour, and outcomes are assessed in terms of attained and maintained capital, defined by more concrete (e.g., mating success, personality traits) and abstract (e.g., influence, received attention) facets, thus extending constructs like embodied capital and social capital theory, which focuses on resources embedded in social relationships. Situations are framed as opportunities to gain capital, and situational factors function as elicitors for gaining and evaluating capital. Combined capital facets should more robustly predict reproductive success and (theoretically) fitness than individual fitness predictors. Different ways of defining and testing these associations are outlined, including a method for empirically examining the psychometric utility of introducing a capital concept. Further theorising and empirical research should more precisely define capital and its facets, and test associations with (correlates of) reproductive success and fitness.
... It has thus come as a surprise to evolutionary biologists that the demographic transition, a set of stages all human societies progress through as they develop and modernize, seems to have caused a maladaptive outcome for humans [1,8,15,19,20]. Specifically, it has been noticed that more affluent humans in more developed countries have fewer offspring, on average, than humans in less developed countries. The leading theory in the field is that this phenomena is peculiar to humans and is indeed maladaptive, as those in more developed countries have fewer offspring, thus not acting in a fitness-maximizing manner [1,6,10]. ...
... Specifically, it has been noticed that more affluent humans in more developed countries have fewer offspring, on average, than humans in less developed countries. The leading theory in the field is that this phenomena is peculiar to humans and is indeed maladaptive, as those in more developed countries have fewer offspring, thus not acting in a fitness-maximizing manner [1,6,10]. ...
Differences in investment into reproduction or offspring rearing are plentiful throughout the world, from the cells inside our bodies to complex sociological interactions among humans. Such differences can lead to profound impacts on species' fitness, fertility, and reproductive rates, sometimes in startling ways. In this paper, we create a simple game-theoretical model to qualitatively investigate the effects of such differential investment. We focus on fertility in human societies and show that more wealthy individuals produce more offspring within a a mating group. However, when assortative mating mechanisms are introduced, this effectively leads to a speciation event, and a higher reproduction rate for poorer individuals is noticed, capturing what we call the "wealthy-to-poor switch". We discuss extensions and implications of this work to nupital gifts in ecology and to clonal competition in cancer cell lines under the influence of treatment.
... Uncovering the processes underpinning the dynamics of contraceptive behaviour is relevant for understanding the demographic transition, defined as the shift from high to low fertility with increasing levels of socioeconomic development in recent and contemporary history (Bongaarts & Watkins, 1996;Borgerhoff Mulder, 1998). Of course, the availability of modern contraception is neither necessary nor sufficient for fertility decline: the European demographic transition started around two centuries before the contraceptive pill was commercialized (Livi-Bacci, 1986) and despite modern contraception being accessible in many parts of sub-Saharan Africa, fertility remains high on average. ...
... Behavioural ecology theory advances that individuals adjust their fertility as a response to an increased perceived trade-off between fertility and investment in own and offspring embodied capital (Kaplan, 1996;Borgerhoff Mulder, 1998;Kaplan et al., 2002). As the risk of death in early childhood decreases, the importance of parental investment for an individual's ability to outcompete their peers increases, especially so in skill-based competitive labour markets (Kaplan, 1996), leading to increased trade-offs between fertility and investment in own and offspring capital and ultimately, fertility decline (Mace, 2008). ...
Numerous evolutionary mechanisms have been proposed for the origins, spread and maintenance of low fertility. Such scholarship has focused on explaining the adoption of fertility-reducing behaviour, especially the use of contraceptive methods. However, this work has yet to engage fully with the dynamics of contraceptive behaviour at the individual level. Here we highlight the importance of considering not just adoption but also discontinuation for understanding contraceptive dynamics and their impact on fertility. We start by introducing contemporary evolutionary approaches to understanding fertility regulation behaviours, discussing the potential for integrating behavioural ecology and cultural evolution frameworks. Second, we draw on family planning studies to highlight the importance of contraceptive discontinuation owing to side-effects for understanding fertility rates and suggest evolutionary hypotheses for explaining patterns of variation in discontinuation rates. Third, we sketch a framework for considering how individual flexibility in contraceptive behaviour might impact the evolution of contraceptive strategies and the demographic transition. We argue that integrating public health and evolutionary approaches to reproductive behaviour might advance both fields by providing (a) a predictive framework for comparing the effectiveness of various public health strategies and (b) a more realistic picture of behaviour by considering contraceptive dynamics at the individual level more explicitly when modelling the cultural evolution of low fertility.
... In addition, in the 'whole country dataset', we did not find sex difference in the relationship between SES and reproductive success. In general, in modern contemporary societies, fertility is lower than in natural/historical populations as a result of demographic transitions (Borgerhoff Mulder, 1998), and consequently, the variance of reproductive success is also expected to be smaller within the two sexes (Hruschka & Burger, 2016). Lower variance of reproductive success in both sexes could make it harder to find evidence for the precondition of the TWH. ...
According to the Trivers–Willard hypothesis (TWH), when the mother's condition around conception influences the future reproductive success of male and female offspring differently, the adjustment of offspring sex ratio (SR) to maternal condition will increase the parents’ fitness. The TWH has been tested in several taxa, including humans where socioeconomic status as an index of condition has been widely used. The results are inconsistent, possibly because the preconditions of the TWH are not always met. To investigate the preconditions and prediction of the TWH in the contemporary Hungarian population, we collected data by an online questionnaire on self-perceived childhood living standard, the number of children and the sex of the respondents’ siblings. We found no sex-specific relationship between reproductive success and childhood living standards, thus the precondition of the TWH was not met. We found no relationship between socioeconomic status and offspring SR when data from the whole country was used, but there was a tendency in the predicted direction when we used data from Budapest and considered the SR of only those family members who were born under similar conditions. Similar approaches should be preferred in the future to avoid noise caused by changing status during the reproductive lifespan.
... This suggests a novel explanation for the demographic transition to reduced fertility. Rather than fertility declines resulting solely from benefits derived from investing more heavily in fewer offspring, transmission of material wealth, or increased lifespan as has been theorized 96 , it may be that the desire to have children is not an evolved motivation. Instead, evolution may have relied on the desires to form romantic/sexual bonds to produce children. ...
Every human and non-human animal must make tradeoffs in investments in terms of time, energy, and resources. The aim of this study was to extrapolate from the types of investments in survival and reproduction that non-human animals make and translate these into human motivations. 16 potential goals were presented to 851 childless, 18–23-year-old adults from 11 world regions in an online study. Each young adult was asked to weight the importance of every goal to his or her ideal life. Weights had to sum to 100, requiring tradeoffs. Results revealed striking agreement across young adults with only four goals weighted above chance: Finding a beloved romantic partner, being physically and emotionally healthy, and earning money or resources. Having lots of sexual partners was the least important goal across all world regions for both sexes. Nevertheless, men more than women valued having many sexual partners, being talented outside work, being physically strong, and having a physically attractive romantic partner. Overall, there was cultural variation in some of the less important goals. Helping young adults achieve success requires understanding their own goals, rather than focusing on popularized depictions of what young adults desire.
... This limitation is not trivial given our stated interest in fertility variations across the globe. As some globally comprehensive studies on PD (e.g., Lutz et al., 2006;Rotella et al., 2021;Sng et al., 2017) and wealth (e.g., Borgerhoff Mulder, 1998;Hackman & Hruschka, 2020) suggest that these patterns extend beyond the west to other parts of the world whereas other studies do not (Luoto, 2019a), it is necessary to conduct further research that accounts for a wider range of countries to confirm the generalizability of the relationships we proposed between PD, income, sex, and fertility rates. Similarly, our data were taken from clients of an online dating company, which also presents problems with representativeness given that dating website users are mostly seeking relationships and thus may not have partners or children, while our objective was to assess the number of children that people have ideally within a long-term relationship context. ...
While previous studies guided by evolutionary life history theory have revealed several important socioecological moderators of the influence of population density (PD) on reproduction, absent is an understanding of how individual-level factors such as personal resources and sex differences might interact and play a role. Using data from a large sample of clients (N = 4,432,440) of an online dating company spanning 317 states nested within 23 countries, we contributed a robust multilevel analysis of life history effects by assessing the interaction between state-level PD and individual-level income on offspring quantity, and we further qualified this analysis by sex. Consistent with previous research, PD was negatively correlated with having children. Consistent with our novel hypotheses, this negative relationship was moderated by income such that the link between PD and low fertility became weaker with increasing levels of income and these patterns were stronger for men than for women. These results held despite controlling for a variety of country-level, state-level, and individual-level confounds. Findings are discussed together with theoretical and practical implications for the management of fertility based on evolutionary life history perspectives.
