Sexual orientation may be influenced by prenatal levels of testosterone and oestrogen. There is evidence that the ratio of the length of 2nd and 4th digits (2D:4D) is negatively related to prenatal testosterone and positively to oestrogen. We report that (a) 2D:4D was lower in a sample of 88 homosexual men than in 88 sex- and age-matched controls recruited without regard to sexual orientation, (b) within the homosexual sample, there was a significant positive relationship between mean 2D:4D ratio and exclusive homosexuality, (c) overall, there was a decrease in 2D:4D from controls to homosexual men to bisexual men and (d) fraternal birth order, a positive predictor of male homosexuality, was not associated with 2D:4D in a sample of 240 Caucasian men recruited without regard to sexual orientation and 45 homosexual men.Further work is needed to confirm the relationships between 2D:4D and sexual orientation. However, these and other recent data tend to support an association between male homosexuality and high fetal testosterone. Very high testosterone levels may be associated with a sexual preference for both men and women.
The ratio between the length of the 2nd and 4th digit (2D:4D) is sexually dimorphic, with mean male 2D:4D lower than mean female 2D:4D. It recently was suggested that 2D:4D is negatively correlated with prenatal testosterone and positively correlated with prenatal estrogen. It is argued that high prenatal testosterone and low estrogen (indicated by low 2D:4D) favors the male fetus and low prenatal testosterone and high estrogen (indicated by high 2D:4D) favors the female fetus. The patterns of expression of 2D:4D are interpreted in terms of sexually antagonistic genes.We report data on the following. (a) reproductive success and 2D:4D from England, Germany, Spain, Hungary (ethnic Hungarians and Gypsy subjects), Poland, and Jamaica (women only). Significant negative associations were found between 2D:4D in men and reproductive success in the English and Spanish samples and significant positive relationships between 2D:4D in women and reproductive success in the English, German, and Hungarian samples. The English sample also showed that married women had higher 2D:4D ratios than unmarried women, suggesting male choice for a correlate of high ratio in women, and that a female 2D:4D ratio greater than male 2D:4D predicted high reproductive success within couples. Comparison of 2D:4D ratios of 62 father:child pairs gave a significant positive relationship. This suggested that genes inherited from the father had some influence on the formation of the 2D:4D ratio. Waist:hip ratio in a sample of English and Jamaican women was negatively related to 2D:4D. (b) Sex and population differences in mean 2D:4D in samples from England, Germany, Spain, Hungary (including ethnic Hungarians and Gypsy subjects), Poland, Jamaica, Finland, and South Africa (a Zulu sample). Significant sex and population differences in mean 2D:4D were apparent.
A woman's reproductive value decreases over her reproductive life span and it is therefore predicted that the likelihood of termination of investment in a child decreases with increasing age. An eventual increase in termination ratio in the oldest age groups, as is often found in abortion statistics, could depend on older women on average having larger families rather than on age per se. We used data on abortions and births in Sweden during 1994 to investigate how abortion ratio is related to age and parity of women. We found that age-specific abortion ratio is U-shaped (i.e. that it is highest for the youngest and for the oldest age groups) in each parity class from zero to four children but that age-dependence breaks down in higher parity classes (5, >/=6). Thus, for each of the parity classes 0-4, the incidence of abortion decreases with age up to a point, but increases again as women approach menopause. This late increase in induced abortion ratio seems to depend on age per se. The data indicate that abortion ratio is an inverse function of fertility, and that investment in new reproduction gradually decreases as a woman approaches menopause. Assuming grandmothering as an important driving force in human life history evolution, such a pattern might indicate that the transition from behavioural investment in one's own children to one's grandchildren is a gradual process similar to the decline in ovarian function.
The role of men's jealousy over a wife's infidelity in precipitating marital conflict and wife abuse is well documented. The role of women's jealousy over a husband's infidelity has received little attention, which is puzzling given high potential costs to women of withdrawal of paternal investment. We address this gap by investigating marital conflict and wife abuse among Tsimane forager farmers of Bolivia. We test predictions derived from male jealousy and paternal disinvestment hypotheses, which consider threats and consequences of infidelity by women (male jealousy hypothesis) and men (paternal disinvestment hypothesis). The paternal disinvestment hypothesis proposes that wife abuse is employed by husbands to limit wives' mate retention effort and maintain men's opportunities to pursue extrapair sexual relationships. Interviews were conducted among husbands and wives in the same marriages using a combination of open-ended and structured items. Spouses agree that the most frequently reported type of marital argument is women's jealousy over a husband's infidelity (N=266 arguments). Roughly 60% of abusive events occurred during arguments over men's diversion of household resources (N=124 abusive events). In multivariate analyses, likelihood of wife abuse is greater in marriages where husbands have affairs, where wives are younger, and where spouses spend more time apart (N=60 husbands, 71 wives). While we find strong support for both male jealousy and paternal disinvestment hypotheses, it is men's infidelity, not women's, that precipitates most instances of marital conflict and wife abuse. We conclude that men's aggression towards their wives facilitates men's diversion of family resources for their selfish interests. Published by Elsevier Inc.
Women's preference for masculine faces varies with hormonal state, sociosexuality, and relationship status, but the underlying mechanisms are poorly understood. We hypothesized that hormones and psychosexual factors (sociosexuality, sexual inhibition/excitation) mediate the perception and evaluation of male faces thereby influencing women's preferences. We used fMRI to measure brain activity in twelve women as they evaluated pictures of male faces (half 30% masculinized, half 30% feminized). Participants were heterosexual women, age 23-28, who were not in a committed relationship and not using hormonal contraception. Women were tested during both the follicular and luteal phase of their menstrual cycle. We found five brain regions related to face and risk processing that responded more to the masculinized than to the feminized faces, including the superior temporal gyrus, precentral gyrus, posterior cingulate cortex, inferior parietal lobule, and anterior cingulate cortex. Increased activation in the anterior cingulate cortex, specifically, may indicate that women perceive masculinized faces to be both more risky and more attractive. We did not see any areas that were more strongly activated by feminized faces. Levels of activation were influenced by hormonal and psychosexual factors. The patterns of hormonally and psychosexually mediated neural activation observed may offer insight into the cognitive processes underlying women's partner preferences.
