Conservatives and liberals have markedly different ideologies. Conservatives, in comparison to liberals, are risk averse and prefer social inequality, traditionally established and familiar in-group values, and familial allegiance. Liberals are risk prone, are open to new views and ways, value equality and out-group relations, and exhibit high independence and self-reliance. We hypothesize that this variation was functional and socially strategic in human evolutionary history. Conservatives, we propose, are familial and in-group specialists, while liberals are out-group specialists. Furthermore, we hypothesize that the different values are caused proximately by attachment style and associated childhood stresses. Accordingly, low avoidant and high secure attachment and associated low childhood stresses ontogenetically generate conservatives, whereas high avoidant and low secure attachment and associated high childhood stresses give rise to liberals. Results from our study of 123 young adults support the hypotheses. We focus on the psychometric scale of conservatism–liberalism but also examine participants' scores on two additional political scales: social dominance orientation and right-wing authoritarianism. We also analyze participants' scores on time preference scales and life expectancy to test whether political values are related to future-versus-present life history tradeoffs or participants' perceptions of the past. We found no support for conservatism–liberalism's relationship to a future-versus-present tradeoff. Conservatism–liberalism, however, is related to how one understands the past in ways that support the notion that the degree of childhood stress affects political values.

The cultural norms of traditional societies encourage behavior that is consistent with maximizing reproductive success but those of modern post-demographic transition societies do not. Newson et al (2005) proposed that this might be because interaction between kin is relatively less frequent in modern social networks. Assuming that people's evaluations of reproductive decisions are influenced by a desire to increase their inclusive fitness, they will be inclined to prefer their kin to make fitness-enhancing choices. Such a preference will encourage the emergence of pronatal cultural norms if social networks are dense with kin. Less pronatal norms will emerge if contact between kin makes up a small proportion of social interactions. This article reports evidence based on role-play studies that supports the assumption of the kin influence hypothesis that evaluations of reproductive decisions are influenced by a desire to increase inclusive fitness. It also presents a cultural evolutionary model demonstrating the long-term effect of declining kin interaction if people are more likely to encourage fitness-enhancing choices when interacting with their kin than with nonrelatives.

Evolutionary principles suggest that there will be differences in the nature of altruism directed toward kin vs. nonkin. The present study sought to explore these differences. Participants were 295 undergraduate students who each completed a questionnaire about help exchanged with siblings, cousins, acquaintances or friends. For siblings, cousins and acquaintances, greater relatedness was associated with higher levels of helping. Friends were an exception, however, receiving as much or more help than kin. Consistent with an evolutionary analysis, as the cost of helping increased, kin received a larger share of the help given, whereas nonkin received a smaller share. For low-cost help, people helped friends more than siblings; for medium-cost help, they helped siblings and friends equally; and for high-cost help, they expressed a greater willingness to help siblings than friends. As expected, the level of reciprocal exchange was higher among acquaintances than among friends; however, there was also an unexpectedly high level of reciprocal exchange among kin.

Evolutionary theory predicts that relatedness will affect family relationships. Previous studies on siblings have mainly focused on sibling differentiation, sibling rivalry, and incest avoidance, and very few have examined the impact of genetic relatedness on the sibling relationship. Using a large data set from the Netherlands (Netherlands Kinship Panel Study), I show that relatedness (full vs. half-sibling) independently influences social investments between siblings. Maternal half-siblings, who are raised together like full siblings (FS), were found to show significantly lower levels of investment than FS. This suggests that a psychological mechanism besides childhood proximity regulates investment in the sibling relationship. Yet, levels of investment were overall higher for maternal than paternal half-siblings, suggesting an important role for childhood co-residence. Results are discussed with reference to kin selection theory.

The Kin Selection Hypothesis for male androphilia posits that genes for male androphilia can be maintained in the population if the fitness costs of not reproducing directly are offset by enhancing inclusive fitness. In theory, androphilic males can increase their inclusive fitness by directing altruistic behavior toward kin, which, in turn, allows kin to increase their reproductive success. Previous research conducted in Western countries has failed to find any support for this hypothesis. The current study tests this basic prediction of the Kin Selection Hypothesis for male androphilia by comparing the altruistic tendencies of androphilic and gynephilic males in the Polynesian nation of Independent Samoa. In Independent Samoa, androphilic males are known locally as fa'afafine. Altruistic tendencies were assessed using a Kin Selection Questionnaire. Comparisons of the altruistic tendencies of fa'afafine and gynephilic men revealed that these two groups did not differ in terms of their overall generosity and allocation of financial resources toward kin, nor did they differ in terms of general neediness or financial resources obtained from kin. Fa'afafine did, however, report greater avuncular tendencies than gynephilic men. Although the greater avuncular tendencies of fa'afafine support the basic prediction of the Kin Selection Hypothesis for male androphilia, further research is needed before one can conclude that these elevated tendencies represent a specially designed adaptation for promoting the fitness of kin. We discuss a number of sociocultural factors that might promote the expression of avuncular tendencies by androphilic males in Independent Samoa. Our results underscore the importance of testing functional hypotheses in evolutionarily appropriate environments.