... Evolutionary theorists also try to make sense of the counterintuitive pattern that apparently higher wealth is associated with lower fertility, when Darwinian reasoning would predict more resources equated to more offspring, and thus increased genetic fitness. The story is complicated and somewhat beyond the scope of this chapter, but part of the problem is due to the conflation of population measures with individual level outcomes (Borgerhoff Mulder, 1998;Mattison and Shenk, 2019). In other words, not all people from rich countries are in fact rich, and when considering individual-level wealth, the relationship is actually non-linear, with the very wealthy and the very low-socioeconomic groups both exhibiting relatively high fertility. ...
Human evolutionary demography is an emerging field blending natural science with social science. This edited volume provides a much-needed, interdisciplinary introduction to the field and highlights cutting-edge research for interested readers and researchers in demography, the evolutionary behavioural sciences, biology, and related disciplines.
By bridging the boundaries between social and biological sciences, the volume stresses the importance of a unified understanding of both in order to grasp past and current demographic patterns. Demographic traits, and traits related to demographic outcomes, including fertility and mortality rates, marriage, parental care, menopause, and cooperative behavior are subject to evolutionary processes. Bringing an understanding of evolution into demography therefore incorporates valuable insights into this field; just as knowledge of demography is key to understanding evolutionary processes. By asking questions about old patterns from a new perspective, the volume—composed of contributions from established and early-career academics—demonstrates that a combination of social science research and evolutionary theory offers holistic understandings and approaches that benefit both fields.
Human Evolutionary Demography introduces an emerging field in an accessible style. It is suitable for graduate courses in demography, as well as upper-level undergraduates. Its range of research is sure to be of interest to academics working on demographic topics (anthropologists, sociologists, demographers), natural scientists working on evolutionary processes, and disciplines which cross-cut natural and social science, such as evolutionary psychology, human behavioral ecology, cultural evolution, and evolutionary medicine. As an accessible introduction, it should interest readers whether or not they are currently familiar with human evolutionary demography.
... Good evolutionary analyses have clearly demonstrated that lowered fertility, greater survival and greater investment in offspring could hypothetically maximize fitness under the right conditions, but, empirically, recent widespread fertility reduction does not maximize fitness in human populations where relevant parameters have been measured (e.g. Borgerhoff Mulder, 1998;Kaplan et al., 1995, Kaplan, 1996Goodman et al., 2012;Bolund and Lumaa, 2017). Nevertheless, the question of whether the fertility transition is adaptive is complicated, because: (1) the fertility transition has proceeded through phases that might have been adaptive in some times and places (Hruschka and Burger, 2015); and (2) the fertility transition may not be permanent (Burger and DeLong, 2016). ...
Human evolutionary demography is an emerging field blending natural science with social science. This edited volume provides a much-needed, interdisciplinary introduction to the field and highlights cutting-edge research for interested readers and researchers in demography, the evolutionary behavioural sciences, biology, and related disciplines.
By bridging the boundaries between social and biological sciences, the volume stresses the importance of a unified understanding of both in order to grasp past and current demographic patterns. Demographic traits, and traits related to demographic outcomes, including fertility and mortality rates, marriage, parental care, menopause, and cooperative behavior are subject to evolutionary processes. Bringing an understanding of evolution into demography therefore incorporates valuable insights into this field; just as knowledge of demography is key to understanding evolutionary processes. By asking questions about old patterns from a new perspective, the volume—composed of contributions from established and early-career academics—demonstrates that a combination of social science research and evolutionary theory offers holistic understandings and approaches that benefit both fields.
Human Evolutionary Demography introduces an emerging field in an accessible style. It is suitable for graduate courses in demography, as well as upper-level undergraduates. Its range of research is sure to be of interest to academics working on demographic topics (anthropologists, sociologists, demographers), natural scientists working on evolutionary processes, and disciplines which cross-cut natural and social science, such as evolutionary psychology, human behavioral ecology, cultural evolution, and evolutionary medicine. As an accessible introduction, it should interest readers whether or not they are currently familiar with human evolutionary demography.
... In response to the incongruity between observed fertility rates and expected optimums for fitness maximization, others have suggested that human reproductive behavior is probably influenced by evolved psychological mechanisms that are sensitive to traits that served as proxies of fitness in the past but have maladaptive outcomes in novel environments (Kaplan et al., 1995). The close relationship between status, resources, and fitness recognized in past human societies and other primate species (Majolo et al., 2012;von Rueden & Jaeggi, 2016) suggests that humans may have evolved to be sensitive to opportunities to optimize status, rather than overall fitness (Alvergne & Lummaa, 2014;Borgerhoff Mulder, 1998;Shenk et al., 2016). Therefore, fertility should be reduced if having additional offspring significantly impedes the parents' ability to improve their existing offspring's status. ...
Contemporary trends in low fertility can in part be explained by increasing incentives to invest in offspring’s embodied capital over offspring quantity in environments where education is a salient source of social mobility. However, studies on this subject have often neglected to empirically examine heterogeneity, missing out on the opportunity to investigate how this relationship is impacted when individuals are excluded from meaningful participation in economic spheres. Using General Social Survey data from the United States, I examine changes in the relationship between number of siblings and college attendance for White and Black respondents throughout the 1900s. Results show that in the early 1900s, White individuals from larger families had a lower chance of completing four years of college education than those from smaller families, whereas the likelihood for Black individuals was more uniform across family sizes. These racial differences mostly converged in the later part of the century. These results may help explain variations in the timing of demographic transitions within different racial groups in the United States and suggest that the benefits of decreasing family size on educational outcomes may be conditional on the specific economic opportunities afforded to a family.
... Many human populations are experiencing rapid demographic, socioeconomic and cultural changes, including demographic transition (Borgerhoff Mulder, 1998), industrialisation, market integration and urbanisation (Mattison & Sear, 2016). These changes affectand are affected byfamily composition and norms governing spousal partner choice. ...
Although still prevalent in many human societies, the practice of cousin marriage has precipitously declined in populations undergoing rapid demographic and socioeconomic change. However, it is still unclear whether changes in the structure of the marriage pool or changes in the fitness-relevant consequences of cousin marriage more strongly influence the frequency of cousin marriage. Here, we use genealogical data collected by the Tsimane Health and Life History Project to show that there is a small but measurable decline in the frequency of first cross-cousin marriage since the mid-twentieth century. Such changes are linked to concomitant changes in the pool of potential spouses in recent decades. We find only very modest differences in fitness-relevant demographic measures between first cousin and non-cousin marriages. These differences have been diminishing as the Tsimane have become more market integrated. The factors that influence preferences for cousin marriage appear to be less prevalent now than in the past, but cultural inertia might slow the pace of change in marriage norms. Overall, our findings suggest that cultural changes in marriage practices reflect underlying societal changes that shape the pool of potential spouses.
... Respecto a la fecundidad, las mejoras en las condiciones de vida debieran haber repercutido en una mayor expresión de la potencialidad reproductora de las personas y en mayores tamaños familiares, circunstancia contraria a la observada en las poblaciones actuales. Mulder (1998) propone varias hipótesis para explicar la relación entre la adopción de medidas limitantes de la natalidad y la eficacia biológica. Una de ellas sugiere que el tamaño de la descendencia obedecería a un compromiso entre la cantidad de descendientes y la calidad de vida de los mismos. ...
Natural selection is one of the main factors that modulate evolution, acting through differential mortality and differential fecundity. The improvement in health conditions has increased people survival, specially in stages previous to reproductive maturity, and has also reduced the family sizes. These changes define the demographic transition and it is postulated that it has reduced the impact of natural selection on human populations.
In Valdepeñas de Jaén (Jaén, España) the changes that gave rise to demographic transition began in the decade 1870-1880, with the highest natality and mortality rates, and culminated around 1930. The opportunity of action of natural selection, measured by Crow’s index, took the maximum value in the years between 1871 and 1900. From then on, a declining trend was observed, mainly due to the reduction in the component of mortality. Although average family size has reduced during the twentieth century, fecundity component doesn’t have significant variations, constituting the most important factor through which natural selection keeps acting on this population.
Demographic transition hasn’t eliminated the capacity of action of natural selection on human populations completely. Elements like pollution effects, emerging epidemics, assisted reproduction or new family models, set new scenarios throughout which Darwin’s natural selection will modulate the evolutionary future of the human species.
La selección natural es uno de los principales factores que modulan la evolución, actuando a través de la mortalidad y la fecundidad diferenciales. La mejora en las condiciones sanitarias ha aumentado la supervivencia de las personas, especialmente en las etapas pre-reproductoras, y ha reducido los tamaños familiares. Estos cambios definen la transición demográfica, y se postula que han reducido el impacto de la selección natural en las poblaciones humanas.
En Valdepeñas de Jaén (Jaén, España), los cambios que dan lugar a la transición demográfica se inician a partir de la década de 1870-1880, cuando se alcanzan las tasas máximas de natalidad y mortalidad, y culminan en torno a 1930. La oportunidad de acción de la selección natural, medida mediante el índice de Crow, alcanza un valor máximo en torno a 1871-1900, adoptando a partir de entonces una tendencia descendente, principalmente debido a la reducción de la componente de la mortalidad. Aunque los tamaños familiares medios se han reducido durante el siglo XX, la componente de la fecundidad no ha presentado grandes variaciones, siendo el factor mediante el que la selección natural sigue operando principalmente en esta población.