We tested the hypothesis that, compared with sociosexually restricted individuals, those with an unrestricted approach to mating would selectively allocate visual attention to attractive opposite-sex others. We also tested for sex differences in this effect. Seventy-four participants completed the Sociosexual Orientation Inventory, and performed a computer-based task that assessed the speed with which they detected changes in attractive and unattractive male and female faces. Differences in reaction times served as indicators of selective attention. Results revealed a Sex X Sociosexuality interaction: Compared with sociosexually restricted men, unrestricted men selectively allocated attention to attractive opposite-sex others; no such effect emerged among women. This finding was specific to opposite-sex targets and did not occur in attention to same-sex others. These results contribute to a growing literature on the adaptive allocation of attention in social environments.
Human fathers are heavily involved in the rearing of children around the world. Early evolutionary explanations focused on the greater need of human children and mothers compared to other species and the consequent increased benefits available to investing fathers and pair-bonded husbands. Contrary to this hypothesis, research suggests that the impact of men's care on the survivorship and physical well-being of juvenile offspring is cross-culturally variable and often unsubstantial. Proper testing of the hypothesis, however, also requires exploring how well children raised with paternal investment fare as adults, compared to those raised in the absence of fathers. We explore this issue among the Tsimane, who exhibit high levels of paternal provisioning and very low divorce rates, by testing the impact of early father death on five measures of adult success: completed height, body mass index (BMI), age of first reproduction, completed fertility for age and number of surviving offspring for age. Of these five tests, a significant effect in the predicted direction was found only for body mass index of adult daughters. Therefore, there is no substantial evidence that Tsimane fathers have a large impact on the success of adult children. We explore alternative explanations for the high levels of paternal involvement and low divorce rates observed among the Tsimane, including the positive effects of men's investments on couple fertility and the constraints imposed by female preferences and the availability of alternative partners.
The human face provides a wealth of information pertaining to the internal state and life-stage history of an individual. Facial width-to-height ratio is a size-independent sexually dimorphic trait, and estimates of aggression made by untrained adults judging own-race faces were positively associated with both facial width-to-height ratio and actual aggressive behavior. Given the significant adaptive value of accurately detecting aggressiveness based on facial appearance, we hypothesized that aggression estimates made by adults and 8-year-olds would be highly correlated with male facial width-to-height ratio even for a face category with which they had minimal experience-other-race faces. For each of the four race and age groups, estimates of aggression were positively correlated with facial width-to-height ratio irrespective of rating own-or other-race faces. Overall, the correlations between facial width-to-height ratio and ratings of aggression were stronger for adults than for children. Sensitivity to facial width-to-height ratio appears to be part of an evolved mechanism designed to detect threats in the external environment. This mechanism is likely broadly tuned and functions independently of experience.
The tradeoff between offspring quantity and offspring quality is at the heart of most evolutionary approaches to the fertility transition, as it is for demographers oriented towards economic explanations for this transition. To date, however, there have been few empirical tests of the key idea that humans trade offspring quantity for quality, and no strictly comparative work designed to identify the specific environmental conditions that favor such a tradeoff. This study suggests that in an East African community where the principal form of intergenerational inheritance is land, intermediate levels of offspring production are favored for women but not men. Women produce approximately the optimal number of surviving children, whereas men produce far fewer than the optimal number. The result highlights the significance of inheritable extrasomatic capital, in conjunction with evolved psychological mechanisms, in shaping fertility strategies that emphasize quality over quantity.
Although unrelated friends are genetically equivalent to strangers, several lines of reasoning suggest that close friendship may sometimes activate processes more relevant to kinship and that this may be especially true for women. We compared responses to strangers, friends, and kin in two studies designed to address distinct domains for which kinship is known to have functional significance: incest avoidance and nepotism. Study 1 examined emotional responses to imagined sexual contact with kin, friends, and strangers. Results revealed that women, compared to men, treated friends more like kin. Study 2 examined benevolent attributions to actual kin, friends, and strangers. Results revealed that women treated friends very much like kin, whereas men treated friends very much like strangers. The current findings support a domain-specific over a domain-general approach to understanding intimate relationships and raise a number of interesting questions about the modular structure of cognitive and affective processes involved in these relationships.
While social-status hierarchies are common to all human societies, status acquisition is relatively understudied in small-scale societies lacking significant material wealth or intergenerational inheritance. Among the Tsimane of Bolivia, a small-scale Amazonian society, we employ a photo-ranking methodology to determine the important predictors of four measures of male social status: success in dyadic physical confrontation, getting one's way in a group, community-wide influence, and respect. The predictors evaluated include age, physical size, skill in food production, level of acculturation, prosocial personality, and social support. We find that physical size best predicts rankings of dyadic fighting ability while social support best predicts getting one's way in a group, community-wide influence, and respect. Level of acculturation, furthermore, is an independent predictor of influence but not respect, and skill in food production is an independent predictor of respect but not influence. The lack of a linear relationship between age and the polyadic social-status measures is evaluated in light of the increasing exposure of the Tsimane to market economies and public education among recent age cohorts. To our knowledge, this study is the first multivariate analysis of social status that considers different determinants of status simultaneously.
Polyandry challenges both traditional sociocultural and evolutionary understandings of marriage. Its existence calls into question the importance of female sexual exclusivity and reproduction in marriage, two fundamental aspects of the affinal bond in most other marital forms. How does polyandrous marriage persist? When does it dissolve? This paper examines marriage patterns in the fraternally polyandrous Tibetan communities of northwestern Nepal, highlighting the most important factors associated with both the maintenance and dissolution of polyandrous unions. Behavioral ecological analyses of polyandry to date have focused on univariate analyses of the economic and fertility correlates of the two primary marital forms observed in nominally polyandrous communities, polyandry and monogamy. In this analysis, multivariate regression and survival analysis are used to extend our understanding of the factors that precipitate partitioning among the polyandrous marriages in the study. Wealth, long thought to be a major force shaping marital decisions in these communities, is modeled in both quantity and quality in this study. This represents an important departure from previous studies, as analyses show that it is not only the amount of wealth that influences the stability of polyandrous marriages, but also the type of wealth and extent to which it is diversified over economic spheres. Univariate analyses show that amount of wealth and the size of the brother/co-husband set are both associated with the probability that a polyandrous marriage will partition. Though these results support earlier findings, multivariate analyses show that controlling for other factors reduces their importance.
We explored the relationships between facial attractiveness and several variables thought to be related to genotypic and phenotypic quality in humans (namely fluctuating asymmetry (FA), body mass index (BMI), health, age). To help resolve some controversy around previous studies, we used consistent measurement and statistical methods and relatively large samples of both female (n=94) and male (n=95) subjects (to be evaluated and measured), and female (n=226) and male (n=153) viewers (to rate attractiveness). We measured the asymmetry of 22 traits from three trait families (eight facial, nine body and five fingerprint traits) and constructed composite asymmetry indices of traits showing significant repeatability. Facial attractiveness was negatively related to an overall asymmetry index in both females and males, with almost identical slopes. Female facial attractiveness was best predicted by BMI and past health problems, whereas male facial attractiveness was best predicted by the socioeconomic status (SES) of their rearing environment. Composite FA indices accounted for a small (<4%) but usually significant percentage of the variation in facial attractiveness in both sexes, when factors related to asymmetry were controlled statistically. We conclude that, although facial attractiveness is negatively related to developmental instability (as measured by asymmetry), attractiveness also signals different aspects of "quality" in the two sexes, independent of FA.
In this paper I evaluate the merit of costly signaling theory (CST) as a paradigm for understanding why men of Ifaluk atoll torch fish. I argue that torch fishing is a handicap that signals men's productivity. Consistent with CST, torch fishing is observed by the predicted audience (women), energetically costly to perform, and a reliable indicator of the frequency a man fishes during the trade wind season. Contrary to expectations of who should benefit from torch fishing and consequently participate, torch fishers are not primarily young and unmarried. Torch fishers, however, are predominately from the matriline that owns the canoe on which they fish, suggesting that torch fishing also signals the productivity of a matriline. Although these results support the possibility that torch fishing is a handicap, no data are presented which demonstrate that torch fishers achieve any gains from sending the costly signal. This shortcoming and other directions for future research on Ifaluk foraging decisions are discussed.
We argue that a central function of religious attendance in the contemporary U.S. is to support a high-fertility, monogamous mating strategy. Although religious attendance is correlated with many demographic, personality, moral, and behavioral variables, we propose that sexual and family variables are at the core of many of these relationships. Numerous researchers have assumed that religious socialization causes people to feel moral reactions and engage in behaviors promoted by religious groups. On our view, mating preferences are centrally involved in individual differences in attraction to religious groups. In a sample of 21,131 individuals who participated in the U.S. General Social Survey, sexual behaviors were the relatively strongest predictors of religious attendance, even after controlling for age and gender. Effects of age and gender on religious attendance were weaker, and substantially reduced when controlling for sexual and family patterns. A sample of 902 college students provided more detailed information on religious, moral, and sexual variables. Results suggest that 1) moral views about sexual behavior are more strongly linked to religious attendance than other moral issues, and 2) mating strategy is more powerful than standard personality variables in predicting religious attendance. These findings suggest that reproductive strategies are at the heart of variations in religious attendance.
Physical attractiveness has been associated with mating behavior, but its role in reproductive success of contemporary humans has received surprisingly little attention. In the Wisconsin Longitudinal Study (1244 women, 997 men born between 1937 and 1940) we examined whether attractiveness assessed from photographs taken at age ~18 predicted the number of biological children at age 53-56. In women, attractiveness predicted higher reproductive success in a nonlinear fashion, so that attractive (second highest quartile) women had 16% and very attractive (highest quartile) women 6% more children than their less attractive counterparts. In men, there was a threshold effect so that men in the lowest attractiveness quartile had 13% fewer children than others who did not differ from each other in the average number of children. These associations were partly but not completely accounted for by attractive participants' increased marriage probability. A linear regression analysis indicated relatively weak directional selection gradient for attractiveness (β=0.06 in women, β=0.07 in men). These findings indicate that physical attractiveness may be associated with reproductive success in humans living in industrialized settings.
Small deviations from bilateral symmetry (a phenomenon called fluctuating asymmetry [FA]) are believed to arise due to an organism's inability to implement a developmental program when challenged by developmental stress. FA thus provides an index of an organism's exposure to adverse environmental effects and its ability to resist these effects. If one wishes to choose an individual with good health and fertility, FA could be used as an index of a potential partner's suitability. To explore whether this theory can be applied to human female bodies (excluding heads), we used a specially developed software package to create images with perfect symmetry. We then compared the relative attractiveness of the normal (asymmetric image) with the symmetric image. When male and female observers rated the images for attractiveness on a scale of 1 to 10, there was no significant difference in attractiveness between the symmetric and asymmetric images. However, in a two-alternative forced-choice experiment, the symmetric image was significantly more popular. The evidence suggests a role for symmetry in the perception of the attractiveness of the human female body.