La transición demográfica no ha eliminado por completo la capacidad de actuación de la selección natural sobre las poblaciones humanas. Elementos como la contaminación y sus efectos, las epidemias emergentes, la reproducción asistida o los nuevos modelos familiares imponen nuevos escenarios a través de los cuales la selección natural de Darwin continuará modulando el devenir evolutivo de la especie humana.
... The rising costs associated with childcare might dissuade many households from expanding their families (14)(15)(16). Another factor to consider is that longer-lasting marriages often result in larger families (15)(16)(17). ...
Background: The number of children born into a family has a significant impact on a mother’s reproductive system’s physiological health and overall well-being. Objectives: This study aimed to explore the effective factors in the number of living children (NLC) among married women in Semnan, Iran, with a particular focus on social, cultural, and economic influences. Methods: This cross-sectional study examined the reproductive histories of 600 married women aged 15 to 49 years from Semnan, Iran. Sample size calculations were based on the principal variable, the expected number of children, using PASS software. The women were selected through a multistage random sampling method from health center lists in 2018 (April-October). The data were collected through interview questionnaires and analyzed using the Quasi-Poisson model. Results: The mean (standard deviation [SD]) age at first marriage for mothers (MAM) and fathers was estimated to be 21.02 (4.80) and 25.10 (4.70) years, respectively. Additionally, the mean (SD) values for the number of expected children (NCEX), living children, and the number of pregnancies were 2.19 (0.96), 1.85 (0.81), and 2.15 (1.03), respectively. Among the variables of interest, only NCEX, the number of pregnancies (with a positive effect), MAM, father’s education, multiple births, and the desire to have children (with a negative effect) significantly influenced NLC. Conclusions: This study recommends implementing an educational program to promote an optimal and ideal family size to prevent adverse pregnancy outcomes. The age of women at marriage is a significant factor, as an increase in women’s age leads to shorter pregnancy intervals, exposing women to complications, such as premature birth, perinatal death, and intrauterine growth restriction. Policymakers should, therefore, encourage early marriages by fostering a culture and providing supportive facilities.
... Economic growth theorists and human evolutionary ecologists alike have addressed the linked dynamics of humans and the resources upon which humans depend. Much of this work has focused on the all-important distinction between increasing resources that are transformed into an increasingly large human population versus increasing resources that are transformed into an increasingly wealthy human population [29,[49][50][51][52][53][54]. ...
We derive and analyse a model with unusual features characterizing human activities over the long-run. First, human population dynamics draw heavily on consumer–resource modelling in ecology in that humans must consume biological resources to produce new humans. Second, the model also draws heavily from economic growth theory in that humans do not simply consume biological resources; they also produce the resources they consume. Finally, humans use two types of technology. Consumption technology affects the rate at which humans can extract resources. Production technology controls how effectively humans convert labour into new resources. The dynamics of both types of technology are subject to cumulative cultural evolutionary processes that allow both technological progress and regress. The resulting model exhibits a wide range of dynamical regimes. That said, the system is routinely sensitive to initial conditions, with wildly different outcomes given the same parameter values. Moreover, the system exhibits a basic fragility in the sense that human activities often lead to the endogenous extinction of the human species. This can happen gently, or it can follow periods of explosive human activity with super-exponential growth that ends in collapse.
This article is part of the theme issue ‘Evolution and sustainability: gathering the strands for an Anthropocene synthesis’.
... Moreover, focusing on these two populations enabled us to assess some of the predictions regarding the effect of market integration and urbanization of traditionally rural communities on childcare practices. It has been assumed that market integration leads to greater reliance on adults (and, in particular, mothers) as caregivers, and reduced socialization with other children of different ages (Colleran & Mace, 2015;Hewlett, 2016;Lawson & Mace, 2011;Mulder, 1998;Padilla-Iglesias & Kramer, 2021;Rogoff et al., 2010;Shenk et al, 2013). This hypothesized shift in the children's interaction partners following market integration is primarily the result of a transition into nuclear family residence, smaller family sizes, and greater schooling which may result in a reduction in the importance of the role of children as caregivers due to them spending a greater amount of time in school. ...
The extent to which toddlers have opportunities to learn in interactive, observational, and independent contexts is thought to vary by culture. However quantitative assessments of cultural variability and of the factors driving intra‐ and inter‐cultural differences in toddler's time allocation are lacking. This paper provides a comparative and quantitative examination of how toddlers spend their time and with whom (adults or children) in two communities (rural Yucatec Maya, urban United States). Additionally, it considers individual factors that predict time allocation. Results demonstrated that Maya toddlers spent more time in independent contexts compared to US toddlers and spent more time exclusively with other children than did US toddlers. Maya toddlers were more likely than US toddlers to spend time observing other people, however, when given the opportunity to observe others there were no differences in visual attentional allocation across cultures. For Maya toddlers maternal schooling related negatively to both time spent with other children and time spent in interactive contexts. The findings highlight the need for researchers to include diverse populations when considering early social experiences as well as assessing factors that may contribute differentially to variations in early experience across cultures.
... The current empirical evidence also supports and extends prior work from evolutionary anthropology and ecology that has proposed links between (a) social status concerns and desires and (b) low fertility ( Boone and Kessler, 1999 ;Borgerhoff Mulder, 1998 ;Hill and Reeve, 2004 ;Rogers, 1990 ;Shenk et al., 2016 ). In the modern world, the cost required to reach the highest-ranking positions, and the expenses for raising competitive offspring and transmitting status to them, are particularly high (see also, Galor, 2012 ;Hill and Reeve, 2004 ), making the tradeoff between status seeking and reproduction especially pronounced. ...
Modern low fertility is an unresolved paradox. Despite the tremendous financial growth and stability in modern societies, birth rates are steadily dropping. Almost half of the world's population lives in countries with below-replacement fertility and is projected for a continued decline. Drawing on life history theory and an evolutionary mismatch perspective, we propose that desire for social status (which is increasingly experienced by individuals in industrialized, modern societies) is a key factor affecting critical reproductive preferences. Across two experimental studies (total N = 719), we show that activating a desire for status can lead people to prefer reproductive tradeoffs that favor having fewer children, thereby predicting preferences for delaying both marriage and having a first child. These data support an evolutionary life history mismatch perspective and suggest a complementary explanation for declining fertility rates in contemporary societies, especially developed and economically advanced ones.
... This could be helpful for our understanding of apparently maladaptive cultural phenomena resulting in low fertility. A prominent and far-reaching example of this is the demographic transition that is occurring in nearly all regions of the world, which can be explained partially but not entirely from various evolutionary perspectives (Borgerhoff Mulder, 1998;Colleran, 2016;Ihara & Feldman, 2004;Mace, 1998). Here is a clear case where selection for high fertility seems to be in direct opposition to cultural selection for high educational attainment (Kong et al., 2017). ...
Human culture changes over time and varies across space. Two main approaches to study cultural evolution have developed in the last fifty years: human behavioural ecology and a suite of perspectives centred on the role of cultural transmission. The latter are often confusingly referred to with the name of the phenomenon they are trying to explain, ‘cultural evolution’. We argue that this is unhelpful and is generating confusion, including the claim that human behavioural ecology disregards cultural evolution. The aim of behavioural ecology is to explain human behaviours, and the vast majority of them are at least to some extent cultural. In addition, culture forms part of the ecology that determines the costs and benefits associated with adopting a behaviour. Thus, human behavioural ecologists have studied cultural evolution from the very beginning, even though they have not focussed on social learning. We explore three examples in detail: kinship systems, religious institutions, and witchcraft belief. We then use the framework offered by Tinbergen’s [1963, Z Tierpsychol, 20(4), 410-433] four evolutionary questions about behaviour to explain how human behavioural ecology and cultural transmission approaches can fruitfully coexist and complement each other. Moreover, we discuss several difficulties with cultural transmission approaches and highlight how the human behavioural ecological view of cultural evolution sometimes diverges from them. We conclude by suggesting that the field can move forward and achieve greater synthesis by exploring how selective processes acting on biological fitness differ from those acting on cultural fitness – and how the two might interact in the cultural evolution of human behaviours.
Link to preprint: https://osf.io/u47tw/
... Researchers from evolutionary demography and biological anthropology have sought to address this puzzle using life-history theory (Borgerhoff Mulder, 1998;Sear et al., 2016). The life-history framework holds that amounts of time and energy available to an organism over its life course are finite, such that time and energy allocated to one purpose over the lifespan, such as growth or somatic maintenance, cannot be allocated to other purposes, such as reproduction (e.g., mate-finding, pregnancy, lactation). ...