Theory of mind is the field devoted to understanding how organisms discern the mental states of others. Because mental states are not directly observable, they can only be inferred from observable features of the actor (such as behavior) and the situational context that the actor is in. Social psychologists, who study theory of mind processes under the rubric of attribution research, have shown that people often make a logical error of inference: The "fundamental attribution error" (FAE) is the tendency to assume that an actor's behavior and mental state correspond to a degree that is logically unwarranted by the situation. The social environment in which theory of mind capacities evolved may have influenced attributional processing in ways that could explain the error. In particular, the error could be caused by a psyche that is designed (1) to consider only those noncorresponding mental states (such as deception) that could have fitness consequences to the mind reader; (2) to bias inferences in a way that reduces the costs of erroneous inferences; or (3) to bias inferences in a way that yields reputational benefits. The existing literature is reviewed in light of these hypotheses.
We investigated whether the attractive facial traits of averageness and symmetry signal health, examining two aspects of signalling: whether these traits are perceived as healthy, and whether they provide accurate health information. In Study 1, we used morphing techniques to alter the averageness and symmetry of individual faces. Increases in both traits increased perceived health, and perceived health correlated negatively with rated distinctiveness (a converse measure of averageness) and positively with rated symmetry of the images. In Study 2, we examined whether these traits signal real, as well as perceived, health, in a sample of individuals for whom health scores, based on detailed medical records, were available. Perceived health correlated negatively with distinctiveness and asymmetry, replicating Study 1. Facial distinctiveness ratings of 17-year-olds were associated with poor childhood health in males, and poor current and adolescent health in females, although the last association was only marginally significant. Facial asymmetry of 17-year-olds was not associated with actual health. We discuss the implications of these results for a good genes account of facial preferences.
The evolution of cooperation through partner choice mechanisms is often thought to involve relatively complex cognitive abilities. Using agent-based simulations I model a simple partner choice rule, the 'Walk Away' rule, where individuals stay in groups that provide higher returns (by virtue of having more cooperators), and 'Walk Away' from groups providing low returns. Implementing this conditional movement rule in a public goods game leads to a number of interesting findings: 1) cooperators have a selective advantage when thresholds are high, corresponding to low tolerance for defectors, 2) high thresholds lead to high initial rates of movement and low final rates of movement (after selection), and 3) as cooperation is selected, the population undergoes a spatial transition from high migration (and a many small and ephemeral groups) to low migration (and large and stable groups). These results suggest that the very simple 'Walk Away' rule of leaving uncooperative groups can favor the evolution of cooperation, and that cooperation can evolve in populations in which individuals are able to move in response to local social conditions. A diverse array of organisms are able to leave degraded physical or social environments. The ubiquitous nature of conditional movement suggests that 'Walk Away' dynamics may play an important role in the evolution of social behavior in both cognitively complex and cognitively simple organisms.
This paper advances an "information goods" theory that explains prestige processes as an emergent product of psychological adaptations that evolved to improve the quality of information acquired via cultural transmission. Natural selection favored social learners who could evaluate potential models and copy the most successful among them. In order to improve the fidelity and comprehensiveness of such ranked-biased copying, social learners further evolved dispositions to sycophantically ingratiate themselves with their chosen models, so as to gain close proximity to, and prolonged interaction with, these models. Once common, these dispositions created, at the group level, distributions of deference that new entrants may adaptively exploit to decide who to begin copying. This generated a preference for models who seem generally "popular." Building on social exchange theories, we argue that a wider range of phenomena associated with prestige processes can more plausibly be explained by this simple theory than by others, and we test its predictions with data from throughout the social sciences. In addition, we distinguish carefully between dominance (force or force threat) and prestige (freely conferred deference).
Low birthweight and the infant's health status are expected to strongly influence the child's reproductive value and, thus, the maternal decisions on the amount and timing of investment. A total of 590 Hungarian primiparous mothers giving birth in the late 1980s were recruited for the longitudinal study. Mothers of high-risk infants shortened the duration of breast-feeding and interbirth intervals, compared to those with an infant of higher survival prospects. The most powerful predictor of the length of the lactation period was the infant's weight at birth, whereas birth spacing was significantly influenced by the health status of the older child. Socioeconomic status had a positive effect on maternal care as well, but it did not change the basic pattern of diminishing maternal care as a function of the infants' low reproductive value. The combination of the above factors resulted in a cumulative effect on maternal investment of mothers with handicapped children of various degrees of risk. An attempt has been made to exclude alternative explanations and to discuss the proximate mechanisms of discriminative parental solicitude.
In this work, we provide evidence based on direct observation of behavior in encounters of opposite-sexed strangers, that women initiate and "control" the outcome. In the first minute of these videotaped 10-min interactions, neither female "solicitation" behavior nor "negative" behavior is strongly related to professed interest in the man, while female "affirmative" behavior at this stage modulates male verbal output in later stages (4-10 min). Although the rate of female courtship-like behavior is significantly higher in the first minute, it is only in the fourth to tenth minute that the rate of female courtship-like behavior is correlated with professed female interest. We hypothesize that this serves as a strategic dynamic reflecting sexual asymmetry in parental investment and the potential cost of male deception to women. Ambiguous protean behavioral strategies veil individuals' intentions and make their future actions unpredictable. These behavioral strategies may result in men's overestimation of female sexual interest.
Darwinists still do not have a full theory of human social organization. Relatedness is a key idea in evolutionary theory. Yet, despite some pioneering studies, few Darwinists have used formal measures of relatedness, and even fewer have used them to describe social organization. Nobody has used relatedness to explain the rise, coherence, and decline of classes, class composition, and class relations. This article does that for one village in southern Thailand over a period of about 120 years. Each class had its own characteristic family type and relations between families, which varied over time. This article also uses the incidence of marriage between kin to show the need to take into account different perspectives on relatedness and social organization.