Globally, mortality and fertility rates generally fall as resource abundance increases.This pattern represents an evolutionary paradox insofar as resource-rich ecological contexts can support higher numbers of offspring, a component of biological fitness. This paradox has not been resolved, in part because the relationships between fertility, life history strategies, reproductive behavior, and socioeconomic conditions are complex and cultural-historically contingent. We aim to understand how we might make sense of this paradox in the specific context of late-twentieth-century, mid–demographic transition Chile. We use distribution-specific generalized linear models to analyze associations between fertility-related life-history traits—number of offspring, ages at first and last reproduction, average interbirth interval, and aver-age number of live births per reproductive span year—and socioeconomic position(SEP) using data from a cohort of 6,802 Chilean women born between 1961 and1970. We show that Chilean women of higher SEP have shorter average interbirth intervals, more births per reproductive span year, later age at first reproduction, earlier ages at last reproduction, and, ultimately, fewer children than women of lower SEP. Chilean women of higher SEP consolidate childbearing over a relatively short time span in the middle of their reproductive careers, whereas women of lower SEP tend to reproduce over the entirety of their reproductive lifespans. These patterns may indicate that different SEP groups follow different pathways toward declining fertility during the demographic transition, reflecting different life-history trade-offs in the process.
... It is now generally agreed that this seems not to be the case in historical and contemporary Western societies (e.g. Borgerhoff Mulder, 1998, Goodman et al., 2012. of cohabitation (especially in early adulthood), and high shares not marrying. Historically, the marriage pattern, with low levels of extramarital births, high levels of non-marriage and late marriage (particularly in times of economic downturns) related to low and late fertility and high levels of non-parenthood. ...
We studied to what extent family lines die out over the course of 122 years based on Swedish population-level data. Our data included demographic and socioeconomic information for four generations in the Skellefteå region of northern Sweden from 1885–2007. The first generation in our sample consisted of men and women born between 1885 and 1899 (N=5,850), and we observed their children, grandchildren, and great-grandchildren. We found that 48% of the first generation did not have any living descendants (great-grandchildren) by 2007. The risk of a family line dying out within the four-generational framework was highest among those who had relatively low fertility in the first generation. Mortality during reproductive years was also a leading reason why individuals in the first generation ended up with a greater risk of not leaving descendants. We identified socioeconomic differences: both the highest-status and the lowest-status occupational groups saw an increased risk of not leaving any descendants. Almost all lineages that made it to the third generation also made it to the fourth generation.
... The current research sheds some light on and adds to the literature addressing the changing patterns in human fertility cycles and family size (e.g., Borgherhoff-Mulder, 1998;Hill and Reeve, 2004 ;Nolin and Ziker, 2016 ). Our findings suggest a way of reconciling how individuals across a range of early childhood environments utilize fast versus slow life history strategies to adapt to present-day economic uncertainty cues in terms of their desired reproductive timing -a potentially major determinant of individuals' actual reproduction and hence, the fertility rates we observe across nations. ...
Why do some people have children earlier compared to others who delay reproduction? Drawing from an evolutionary, life history theory perspective, we posited that reproductive timing could be influenced by economic uncertainty and childhood socioeconomic status (SES). For individuals lower in childhood SES, economic uncertainty influenced the desire to reproduce earlier compared to individuals higher in childhood SES. Furthermore, the decision regarding reproductive timing was influenced by tradeoffs between earlier reproduction or furthering one's education or career. Overall, economic uncertainty appears to shift individuals into different life history strategies as a function of childhood SES, suggesting how ecological factors and early life environment can influence fertility-related decisions at the individual level and may contribute to the highly variable fertility patterns observed across countries.
... Thus, in anthropological demography, culture can be considered as a distal variable that determines the action of the proximate ones, which on their turn represent the biology of reproduction expressed in terms of age at birth of the first child, lactation duration, and others (Roth 2004). However, humans are opportunistic living organisms, which respond to the environmental stress and develop new adaptive reproductive strategies and behaviors (Bates 2005;Kaplan and Lancaster 2003;Mosley and Chen 1984;Mulder 1998;Mueller 2001;Potts 1997;Retaroli 1990;Scagy 1973;Turke 1989). In that way, culture is a crucial variable for understanding demographic behavior. ...
The examination of demographic patterns and their variables is essential for our understanding of the anthropological and cultural processes that shape modern societies. Greek Thrace plays an important role in such analyses due to the cultural and socioeconomic variability it presents. This study is based on vital registration data of the 20th century, derived by various populations that inhabit the Department of Rhodopi, which are defined by discrete socio-economic, cultural, and historic backgrounds. We analyze and present various aspects of the social lives observed in these populations: marriage patterns, mean age at marriage of the spouses, mean age of mothers at first and last child, reproductive span, the children ever born, as well as other parameters related to reproduction. Our analyses show that several changes in marriage patterns have been undertaken throughout time, indicating an ongoing fertility transition. In that way, this study contributes to our knowledge on the anthropological populations from this part of Greece in relation to their socio-economic environment and it proposes possible analogies and differences that affected the evolution of the embedded populations.
... Humans, in particular, show a suite of both behavioural and physical traits that appear to hinder fitness; examples include behaviours such as the tendency to over-consume fatty foods (Neel, 1962;Hales & Barker, 2001;Speakman, 2013), the prolonged use of ineffective (e.g. homeopathic) medicine (Tanaka et al., 2009), engagement in harmful bodily practices (Koziel et al., 2010;Howard & Gibson, 2017;Wander, 2017), and the negative correlation between wealth and fertility (Borgerhoff Mulder, 1998). ...
Despite appearing to be maladaptive, the human menopause and prolonged post-reproductive lifespan are thought to have been shaped in our evolutionary history by natural selection. As a result, there has been a great deal of research looking at the inclusive fitness benefits of a post-reproductive lifespan. However, there are still many things we do not know about menopause, such as whether current variation in menopause timing is the result of evolutionary trade-offs, whether menopause symptoms require an evolutionary explanation, and how post-reproductive care functions in a sample of women from the United Kingdom. This thesis focuses on trying to fill these identified gaps in the literature using data from the United Kingdom, United States, and China. I find no evidence for menopause symptoms being facultative, nor that menopause timing varies in the way predicted by current evolutionary models. However, I do find that a later menopause is predicted by an increased likelihood of pregnancy, suggesting an energetic trade-off. Further, I show that menopause symptoms predictably vary relative to one’s ecology, with a more stressful environment predicting worse symptoms. When looking at caring behaviour, I found evidence in favour of it being facultative relative to fecundity status, with pre-menopausal women caring more for their parents, while post-menopausal women spent more time caring for their grandchildren. Finally, I present evidence for an earlier menopause predicting a greater number of grandchildren, suggesting that women are able to offset the costs of being post-reproductive by increasing indirect fitness. Results from this thesis suggest that many aspects of the menopausal transition are plastic, and often vary in a way predicted by evolutionary theory. Through understanding these trends, it may those who experience menopause more autonomy over the transition. Further, my research on fecundity status and caring behaviours demonstrates the behavioural implications of energetic trade-offs.
... Some have argued that the dramatic decline in family size that has been observed in most parts of the world provides evidence against an evolutionary understanding of human behavior (e.g., Vining 1986), but HBE sees this as an opportunity to better understand which environmental factors lead to this unique behavioral response. Indeed, many HBE researchers examine reproductive decision-making to understand the demographic transition (e.g., Borgerhoff Mulder 1998). ...
Researchers across the social sciences have long been interested in families. How people make decisions such as who to marry, when to have a baby, how big or small a family to have, or whether to stay with a partner or stray are questions that continue to interest economists, sociologists, demographers, and anthropologists. Human families vary across the globe; different cultures have different marriage practices, different ideas about who raises children, and even different notions of what a family is. Human behavioral ecology is a branch of anthropology that is particularly interested in cultural variation of family systems and how these differences impact upon the people that inhabit them; the children, parents, grandparents. It draws on evolutionary theory to direct research and generate testable hypotheses to uncover how different ecologies, including social contexts, can explain diversity in families. In this Special Issue on the behavioral ecology of the family, we have collated a selection of papers that showcase just how useful this framework is for understanding cultural variation in families, which we hope will convince other social scientists interested in family research to draw upon evolutionary and ecological insight in their own work.
... While low fertility never appears to have a fitness payoff, in market economies it does seem to have an economic dividend. Indeed one feature of most fertility transitions is a negative association between wealth and fertility [21][22][23], whereby low fertility is associated with greater levels of wealth. While this negative relationship was once deemed an intractable problem for human evolutionary sciences, recent re-evaluations have found rather a weakened, variable, and clustered relationship between wealth and fertility [9,[24][25][26][27]. ...
Mixed economies provide a unique context for testing theories of fertility change. Because they have a stake in two traditions, mixed-economy households balance the demands of both a labor-based subsistence economy, which benefits from a large family, and a wage-labor economy, which benefits from reduced fertility. Additionally, household size changes over the course of its life-cycle and shapes available economic opportunities. Here we argue that in mixed economies, fertility may reflect opportunities for livelihood diversity rather than simply responding to the restricted socioeconomic benefits of small families. While low fertility may in some cases have an economic benefit, low fertility can also limit the livelihood diversity of a household which is a key strategy for long-term economic success. We test this prediction with longitudinal data from a Maya community undergoing both a sustained decline in fertility and rapid integration into the market economy. Using household-level fertility, number of adults, and livelihood diversity at two time points, we find that household size is positively related to livelihood diversity, which in turn is positively related to household income per-capita. However, household size also has a negative association with income per capita. The results reflect a balancing act whereby households attempt to maximize the economic diversity with as few members as possible. Broadly, these results suggest that theories of fertility decline must account for how households pool resources and diversify economic activities in the face of increasing market integration, treating fertility as both an outcome and an input into economic and reproductive decision-making.
... Such contextual factors shape relationships between family members, and vary strongly across time and space. Large-scale social changes, such as industrialization and modernization, alter the fundamental conditions human nature faces (Borgerhoff Mulder, 1998;Stulp & Barrett, 2016), and, in turn, structure the ways kin effects are manifested. For instance, in pre-industrial societies, couples often co-resided either with the husband's parents (patrilocality) or the wife's parents (matrilocality), whereas neo-locality, in which the couple did not coreside with either lineage, was more unusual. ...
Objective
We investigate the association between the geographic proximity of the grandparents on net marital fertility and maternal survival in Sweden, 1900–1910, within the framework of the cooperative-breeding-hypothesis.
Methods
Data were derived from Swedish full-count censuses (1880–1910) and the Swedish Death Index. Married couples were linked to their parental households. Poisson and logistic regression analyses were used to investigate the association between the geographical proximity of the grandparents on net marital fertility, which we measured as the number of surviving children born between 1900 and 1910, and the mother's survival. Models were fitted with and without fixed effects to assess the effects of unobserved characteristics shared at the parish and the family level.
Results
The results indicate that net fertility and maternal survival increased with the husband's parents' geographic proximity. In contrast, we found no evidence that the geographic proximity of the wife's parents was associated with increased fertility or maternal survival. Rather, the presence of the mother's parents in the household lowered net fertility and reduced maternal survival.
Conclusions
This study provides evidence that kin proximity was associated with fertility and mortality of married women, and that the associations differed for paternal and maternal kin in the societal context of Swedish nuclear families (1900–1910). However, the patterns of kin proximity that we identified were correlated with characteristics such as socioeconomic status, occupation, and wealth, which also exhibited strong correlations with fertility and survival. Future research assessing the effects of kinship on demographic developments must therefore carefully consider the socio-environmental context.
... Finally, our cohort analysis captures the well-known reversal of wealth effects on fertility, where wealth and status become decoupled from fertility 13,[82][83][84] . Over the course of four cohorts of Maya women, sharing-group size, land wealth and education all move from having relatively positive effects on fertility to having negative influences on fertility behaviors. ...
The adoption of contraception often coincides with market integration and has transformative effects on fertility behavior. Yet many parents in small-scale societies make decisions about whether and when to adopt family planning in an environment where the payoffs to have smaller families are uncertain. Here we track the fertility of Maya women across 90 years, spanning the transition from natural to contracepting fertility. We first situate the uncertainty in which fertility decisions are made and model how childbearing behaviors respond. We find that contraception, a key factor in cultural transmission models of fertility decline, initially has little effect on family size as women appear to hedge their bets and adopt fertility control only at the end of their reproductive careers. Family planning is, however, associated with the spread of lower fertility in later cohorts. Distinguishing influences on the origin versus spread of a behaviour provides valuable insight into causal factors shaping individual and normative changes in fertility.
Every human and non-animal must make tradeoffs in investments in terms of time, energy, and resources. The aim of this study was to extrapolate from the types of investments in survival and mating that non-human animals make and translate these into human motivations. 16 potential goals were presented to 851 childless, 18-23-year-old adults from 11 world regions in an online study. Each young adult was asked to weight the importance of every goal to his or her ideal life. Weights had to sum to 100, requiring tradeoffs. Results revealed striking agreement across young adults with only four goals weighted above chance. The two most important goals were having a mutually loving romantic relationship and being healthy followed by earning money or resources, with lots of sexual partners the least important goal. Helping young adults achieve success requires understanding their goals, rather than focusing on popular media’s depictions of what young adults should desire.
Human evolutionary demography is an emerging field blending natural science with social science. This edited volume provides a much-needed, interdisciplinary introduction to the field and highlights cutting-edge research for interested readers and researchers in demography, the evolutionary behavioural sciences, biology, and related disciplines.
By bridging the boundaries between social and biological sciences, the volume stresses the importance of a unified understanding of both in order to grasp past and current demographic patterns. Demographic traits, and traits related to demographic outcomes, including fertility and mortality rates, marriage, parental care, menopause, and cooperative behavior are subject to evolutionary processes. Bringing an understanding of evolution into demography therefore incorporates valuable insights into this field; just as knowledge of demography is key to understanding evolutionary processes. By asking questions about old patterns from a new perspective, the volume—composed of contributions from established and early-career academics—demonstrates that a combination of social science research and evolutionary theory offers holistic understandings and approaches that benefit both fields.
Human Evolutionary Demography introduces an emerging field in an accessible style. It is suitable for graduate courses in demography, as well as upper-level undergraduates. Its range of research is sure to be of interest to academics working on demographic topics (anthropologists, sociologists, demographers), natural scientists working on evolutionary processes, and disciplines which cross-cut natural and social science, such as evolutionary psychology, human behavioral ecology, cultural evolution, and evolutionary medicine. As an accessible introduction, it should interest readers whether or not they are currently familiar with human evolutionary demography.
Human evolutionary demography is an emerging field blending natural science with social science. This edited volume provides a much-needed, interdisciplinary introduction to the field and highlights cutting-edge research for interested readers and researchers in demography, the evolutionary behavioural sciences, biology, and related disciplines.
By bridging the boundaries between social and biological sciences, the volume stresses the importance of a unified understanding of both in order to grasp past and current demographic patterns. Demographic traits, and traits related to demographic outcomes, including fertility and mortality rates, marriage, parental care, menopause, and cooperative behavior are subject to evolutionary processes. Bringing an understanding of evolution into demography therefore incorporates valuable insights into this field; just as knowledge of demography is key to understanding evolutionary processes. By asking questions about old patterns from a new perspective, the volume—composed of contributions from established and early-career academics—demonstrates that a combination of social science research and evolutionary theory offers holistic understandings and approaches that benefit both fields.
Human Evolutionary Demography introduces an emerging field in an accessible style. It is suitable for graduate courses in demography, as well as upper-level undergraduates. Its range of research is sure to be of interest to academics working on demographic topics (anthropologists, sociologists, demographers), natural scientists working on evolutionary processes, and disciplines which cross-cut natural and social science, such as evolutionary psychology, human behavioral ecology, cultural evolution, and evolutionary medicine. As an accessible introduction, it should interest readers whether or not they are currently familiar with human evolutionary demography.
Human evolutionary demography is an emerging field blending natural science with social science. This edited volume provides a much-needed, interdisciplinary introduction to the field and highlights cutting-edge research for interested readers and researchers in demography, the evolutionary behavioural sciences, biology, and related disciplines.
By bridging the boundaries between social and biological sciences, the volume stresses the importance of a unified understanding of both in order to grasp past and current demographic patterns. Demographic traits, and traits related to demographic outcomes, including fertility and mortality rates, marriage, parental care, menopause, and cooperative behavior are subject to evolutionary processes. Bringing an understanding of evolution into demography therefore incorporates valuable insights into this field; just as knowledge of demography is key to understanding evolutionary processes. By asking questions about old patterns from a new perspective, the volume—composed of contributions from established and early-career academics—demonstrates that a combination of social science research and evolutionary theory offers holistic understandings and approaches that benefit both fields.
Human Evolutionary Demography introduces an emerging field in an accessible style. It is suitable for graduate courses in demography, as well as upper-level undergraduates. Its range of research is sure to be of interest to academics working on demographic topics (anthropologists, sociologists, demographers), natural scientists working on evolutionary processes, and disciplines which cross-cut natural and social science, such as evolutionary psychology, human behavioral ecology, cultural evolution, and evolutionary medicine. As an accessible introduction, it should interest readers whether or not they are currently familiar with human evolutionary demography.
I used psychopathy to provide an illustration of how we can explore the evolution of behavioral traits on phenotypic data. But looking at my own work in the field, I never thought that the main contribution of my research is empirical data. I never thought that someone should teach students that psychopathy or any other behavioral trait has fitness-related outcomes based on the data that I collected. This is not only based on the fact that the research designs that I applied have important methodological limitations like nonrepresentative samples and cross-sectional designs. As I mentioned in a chapter where I described the basic tenets of behavioral ecology, we observe only the snapshots of evolution—the data on the associations between behavioral traits and fitness in a certain moment in time in a given population. These data are invaluable for understanding of the microevolutionary processes on behavioral traits but they are hardly sufficient to make reliable conclusions on these processes. In fact, I would like to go even further: even when observing these exact data, we do not see evolutionary processes, we observe potential processes—the ones that may or may not exist in the population (and yes, I think that we see potential processes even if we have representative samples and prospective research designs). These processes are probabilistic and uncertain phenomena (like any others, needless to say), even in the populational state that we speak of them; projecting them into the future carries an even larger margin of uncertainty. But this does not diminish the importance of these processes in any sense—potential realities are important as the real ones (just ask the surrealists). But it does provide us with indications of what main contributions of our research may be.