Relative social status strongly regulates human behavior, yet this factor has been largely ignored in research on risky decision-making. Humans, like other animals, incur risks as they compete to defend or improve their standing in a social group. Among men, access to culturally important resources is a locus of intrasexual competition and a determinant of status. Thus, relative status should affect men's motivations for risk in relevant domains. Contrasting predictions about such effects were derived from dominance theory and risk-sensitive foraging theory. Experiments varied whether subjects thought they were being observed and evaluated by others of lower, equal, or higher status, and whether decisions involved resources (status relevant) or medical treatments (status irrelevant). Across two experiments, men who thought others of equal status were viewing and evaluating their decisions were more likely to favor a high risk/high gain means of recouping a monetary loss over a no risk/low gain means with equal expected value. Supporting predictions from dominance theory, this motivation for risk-taking appeared only in the equal status condition, only for men, and only for resource loss problems. Taken together, the results support the idea that motivational systems designed to negotiate a status-saturated social world regulate the cognitive processes that generate risky decision-making in men.
Humans and other animals have a variety of psychological abilities tailored to the demands of asocial foraging, that is, foraging without coordination or competition with other conspecifics. Human foraging, however, also includes a unique element, the creation of resource pooling systems. In this type of social foraging, individuals contribute when they have excess resources and receive provisioning when in need. Is this behavior produced by the same psychology as asocial foraging? If so, foraging partners should be judged by the same criteria used to judge asocial patches of resources: the net energetic benefits they provide. The logic of resource pooling speaks against this. Maintaining such a system requires the ability to judge others not on their short-term returns, but on the psychological variables that guide their behavior over the long-term. We test this idea in a series of five studies using an implicit measure of categorization. Results showed that (1) others are judged by the costs they incur (a variable not relevant to asocial foraging) whereas (2) others are not judged by the benefits they provide when benefits provided are unrevealing of underlying psychological variables (despite this variable being relevant to asocial foraging). These results are suggestive of a complex psychology designed for both social and asocial foraging.
This pilot study explores the degree of solidarity felt between full and half siblings who are raised in a Mormon Fundamentalist polygamous community. The community under study is unique in that, at the level of official culture, it actively promotes full and half sibling solidarity through an ethos that strives to downplay genetic differences in favor of a harmonious family living together in one household. This community is an ideal cultural setting in which to examine the suitability of inclusive fitness theory for understanding the factors that promote family cohesion, sibling solidarity, and rivalry. Our main question becomes: is the degree of sibling solidarity a manifestation of genetic closeness or a natural byproduct of emotional closeness that arises from being raised together? We found evidence for more solidarity between full siblings than between half siblings. Our data suggest that, despite the force of religious ideals, and notwithstanding the continued close physical proximity of half siblings in the polygamous family, there is a pronounced clustering of feeling and affection in the polygamous family that is consistent with inclusive fitness theory.
Restrictive eating attitudes and behaviors have been hypothesized to be related to processes of intrasexual competition. According to this perspective, within-sex competition for status serves the adaptive purpose of attracting mates. As such, status competition salience may lead to concerns of mating desirability. For heterosexual women and gay men, such concerns revolve around appearing youthful and thus, thinner. Following this logic, we examined how exposure to high-status and competitive (but not thin or highly attractive) same-sex individuals would influence body image and eating attitudes in heterosexual and in gay/lesbian individuals. Results indicated that for heterosexuals, intrasexual competition cues led to greater body image dissatisfaction and more restrictive eating attitudes for women, but not for men. In contrast, for homosexual individuals, intrasexual competition cues led to worse body image and eating attitudes for gay men, but not for lesbian women. These findings support the idea that the ultimate explanation for eating disorders is related to intrasexual competition.
We propose that a "social exchange heuristic" is as important as the cheater detection mechanism for attaining mutual cooperation in social exchange. The social exchange heuristic prompts people to perceive a mixed-motive situation, such as the Prisoner's Dilemma (PD), as an Assurance Game (AG) situation in which cooperation is a personally better choice than defection insofar as the partner is cooperating as well. We demonstrate the operation of the social exchange heuristic through a comparison of the ordinary one-shot, simultaneous PD with the one-shot, sequential PD. Participants in the current experiments, involving a total of 261 volunteers, committed a logical error in the direction of favoring mutual cooperation as the situation involved more serious consequences. This result strongly suggests the operation of a domain specific "bias" that encourages pursuit of mutual cooperation in social exchange.
As biological and linguistic diversity, the world's cultural diversity is on decline. However, to date there are no estimates of the rate at which the specific cultural traits of a group disappear, mainly because we lack empirical data to assess how the cultural traits of a given population change over time. Here we estimate changes in cultural traits associated to the traditional knowledge of wild plant uses among an Amazonian indigenous society. We collected data among 1151 Tsimane' Amerindians at two periods of time. Results show that between 2000 and 2009, Tsimane' adults experienced a net decrease in the report of plant uses ranging from 9% (for the female subsample) to 26% (for the subsample of people living close to towns), equivalent to a 1 to 3 % per year. Results from a Monte Carlo simulation show that the observed changes were not the result of randomness. Changes were more acute for men than for women and for informants living in villages close to market towns than for informants settled in remote villages. The Tsimane' could be abandoning their traditional knowledge as they perceive that this form of knowledge do not equip them well to deal with the new socio-economic and cultural conditions they face nowadays.
Little is known about the physiological and behavioral changes that expectant fathers undergo prior to the birth of their babies. We measured hormone concentrations and responses to infant stimuli in expectant and new fathers living with their partners to determine whether men can experience changes that parallel the dramatic shifts seen in pregnant women. We obtained two blood samples from couples at one of four times before or after the birth of their babies. After the first sample, the couples were exposed to auditory, visual, and olfactory cues from newborn infants (test of situational reactivity). Men and women had similar stage-specific differences in hormone levels, including higher concentrations of prolactin and cortisol in the period just before the births and lower postnatal concentrations of sex steroids (testosterone or estradiol). Men with more pregnancy (couvade) symptoms and men who were most affected by the infant reactivity test had higher prolactin levels and greater post-test reduction in testosterone. Hormone concentrations were correlated between partners. This pattern of hormonal change in men and other paternal mammals, and its absence in nonpaternal species, suggests that hormones may play a role in priming males to provide care for young.