In this chapter I introduce the field of behavioral ecology of personality or evolutionary personality ecology. The main conceptual framework is based on three topics, sometimes called evolutionary puzzles of personality: evolutionary explanations of inter-individual differences in behavior, cross-situational and temporal consistency in behavior, and associations between functionally different personality traits. Afterwards, I present the phenotypic associations between personality traits (based mostly on the Big Five/Five Factor Model of personality) and fertility in humans. The main part of the chapter is dedicated to the application of conceptual models from animal BE in explaining three evolutionary puzzles of personality in humans. At the end of the chapter I describe the extensions of the presented framework to other human behavioral traits like intelligence, psychopathological traits, social attitudes, and values.KeywordsBehavioral ecology of personalityEvolutionary puzzles of personalityBig FiveFive factor modelPersonality and fitness
The existence and characteristics of evolutionary tradeoffs in human populations are described in this chapter. Firstly I cover fertility-longevity tradeoff, the problems of its empirical detection and the possible explanation of this problem (a possibility of nonlinear association between these two fitness components), and the implications of the tradeoff for the evolution of menopause. The characteristics of the tradeoff based on the age of first reproduction are presented afterwards: fertility benefits of early reproduction followed by the health costs for females, the links between first and last reproduction, and the detrimental effects of delaying first reproduction after the age of 30. Another major tradeoff is the one between quantity and quality of offspring: I describe the effects of parental care for offspring’s longevity (especially in preindustrial populations), together with a lack of evidence for the link between parental care and offspring’s fertility. Finally, I depict mating-parenting tradeoff and its characteristics in humans.KeywordsFertility-longevity tradeoffAge of first reproduction tradeoffQuantity-quality tradeoffMating-parenting tradeoff
This chapter is dedicated to the behavioral ecological analysis of psychopathy. Firstly I describe psychopathy as a behavioral syndrome consisting of several traits (manipulativeness, emotional superficiality, lack of behavioral control, and sometimes antisocial behavior), and the most prominent instruments for psychopathy measurement. Afterwards, I briefly describe the nomological network of psychopathy, by depicting the relations between psychopathy, criminal behavior, aggressiveness, violence, moral behavior, social interactions, psychopathology, intelligence, executive functioning, and career choices. Genetic, neurobiological, and environmental precursors of psychopathy are described as well, with an emphasis on the maltreatment and deprivation in childhood as the facilitators of the psychopathy development. Evolutionary considerations of psychopathy are described: the role of assortative mating, faster life history and pace of life, presence of psychopathy in chimpanzees (Pan troglodytes), and the relations between psychopathy and health. Considering the importance of reproductive success as a core fitness component, I describe the associations between psychopathy and fertility in a separate subchapter. Finally, I analyze the existing empirical data and show the application of behavioral ecological models in explaining the first evolutionary puzzle of personality (the maintenance of inter-individual variation in behavior) as applied to psychopathy.KeywordsPsychopathy definitionPsychopathy measurementNomological network of psychopathyEvolution of psychopathyBehavioral ecology of psychopathy
In this chapter, I cover several topics related to evolutionary explanations of the processes emerging in human families. Sex differences in parental care as a part of sex roles are analyzed from a viewpoint of sexual selection. I explain the origins of markedly heightened parental care in humans and the conditions influencing parental investment—ecological conditions, parental characteristics, and offspring traits. I describe the importance of grandparental care for offspring and grandoffspring fitness, the conditions that may bias grandparental investment, and the role of this investment in the evolution of human longevity. Various parent-offspring interactions are described including the parent-offspring conflict (highlighting the parental control in offspring’s mating), the cooperation between parents and offspring, and parental effects in general. Afterwards, I describe reproductive motivation, its importance in exploring the evolution of behavioral traits, and provide examples of how we can empirically measure reproductive motivation. This chapter ends with the more detailed descriptions of the demographic transition’s roots and evolutionary explanations of demographic transition by comparing alternative hypotheses regarding this complex phenomenon.KeywordsParental careGrandparental careParent-offspring conflictReproductive motivationDemographic transition
Life History Theory (LHT) represents one of the most prominent conceptual frameworks in evolutionary social sciences—its basic assumptions are described in this chapter with an emphasis on the hypothesized fast-slow continuum. LHT has somewhat different usage in evolutionary psychology, compared to evolutionary biology and HBE; thus, evolutionary psychological view of LHT and its criticisms are presented afterwards. I provide detailed analysis of the covariations between various life history traits (body mass, maturation age, onset of sexual behavior, age of first reproduction, fertility, and parental investment) and the level of their congruence with the continuum. Furthermore, I explore the existing data on the associations between ecological context and life history in light of a hypothesis that harsher environments trigger fast life history trajectory. I present my own approach to life history, namely the network approach, where life history traits and their covariations are viewed as the dynamic systems on a population level. Finally, I address LHT criticisms and unresolved questions—problems of transferring assumptions from the between-species to between-individual level, unsuitability of fast-slow continuum to adequately describe empirical data, and the problem of measurement of life histories in humans.KeywordsLife History TheoryFast-slow continuumHarsh environmentNetwork analysis
This chapter covers evolutionary behavioral sciences in general, both animal and human disciplines. Firstly I briefly sketch the historical development and basic research tenets of (animal) behavioral ecology. Afterward, I depict human behavioral ecology (HBE) and its conceptual foundations; I use The Tsimane Health and Life History Project as the example of ethnographic research in HBE and provide arguments for the existence of natural selection in contemporary human populations. I present critiques of HBE and major unresolved questions including the accusations of a rigid view on human nature, the lack of connection with animal BE and cultural evolution research, a potential problem with fertility estimates, and an issue of phenotypic gambit. Afterwards, the conceptual foundations of Evolutionary Psychology (EP) are presented (the view of brain as a computer, massive modularity principle, Environment of Evolutionary Adaptiveness, adaptive lag and evolutionary mismatch, search for human universals) followed by the major criticisms of this discipline (the problem of declaring a trait as an adaptation, problem of inferring evolutionary processes on available data, and problem of Environment of Evolutionary Adaptiveness). Finally, I compare HBE and EP and highlight both the differences between the fields and their complementarity in the exploration of behavioral evolution.KeywordsEvolutionary social sciencesBehavioral ecologyEvolutionary psychology
Human culture changes over time and varies across space. Two main approaches to study cultural evolution have developed in the last fifty years: human behavioural ecology and a suite of perspectives centred on the role of cultural transmission. The latter are often confusingly referred to with the name of the phenomenon they are trying to explain, ‘cultural evolution’. We argue that this is unhelpful and is generating confusion, as evident in the assertion that human behavioural ecology disregards cultural evolution. The aim of behavioural ecology is to explain human behaviours, and the vast majority of them are at least to some extent cultural. In addition, culture forms part of the ecology that determines the costs and benefits associated with adopting a behaviour. Thus, human behavioural ecologists have studied cultural evolution from the very beginning, even though they have not focussed on social learning. We explore three examples in detail: kinship systems, religious institutions, and witchcraft belief. We then use the framework offered by Tinbergen’s [1963, Z Tierpsychol, 20(4), 410–433] four evolutionary questions about behaviour to explain how human behavioural ecology and cultural transmission approaches can complement each other. Moreover, we discuss several difficulties with cultural transmission approaches and highlight how the human behavioural ecological view of cultural evolution sometimes diverges from them. We conclude by suggesting that the field can move forward and achieve greater synthesis by exploring how selective processes acting on biological fitness differ from those acting on cultural fitness – and how the two might interact in the cultural evolution of human behaviours.
Evolutionary approaches assert that human behavior is driven by the motivation to survive, reproduce, and ensure the survival of one’s offspring. From an evolutionary perspective, the low fertility observed in many countries today thus seems rather perplexing. At least two phenomena need to be explained. First, why has human fertility in many countries declined to below replacement levels, if people are genetically programmed to have as many children as they can? Second, why do people continue to strive for higher social status, even though status is no longer or even negatively related to fertility? In this chapter, I review evolutionary approaches to reproduction and consider the adaptiveness of low fertility and status-seeking.
Women's fertility is the focus of most demographic analyses, for in most mammals, and in many preindustrial societies, variance in male fertility, while an interesting biological phenomenon, is irrelevant. Yet in monogamous societies, the reproductive ecology of men, as well as that of women, is important is creating reproductive patterns. In nineteenth-century Sweden, the focus of this study, male reproductive ecology responded to resource conditions: richer men had more children than poorer men. Men's fertility also interacted with local and historical factors in complex ways to have significant impact on population growth. As a result, "the" demographic transition was local, and locally reversible, in Sweden. Results cannot be simply translated from nineteenth-century studies to current attempts to promote fertility decline, because today, male and female resource-fertility curves differ in shape, not only in magnitude. When we translate studies of fertility decline, it is important to study individual fertility and to discern whether, in any particular case, male and female patterns are similar.
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.
What reproductive strategies are favoured by natural selection in a world where wealth can be inherited? Heritable wealth introduces several interesting wrinkles. First, there is the trade-off between the number of one’s children and their wealth. A parent cannot simultaneously maximise both. Second, there is the question of how fitness should be defined. It makes no sense to equate fitness with the number of children, because the parent whose children are many may lose in competition with parents whose children are fewer but wealthier.
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.
The evolutionary biological hypothesis that culturally defined values and goals are proximate means of enhancing reproductive success is tested on data from the Mukogodo, a small group of Maa-speaking pastoralists in north-central Kenya who value the accumulation of livestock. The results support the prediction that, at least among males, livestock wealth should correlate with reproductive success. This correlation appears to be due mainly to greater polygyny among wealthier men. Lower age at first marriage among wealthier men may also contribute to the correlation between livestock wealth and reproductive success. The association between livestock wealth and reproductive success does not appear to be due to the productivity of wives and children, to bridewealths obtained when daughters marry, or to the effects of wealth on the reproductive success of men's wives.
Not available in German. Since the fall of the wall, East Germans have drastically changed their demographic behaviour. Marriages and births have dropped to an unprecedented low level. Our paper tracks birth rates of the East German population, past, present, and future. We propose a imulation model of future cohort fertility. The hyptheses we develop build on the historical record of reproductive behaviour in the German Democratic Republic (GDR) since 1960 and on an analysis of the pattern of change between 1990 and 1994. The particular emphasis lies on the assumption that East German couples will rapidly westernize their family size by trying to reach completed fertility levels of the corresponding West German cohort. This implies that the resulting adaptation process includes the post-unification crisis as a logical first step.
Age is the characteristic variable of population analysis; much of the theory of this volume will be concerned with the effect of age distribution, either fixed or changing. Abstraction is necessary in demographic as in other theory; is it possible to abstract even from age and still obtain results of value?
An interpretation of fertility transition is offered applying facts and theories of evolution to concepts and relationships in the literature on fertility determinants. The discussion is centered on a kin hypothesis involving five propositions. (1) Resource insolvency brought on by the birth of children has been an important selective pressure throughout human evolution, and as a result humans have evolved to strive for social and economic success. (2) In traditional societies, extended kinship networks function to disperse the costs of childrearing among an array of relatives. (3) The pursuit of social and economic success in societies undergoing modernization leads to the breakdown of these kinship networks. Simultaneously, there is a tremendous increase in the complexity and number of routes to social and economic success. (4) Following the breakdown of extended kinship networks, childrearing costs are concentrated on parents, thus potentially constraining the pursuit of social and economic success. (5) Also following the breakdown of extended kinship networks, resources formerly controlled by the kin group (usually its elders) come under the control of young adults, enabling them to concentrate resources on a small number of children. Once some parents concentrate their resources on small numbers of children, other parents must follow suit if their offspring are to be socially competitive. Preliminary support for the kin hypothesis is established with data from the demographic and anthropological literature. -Author
The view that individual choice is fundamental to the analysis of modern reproductive behavior has been largely unchallenged. This article proposes that certain characteristics of demographic change in Western Europe between 1870 and 1960 suggest a significant role for "others"--kin, friends, and neighbors--in accounting for demographic behavior. Moreover, demographic as well as linguistic patterns suggest that while in the past the relevant "others" were members of local community, in the present the relevant community is largely national.
Experimental, theoretical, psychological, and economic barriers have caused physicians to rely on biomedical treatments for infertility at the exclusion of more environmentally oriented ones (e.g., psychosocial stress therapy). An evolutionary model is described for the origin of reproductive failure, suggesting why mammals evolved to be reproductively responsive to the environment and why psychosocial stress should have an especially strong impact on fertility problems. A study of the causal role of psychosocial stress in infertility is then summarized. The paper concludes with implications for future directions for the treatment of infertility and related human reproductive problems.
A family reconstitution study of the Krummhörn population (Ostfriesland, Germany, 1720-1874) reveals that infant mortality and children's probabilities of marrying or emigrating unmarried are affected by the number of living same-sexed sibs in farmers' families but not in the families of landless laborers. We interpret these results in terms of a "local resource competition" model in which resource-holding families are obliged to manipulate the reproductive future of their offspring. In contrast, families that lack resources have no need to manipulate their offspring and are more likely to benefit from allowing their offspring to capitalize on whatever opportunities to reproduce present themselves.
The relationship between the energy expended per offspring, fitness of offspring, and parental fitness is presented in a two-dimensional graphical model. The validity of the model in determining an optimal parental strategy is demonstrated analytically. The model applies under various conditions of parental care and sibling care for the offspring but is most useful for species that produce numerous small offspring which are given no parental care.
Blurton Jones and Sibly (1978) developed a model of costs (weight of food and baby carried while foraging) of !Kung women's reproduction under ecological-economic constraints that were described by Lee (1972). Predictions are drawn from this model and tested on Howell's (1979) data from reproductive histories of 172 individual women.Women were rated on a scale of dependence on bush or cattlepost foods. The ratings were compared with what is known about the places at which the women gave birth to their children. Agreement was good and the population was divided into two groups for study: 65 women most dependent on bush foods and 70 women substantially dependent on agricultural produce. Thirty-seven women of uncertain or intermediate status were omitted.As predicted by the model, among the women who were dependent on bush foods: (1) first interbirth intervals (IBI) were shorter than were later IBI; (2) the survivorship of children in first IBIs was not strongly related to length of IBI, with shorter IBI giving as good survivorship as longer IBI; (3) IBIs lengthen as the number of surviving children increases, until the fourth child; (4) after the fourth child IBIs do not differ significantly although they tend to be shorter (contrary to the prediction, a null hypothesis whose statistical support is consequently poor); (5) for IBIs after the first IBI, mortality increased markedly as IBI decreased; (6) mortality was even more closely related to backloads entailed by each IBI, as calculated by Blurton Jones and Sibly (1978); (7) mortality was less closely related to backloads calculated from an alternative version of the model from which weight of food was excluded; (8) as reported by Howell (1979), a new pregnancy followed rapidly after the death of the preceeding child but (9) as predicted, a new pregnancy did not follow so fast after the death of older children; (10) for the cattlepost women IBIs were shorter than for women dependent mainly on bush foods; and (11) there was no significant relationship between IBI and mortality for the cattlepost women, and mortality at short IBI was lower than in bush women.The assumption that the benefit accruing from more births will be balanced against the costs (costs to survivorship assumed to result from the work entailed by caring for each child) was successful in giving many predictions confirmed by the data. The significance and limitations of this “optimization” study are discussed.
In the theoretical structure of new home economics models, human fertility decisions are taken so as to maximize satisfaction provided to the parents by the presence and well-being of their children and by other competing commodities. Nevertheless, the concept of utility of children fails to specify why parents benefit from the presence and well-being of their progeny. In this study, fertility choices are analyzed from an evolutionary perspective. Parents are thus expected to adopt a lifetime fertility schedule `that maximizes fitness, defined here as the product of quantity with quality of offspring. Quality refers to the probability that reproduction occurs in the next generation, a variable that is thought to depend on the amount of resources allocated by the parents to the progeny. Using multiperiod dynamic models of fertility, it is shown that an increase in wealth and income reduces time to first conceptions and increases total conceptions. However, the relationship between offspring number and income is not always positive as changes in child quality also occur. Uncertainty about future economic status favors more conservative fertility schedules. Qualitative predictions of the models are generally supported by available evidence, although much work remains to be done to estimate the models quantitatively.
This paper has two interrelated goals. The first is to offer a general theory of fertility and parental investment across a broad spectrum of human societies. The second is to provide a perspective that unifies traditionally separate domains of anthropology. The basic foundation for the analysis is life history theory and evolutionary biological models of optimal fertility regulation. This tradition is combined with human capital theory in economics to produce a more general theory of investments in embodied capital within and between generations. This synthesis results in a series of optimality models to examine the decision processes underlying fertility and parental investment upon which natural selection is expected to act. Those models are then applied to the hunting and gathering lifeway. This analysis focuses both on problems that all hunting and gathering peoples face and on the production of variable responses in relation to variable ecologies. Next, this consideration of optimal parental investment and fertility behavior in hunter-gatherers is united with existing models of the proximate determinants of human fertility. The analysis of proximate mechanisms is based on the idea that natural selection acts on the final phenotypic outcome of a coordinated system of physiological, psychological and cultural processes. The important conditions affecting parental investment and fertility in modern socioeconomic contexts are then discussed. An explanation of modern fertility and parental investment behavior in terms of the interaction of those conditions with the physiological and psychological mechanisms that evolved during our hunting and gathering history is proposed. The proposal is that skills-based competitive labor markets increase the value of parental investment in children and motivate better-educated, higher income parents to invest more per child than their less-educated, lower-earning counterparts. It is also suggested that the deviation from fitness maximization associated with low modern fertility is due to excess expenditures on both parental and offspring consumption, indicating that our evolved psychology is responding to cues in the modern environment that are not directly related to the fitness impacts of consumption. © 1996 Wiley-Liss, Inc.