Costly signaling theory (CST) offers an explanation of generosity and collective action that contrasts sharply with explanations based on conditional reciprocity. This makes it particularly relevant to situations involving widespread unconditional provisioning of collective goods. We provide a preliminary application of CST to ethnographic data on turtle hunting and public feasting among the Meriam of Torres Strait, Australia. Turtle hunting appears to meet the key conditions specified in CST: it is (1) an honest signal of underlying abilities such as strength, risk-taking, skill, and leadership; (2) costly in ways not subject to reciprocation; (3) an effective means of broadcasting signals, since the collective good (a feast) attracts a large audience; and (4) seems to provide benefits to signalers (turtle hunters) as well as recipients (audience). We conclude with some suggestions as to the broader implications of this research, and the costly signaling paradigm in general, for understanding collective action and generosity in human social groups.
We propose that intuitions about modern mass-level criminal justice emerge from evolved mechanisms designed to operate in ancestral small-scale societies. By hypothesis, individuals confronted with a crime compute two distinct psychological magnitudes: one that reflects the crime's seriousness and another that reflects the criminal's long-term value as an associate. These magnitudes are computed based on different sets of cues and are fed into motivational mechanisms regulating different aspects of sanctioning. The seriousness variable regulates how much to react (e.g., how severely we want to punish); the variable indexing the criminal's association value regulates the more fundamental decision of how to react (i.e., whether we want to punish or repair). Using experimental designs embedded in surveys, we validate this theory across several types of crime and two countries. The evidence augments past research and suggests that the human mind contains dedicated psychological mechanisms for restoring social relationships following acts of exploitation.
Recent research on kin investment as a reproductive strategy is based on the idea that differences in grandparental caregiving directly reflect degrees of differential grandpaternal versus grandmaternal certainty. In a cross-cultural study in Greece and Germany, 544 subjects (318 Greeks, 208 Germans, 18 of other origins) were asked for an assessment of their grandparents' (GPs') caregiving. In Germany and urban Greece (modern Western societies), the maternal GPs were rated as more intensive caregivers than the paternal GPs, but this was not the case in rural Greece, where paternal GPs provided more care. However, in all groups, grandmothers were more caring than grandfathers. Thus, contrary to previous theory and research, these two effects must be clearly distinguished, and may be explained by (1) more intense female caregiving in humans (as in other viviparous mammals) and (2) a socially engendered favoring of maternal relatives in Western industrial societies as opposed to the favoring of paternal GPs seen in the patrilateral culture of rural Greece.
In this paper, I present evidence for a robust and quite general force of selection on the human life cycle. The force of selection acts in remarkably invariant ways on human life histories, despite a great abundance of demographic diversity. Human life histories are highly structured, with mortality and fertility changing substantially through the life cycle. This structure necessitates the use of structured population models to understand human life history evolution. Using such structured models, I find that the vital rates to which fitness is most sensitive are pre-reproductive survival probabilities, particularly the survival of children ages 0-4. The fact that the preponderance of selection falls on transitions related to recruitment combined with the late age at first reproduction characteristic of the human life cycle, creates a fitness bottleneck out of recruitment. Because of this, antagonistic pleiotropy with any trait that detracts from the constituent transitions to recruitment is expected. I explore the predictors of variation in the force of selection on early survival. High fertility increases the selective premium placed on early survivorship, while high life expectancy at birth decreases it.
Psychopaths are manipulative, impulsive, and callous individuals with long histories of antisocial behavior. Two models have guided the study of psychopathy. One suggests that psychopathy is a psychopathology, i.e., the outcome of defective or perturbed development. A second suggests that psychopathy is a life-history strategy of social defection and aggression that was reproductively viable in the environment of evolutionary adaptedness (EEA). These two models make different predictions with regard to the presence of signs of perturbations or instability in the development of psychopaths. In Study 1, we obtained data on prenatal, perinatal, and neonatal signs of developmental perturbations from the clinical files of 643 nonpsychopathic and 157 psychopathic male offenders. In Study 2, we measured fluctuating asymmetry (FA, a concurrent sign of past developmental perturbations) in 15 psychopathic male offenders, 25 nonpsychopathic male offenders, and 31 male nonoffenders. Psychopathic offenders scored lower than nonpsychopathic offenders on obstetrical problems and FA; both psychopathic and nonpsychopathic offenders scored higher than nonoffenders on FA. The five offenders from Study 2 meeting the most stringent criteria for psychopathy were similar to nonoffenders with regard to FA and had the lowest asymmetry scores among offenders. These results provide no support for psychopathological models of psychopathy and partial support for life-history strategy models.
Fetal and adult testosterone may be important in establishing and maintaining sex-dependent abilities associated with male physical competitiveness. There is evidence that the ratio of the length of the 2nd and 4th digits (2D:4D) is a negative correlate of prenatal and adult testosterone. We use ability in sports, and particularly ability in football, as a proxy for male physical competitiveness. Compared to males with high 2D:4D ratio, men with low ratio reported higher attainment in a range of sports and had higher mental rotation scores (a measure of visual-spatial ability). Professional football players had lower 2D:4D ratios than controls. Football players in 1st team squads had lower 2D:4D than reserves or youth team players. Men who had represented their country had lower ratios than those who had not, and there was a significant (one-tailed) negative association between 2D:4D and number of international appearances after the effect of country was removed. We suggest that prenatal and adult testosterone promotes the development and maintenance of traits which are useful in sports and athletics disciplines and in male:male fighting.
The jus primae noctis was, in the European late medieval context, a widespread popular belief in an ancient privilege of the lord of the manor to share the wedding bed with his peasants' brides. Symbolic gestures, reflecting this belief, were developed by the lords and used as humiliating signs of superiority over the dependent peasants in the fifteenth century, a time of diminishing status differences. Actual intercourse in the exercise of the alleged right is difficult to prove, and there is no hard evidence to suggest that it ever actually happened. However, the symbolic gestures can be best interpreted as a male power display, with a basis in the psychology of coercive social dominance, male competition, and male desire for sexual variety. Several non-European cultures have accounts of a similar custom related to a young girl's first sexual intercourse: ritual defloration by chiefs, priests, or strangers. This non-European custom differs from the jus primae noctis in its proximate details, but seems from an ultimate point of view, to be in conformity with the European evidence. In this article the origin, development, and relationship of both customs are discussed and interpreted in light of recent evolutionary studies of primate behavior and sexual psychology.