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.
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.
A family reconstitution study of the Krummhörn population (Ostfriesland, Germany, 1720–1874) reveals that infant mortality
and children’s probabilities of marrying or emigrating unmarried are affected by the number of living same-sexed sibs in farmers’
families but not in the families of landless laborers. We interpret these results in terms of a “local resource competition”
model in which resource-holding families are obliged to manipulate the reproductive future of their offspring. In contrast,
families that lack resources have no need to manipulate their offspring and are more likely to benefit from allowing their
offspring to capitalize on whatever opportunities to reproduce present themselves.
Maximizing reproductive success involves having as many children as possible that can themselves reproduce successfully. Thus, when Gabbra parents decide to have another baby, they must trade off the probability that they will be able to afford to raise the child and marry it off successfully when it reaches maturity against the risk that feeding and raising that child would diminish the family herd, harming the marriage prospects of other children and possibly even leading to household destitution. Here I use a dynamic, state-dependent optimality model to analyze this trade-off. The decision to have another baby depends on household wealth and the number of children they already have. Parents should not necessarily reproduce at the maximum rate to maximize reproductive success, and the costs of marrying off a child have a large impact on the optimal family size. In the Gabbra, the cost of marrying off boys greatly exceeds the cost of marrying off girls. An analysis of demographic data from Gabbra households with a living husband and a first wife that had reached menopause show that probability of remarriage is strongly dependent on the number of children the first wife had. Number of sons has a much greater influence than number of daughters on the probability of a second marriage, as predicted by the model. Men are attempting to create the optimal family.
The “Leslie matrix” of demography is extended to deal with categories of wealth, rather than age, and is used to build an evolutionary model of the effect of heritable wealth on reproductive decisions. Optimal reproductive strategies are assumed to be those that maximize the long-term rate of growth in the numbers of one's descendents. In poor environments, the optimal strategy is to maximize the wealth inherited by each offspring, which requires limiting their numbers. In rich environments, on the other hand, it pays to maximize the number of offspring. Strong positive correlations between wealth and the number of offspring are predicted only in rich environments. Therefore, evidence that the rich reproduce more slowly than the poor is not inconsistent with the hypothesis that reproductive strategies have been shaped by evolution.
Abstract In Western society the process of fertility decline is often regarded as an innovation process. The assumptions behind this approach seem rather questionable, and the diffusion lags or gradients of limited importance. Both Swedish and other European data are used as illustrations. It is suggested that the decline be treated within the wider sociological perspective of a time-consuming adjustment or change process, not necessarily starting from a position of completely uncontrolled fertility within marriage. The situation in to-day's high fertility populations is briefly discussed against this background.
How do biological, psychological, sociological, and cultural factors combine to change societies over the long run? Boyd and Richerson explore how genetic and cultural factors interact, under the influence of evolutionary forces, to produce the diversity we see in human cultures. Using methods developed by population biologists, they propose a theory of cultural evolution that is an original and fair-minded alternative to the sociobiology debate.
In this paper we develop a model that examines fertility and childhood mortality patterns and their relationship to environmental variables. Interactions among environmental variables can account for different fertility patterns and different mixes of these variables can produce similar patterns of fertility. Our model attempts to quantify the idea that there is a trade-off between producing a few children likely to survive to reproductive age and producing a greater number of children with lower chances for survival. The optimum mix of these strategies depends on environmental characteristics. We use the model to make predictions about fertility and mortality patterns among two Bushmen populations of southern Africa—the Ghanzi and Ngamiland !Kung—using data collected by Harpending in 1967–1968. The results do not support explanations of the low fertilities observed among !Kung Bushmen women, in whom it is thought that fitness is maximized by limiting fertility, and show no relationship between mortality and family size in either !Kung population. Instead, the number of offspring reaching reproductive age in both populations increases as their completed family size increases.
We examine the effects of sex, birth order, and paternal investment on mortality. No sex ratio differences and no differences in mortality by sex or birth order are present. Infant mortality among women who married more than once is significantly higher than among women who married once, suggesting that paternal care has a significant effect.
An adaptive model for the evolution of reproductive failure predicted psychosocial stress to increase as anatomic causes of infertility decrease. The nonanatomic infertility group in our study reported greater psychosocial stress than intermediate (P < 0.008) or anatomic groups (P < 0.0005). Controls, women with nonanatomic etiologies who were not attempting pregnancy, also reported higher psychosocial stress than the anatomic group (P < 0.007). Results are consistent with the hypothesis that psychosocial distress contributes significantly to the etiology of some forms of infertility.
After a promising start some three decades ago, the application of micro-economic analysis to fertility studies has proved disappointing. It has not led to an increased understanding of fertility decisions nor to the policy insights which had been expected. This paper considers the reasons for this disappointment. It reviews briefly the development of the now dominant version of the economic approach to fertility analysis, the so-called "Chicago Model". It concludes that several basic conceptual and theoretical weaknesses of this approach have led it up a blind alley. The paper concludes with suggestions for new assumptions and approaches which may make the theory more relevant for policy programmes.
Social-group forerunners of fertility control in Europe, in The Decline of Fertility in Europe Social versus reproductive success – the central theoretical problem of human sociobiology
- M Livi-Bacci
Livi-Bacci, M. (1986) Social-group forerunners of fertility control
in Europe, in The Decline of Fertility in Europe (Coale, A.J. and
Watkins, S.C., eds), pp. 182–200, Princeton University Press
4 Vining, D.R. (1986) Social versus reproductive success – the central
theoretical problem of human sociobiology, Behav. Brain Sci. 9,
167–260
The life histories of men in Albuquerque: an evolutionary–economic analysis of parental investment and fertility in modern society, in Human Behavior and Adaptation: an Anthropological Perspective
- H S Kaplan
- J B Lancaster
Kaplan, H.S. and Lancaster, J.B. The life histories of men in
Albuquerque: an evolutionary–economic analysis of parental
investment and fertility in modern society, in Human Behavior and
Adaptation: an Anthropological Perspective (Cronk, L.,
Chagnon, N.A. and Irons, W., eds), Aldine de Gruyter (in press)
An adaptive model of human reproductive rate: why people have small families
- R Mace
Mace, R. An adaptive model of human reproductive rate:
why people have small families, in Human Behavior and
Adaptation: an Anthropological Perspective (Cronk, L., Chagnon, N.A.
and Irons, W., eds), Aldine de Gruyter (in press)
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16 Conrad, C., Lechner, M. and Werner, W. (1996) East German fertility
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Dynamic Modeling in Behavioral Ecology When to have another baby: a dynamic model of reproductive decision-making and evidence from Gabbra pastoralists
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reproductive decision-making and evidence from Gabbra
pastoralists, Ethol. Sociobiol. 17, 263–273
For love or money: the evolution of reproductive and material motivations The decline of fertility: innovation or adjustment process
- A R Rogers
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reproductive and material motivations, in Human Reproductive
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- R Boyd
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Process, University of Chicago Press
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Fertility strategy for the individual and the group
- C Wrigley
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primogeniture, Hum. Nat. 4, 1–45
Modes of production, secularization, and the pace of fertility decline in Western Europe, 1870–1930, in The Decline of Fertility in Resource competition and reproduction: the relationship between economic and parental strategies in the Krummhorn population
- R Lesthaeghe
- C E Wilson
- R Dunbar
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secularization, and the pace of fertility decline in Western
Europe, 1870–1930, in The Decline of Fertility in Europe (Coale, A.J.
and Watkins, S.C., eds), pp. 261–292, Princeton University Press
12 Voland, E. and Dunbar, R. (1995) Resource competition and reproduction: the relationship between economic and parental strategies
in the Krummhorn population (1720–1874), Hum. Nat. 6, 33–49
To marry again or not? A dynamic model for demographic transition
- B Luttbeg
- M Borgerhoff Mulder
- M Mangel
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(Cronk, L., Chagnon, N.A. and Irons, W., eds), Aldine de Gruyter
(in press)
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- N Keyfitz
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The life histories of men in Albuquerque: an evolutionary-economic analysis of parental investment and fertility in modern society
- H S Kaplan
- J B Lancaster
Kaplan, H.S. and Lancaster, J.B. The life histories of men in
Albuquerque: an evolutionary-economic analysis of parental
investment and fertility in modern society, in Human Behavior and
Adaptation: an Anthropological Perspective (Cronk, L.,
Chagnon, N.A. and Irons, W., eds), Aldine de Gruyter (in press)