In a between-group study, men were much more likely to profess willingness to donate sperm if it would be used solely for research purposes (67% yes) than if it would be used for reproduction through the process of donor insemination (24%). Offered several reasons why one might be willing to be a sperm donor, men nominated “money” and “helping out a couple in need” most often and “the chance to produce children” least often, whereas “the knowledge that it might produce children that I may never meet” was the most popular of several proposed rationales for not donating. These results are discussed in light of the specificity of male sexual psychologies and the possible costs associated with sperm donation, within the more general framework of identifying the circumstances in which men report the willingness to donate sperm.
The maintenance of personality variation remains an unexplained puzzle in evolutionary biology. Despite evidence among non-humans that personality variation affects fitness, few data exist to assess the personality-fitness relationship in humans. Among Tsimane forager-horticulturalists (n = 632), we test whether personality traits (assessed using a 43-item Big Five Inventory administered orally in native language) predict fertility, offspring survivorship, age of first reproduction, and other fitness correlates (extramarital affairs, conflicts, social visitation, food production, and several health measures). Among men, several personality factors associate with higher fertility, more time spent producing food and social visitation. Among women, the relationship between personality and fitness varies across regions of Tsimane territory. The only case of an intermediate personality level associated with highest fitness was found for Industriousness in men. We find that personality factors positively associated with fitness do not associate with greater health costs, although greater Extraversion and Openness may lead to more conflicts among men. Factor heritability ranges from 60% for Prosociality and Extraversion to 8% for Neuroticism. We interpret our results in light of evolutionary models that explain maintenance of personality variation, including incomplete directional selection, mutation-selection balance, condition-dependent reaction norms and fluctuating selection based on sex or spatial variability in selection pressures.
Based on Silverman and Eals' hunter-gatherer theory of the origin of sex-specific spatial attributes, the present research sought to identify the evolved mechanisms involved in hunting that contribute to the dimorphism. The focus of these studies was the relationship between three-dimensional mental rotations, the spatial test showing the largest and most reliable sex difference favoring males, and wayfinding in the woods. Space constancy was presumed to be the evolved mechanism fundamental to both of these abilities. Measures of wayfinding were derived by leading subjects individually on a circuitous route through a wooded area, during which they were stopped at prescribed places and required to set an arrow pointing in the direction the walk began. As well, subjects were eventually required to lead the experimenters back to the starting point by the most direct route. In support of the hypotheses, males excelled on the various measures of wayfinding, and wayfinding was significantly related across sexes to mental rotations scores but not to nonrotational spatial abilities or general intelligence.
Fifty-five men participating in a domestic violence treatment program agreed to complete a questionnaire and rate the degree to which their children looked like them. Ratings of paternal resemblance were positively correlated with the self-reported quality of the men's relationships with their children and inversely proportional to the severity of injuries suffered by their spouses. Analogous results were also found for the men's experience with their parents. We suggest that these results reflect men's use of paternal resemblance to assess paternity.
Testosterone plays an important role in mediating male reproductive trade-offs in many vertebrate species, augmenting muscle and influencing behavior necessary for male-male competition and mating-effort. Among humans, testosterone may also play a key role in facilitating male provisioning of offspring as muscular and neuromuscular performance are deeply influenced by acute changes in testosterone. This study examines acute changes in salivary testosterone among 63 Tsimane men ranging in age from 16-80 (mean 38.2) years during one-hour bouts of tree-chopping while clearing horticultural plots. The Tsimane forager-horticulturalists living in the Bolivian Amazon experience high energy expenditure associated with food production, have high levels of parasites and pathogens, and display significantly lower baseline salivary testosterone than age-matched US males. Mixed-effects models controlling for BMI and time of specimen collection reveal increased salivary testosterone (p<0.001) equivalent to a 48.6% rise, after one hour of tree chopping. Age had no effect on baseline (p=0.656) or change in testosterone (p=0.530); self-reported illness did not modify testosterone change (p=0.488). A comparison of these results to the relative change in testosterone during a competitive soccer tournament in the same population reveals larger relative changes in testosterone following resource production (tree chopping), compared to competition (soccer). These findings highlight the importance of moving beyond a unidimensional focus on changes in testosterone and male-male aggression to investigate the importance of testosterone-behavior interactions across additional male fitness-related activities. Acutely increased testosterone during muscularly intensive horticultural food production may facilitate male productivity and provisioning.
Previous studies with adult humans and non-human animals revealed more rapid fear learning for spiders and snakes than for mushrooms and flowers. The current experiments tested whether 11-month-olds show a similar effect in learning associative pairings between facial emotions and fear-relevant and fear-irrelevant stimuli. Consistent with the greater incidence of snake and spider phobias in women, results show that female but not male infants learn rapidly to associate negative facial emotions with fear-relevant stimuli. No difference was found between the sexes for fear-irrelevant stimuli. The results are discussed in relation to fear learning, phobias, and a specialized evolved fear mechanism in humans.
Costly signaling has been proposed as a possible mechanism to explain food sharing in foraging populations. This sharing-as-signaling hypothesis predicts an association between sharing and status. Using exponential random graph modeling (ERGM), this prediction is tested on a social network of between-household food-sharing relationships in the fishing and sea-hunting village of Lamalera, Indonesia. Previous analyses (Nolin 2010) have shown that most sharing in Lamalera is consistent with reciprocal altruism. The question addressed here is whether any additional variation may be explained as sharing-as-signaling by high-status households. The results show that high-status households both give and receive more than other households, a pattern more consistent with reciprocal altruism than costly signaling. However, once the propensity to reciprocate and household productivity are controlled, households of men holding leadership positions show greater odds of unreciprocated giving when compared to households of non-leaders. This pattern of excessive giving by leaders is consistent with the sharing-as-signaling hypothesis. Wealthy households show the opposite pattern, giving less and receiving more than other households. These households may reciprocate in a currency other than food or their wealth may attract favor-seeking behavior from others. Overall, status covariates explain little variation in the sharing network as a whole, and much of the sharing observed by high-status households is best explained by the same factors that explain sharing by other households. This pattern suggests that multiple mechanisms may operate simultaneously to promote sharing in Lamalera and that signaling may motivate some sharing by some individuals even within sharing regimes primarily maintained by other mechanisms.
In most human foraging societies, the meat of large animals is widely shared. Many assume that people follow this practice because it helps to reduce the risk inherent in big game hunting. In principle, a hunter can offset the chance of many hungry days by exchanging some of the meat earned from a successful strike for shares in future kills made by other hunters. If hunting and its associated risks of failure have great antiquity, then meat sharing might have been the evolutionary foundation for many other distinctively human patterns of social exchange. Here we use previously unpublished data from the Tanzanian Hadza to test hypotheses drawn from a simple version of this argument. Results indicate that Hadza meat sharing does not fit the expectations of risk-reduction reciprocity. We comment on some variations of the "sharing as exchange" argument; then elaborate an alternative based partly on the observation that a successful hunter does not control the distribution of his kill. Instead of family provisioning, his goal may be to enhance his status as a desirable neighbor. If correct, this alternative argument has implications for the evolution of men's work.
Kenrick et al.'s experiments demonstrate that men who view photographs of physically attractive women or Playboy centerfolds subsequently find their current mates less physically attractive and become less satisfied with their current relationships. What then would be the cumulative effect of being exposed to young, attractive women on a daily basis? Would there be any real consequences to the men's dissatisfaction with their relationships? Secondary school teachers and college professors come in contact with more young women at the peak of their reproductive value than others do. The analysis of a large, representative data set from the United States indicates that, while men in general are less likely to be divorced than women, and secondary school teachers and college professors in general are less likely to be divorced than others, simultaneously being male and being a secondary school teacher or college professor statistically increases the likelihood of being divorced (p <.05). We contend that the contrast effect that Kenrick et al. find in their experiments is cumulative and has real consequences.
Human speech evidently conveys an adaptive advantage, given its apparently rapid dissemination through the ancient world and global use today. As such, speech must be capable of altering human biology in a positive way, possibly through those neuroendocrine mechanisms responsible for strengthening the social bonds between individuals. Indeed, speech between trusted individuals is capable of reducing levels of salivary cortisol, often considered a biomarker of stress, and increasing levels of urinary oxytocin, a hormone involved in the formation and maintenance of positive relationships. It is not clear, however, whether it is the uniquely human grammar, syntax, content and/or choice of words that causes these physiological changes, or whether the prosodic elements of speech, which are present in the vocal cues of many other species, are responsible. In order to tease apart these elements of human communication, we examined the hormonal responses of female children who instant messaged their mothers after undergoing a stressor. We discovered that unlike children interacting with their mothers in person or over the phone, girls who instant messaged did not release oxytocin; instead, these participants showed levels of salivary cortisol as high as control subjects who did not interact with their parents at all. We conclude that the comforting sound of a familiar voice is responsible for the hormonal differences observed and, hence, that similar differences may be seen in other species using vocal cues to communicate.
This was a follow-up study of earlier reported findings by the present investigators suggesting, albeit equivocally, that separation during early childhood inhibited later sexual acts of a potentially procreative nature between siblings but did not deter other sexual activity. The present study surveyed 170 subjects, mostly in the Toronto area, by telephone and mail. Respondents reporting potentially procreative, postchildhood sexual acts (attempted or completed genital intercourse) with siblings were compared with those reporting sexual relationships excluding procreative acts, and a third sample reporting no postchildhood sibling sexual behavior. Consonant with expectations from the earlier study, prolonged separation during early childhood was associated with procreative postchildhood sexual activity but not with other postchildhood sexual activity. Contrary to predictions, however, both sexual activity groups reported significantly more nudity and physical contact with siblings during childhood than subjects reporting no sexual activity. The findings are discussed in terms of a revised version of the Westermarck hypothesis, which is consistent with a domain-specific approach to evolved incest avoidance mechanisms.
Spices are aromatic plant materials that are used in cooking. Recently it was hypothesized that spice use yields a health benefit: cleansing food of parasites and pathogens before it is eaten, thereby reducing food poisoning and foodborne illnesses. In support, most spices have antimicrobial properties and use of spices in meat-based recipes is greatest in hot climates, where the diversity and growth rates of microorganisms are highest. A critical prediction of the antimicrobial hypothesis is that spices should be used less in preparing vegetables than meat dishes. This is because cells of dead plants are better protected physically and chemically against bacteria and fungi than cells of dead animals (whose immune system ceased functioning at death), so fewer spices would be necessary to make vegetables safe for consumption. We tested this corollary by compiling information on 2129 vegetable-only recipes from 107 traditional cookbooks of 36 countries. Analyses revealed that spice use increased with increasing ambient temperature, but less dramatically than in meat-based recipes. In all 36 countries, vegetable dishes called for fewer spices per recipe than meat dishes; 27 of these differences were significant. Of 41 individual spices, 38 were used less frequently in vegetable recipes; 30 of these differences were significant. Proportions of recipes that called for >1 spice and >1 extremely potent antimicrobial spice also were significantly lower for vegetable dishes. By every measure, vegetable-based recipes were significantly less spicy than meat-based recipes. Within-country analyses control for possible differences in spice plant availability and degrees of cultural independence. Results thus strongly support the antimicrobial hypothesis.