Almost by definition, “popular culture” reflects the effects of most people imitating those around them. At the same time, trends and fashions are constantly changing, with future outcomes potentially irrational and nearly impossible to predict. A simple null model, which captures these seemingly conflicting tendencies of conformity and change, involves the random copying of cultural variants between individuals, with occasional innovation. Here, we show that the random-copying model predicts a continual flux of initially obscure new ideas (analogous to mutations) becoming highly popular by chance alone, such that the turnover rate on a list of most popular variants depends on the list size and the amount of innovation but not on population size. We also present evidence for remarkably regular turnover on “pop charts”—including the most popular music, first names, and dog breeds in 20th-century United States—which fits this expectation. By predicting parametric effects on the turnover of popular fashion, the random-copying model provides an additional means of characterizing collective copying behavior in culture evolution.

In humans, paternal investment is highly variable and is modulated by paternity uncertainty. Facial phenotypic similarity between a father and a child is one possible paternity indicator. However, whether such paternal-biased traits are expressed in children is unclear, as previous empirical results are contradictory. Therefore, we quantified the facial resemblance between a child and each of his or her parents, from birth to 6 years old. Resemblance was assessed from pictures of the face by nonrelated judges. We found that, at all ages, children resemble both their parents more than would be expected by chance, although there is a differential resemblance toward one or the other parent depending on the age and sex of the child. For newborns, boys and girls resemble their mothers more, this differential resemblance persisting through time for girls. For boys, an inversion occurs and they resemble their fathers more between 2 and 3 years of age. The resemblance ascribed by the parents shows that, at birth, mothers ascribe a resemblance to the father, as previously found, although assessment by external judges revealed the opposite. These results suggest that facial appearance is a cue for kin recognition between a father and a child. Patterns of differential resemblance are discussed within the context of evolutionary theories on parental investment.

People signal status by producing, distributing, or consuming goods. Behavioral ecologists working with foragers stress signaling by production (e.g., supply of wildlife), whereas economists working in industrial economies stress signaling by individual consumption or expenditures. As foraging economies experience economic transformations, one expects greater reliance on individual consumption compared with production to signal status. We test two hypotheses: if people signal by individual consumption, they will allocate a higher share of their monetary expenditures to luxuries or to visible durable goods (Hypothesis 1) and the propensity to signal by individual consumption will be more salient among people closer to market towns (Hypothesis 2). To test the hypotheses, we draw on data from a native Amazonian society of foragers and slash-and-burn farmers in Bolivia (Tsimane') undergoing increasing exposure to the market economy. The sample included 161 women and 257 men 16+ years of age in 13 villages. The dependent variable was the share of total monetary expenditures allocated to different types of durable goods (e.g., clothing, luxuries, highly visible and less visible goods) during the previous year. Separate OLS regressions were used for women and men. We found support for Hypothesis 1. Higher levels of total monetary expenditures bore a positive association with the share of expenditures allocated to luxury goods and a negative association with expenditures allocated to less visible durable goods. Only among women did we find a positive association between total expenditures and the share of expenditures allocated to highly visible goods. We found no support for Hypothesis 2.

It has been proposed that selection has shaped the human mind to be predictably biased in domains where the costs of false-positive and false-negative errors have been asymmetrical throughout human evolutionary history. Using this logic, the current study predicts that men and women systematically overestimate the degree to which members of the opposite sex find their same-sex mating competition desirable. Ten photographs of opposite-sex targets were shown to a sample of men (n=123) and women (n=159), and they were asked questions pertaining to each target's desirability as a mate. The same photographs, this time with sex of target and participant being the same, were shown to a second group of men (n=105) and women (n=103), and they were asked to estimate the desirability of the depicted individuals to members of the opposite sex. Consistent with the mate competition overestimation bias hypothesis, men and women consistently overestimated the degree to which members of the opposite sex find members of their same sex attractive and desirable as potential mates.

Human physical attractiveness appears to be an important signal of mate value that is utilized in mate choice We argue that performance-related physical fitness (PF) was an important facet of ancestral male mate value and, therefore, that a positive relationship exists between PF and physical attractiveness as well as mating success. We investigated these relationships in a sample of 80 young men. In line with our predictions, we found that (i) a composite measure of PF correlated substantially with body attractiveness (r=.43, after controlling for confounds) but not with facial attractiveness; (ii) PF was positively related to various measures of self-reported mating success (rS ≈ .22); (iii) the relationship between PF and self-reported mating success was partly mediated by body attractiveness. We conclude it is a key function of men's body attractiveness to signal their PF and that men's faces and bodies signal different facets of mate value.

Despite many empirical studies of children killed by parents, there has been little theoretical progress. An examination of 378 cases in a national register revealed that circumstances differed for genetic parents versus stepparents. Infants were at greatest risk of filicide, especially by genetic mothers. Genetic mothers who killed offspring, especially older children, disproportionately had a mental illness and received relatively short sentences, if convicted. Filicides by genetic fathers were disproportionately accompanied by marital discord, suicide, and uxoricide. Filicides by stepparents were disproportionately common and likely to involve ongoing abuse and death by beating. Moreover, if parents also had genetic offspring, their stepchildren were at increased risk of ongoing abuse and neglect prior to death. Poor child health appeared to increase the risk of filicide by genetic mothers, especially as remaining opportunities for childbearing diminished. Although each finding might be consistent with existing lay accounts of filicide (depression, socioeconomic stress, etc.), together, they yielded a pattern uniquely consistent with selectionist accounts based mainly on parental investment theory.

Punishment has been proposed as being central to two distinctively human phenomena: cooperation in groups and morality. Here we investigate moralistic punishment, a behavior designed to inflict costs on another individual in response to a perceived moral violation. There is currently no consensus on which evolutionary model best accounts for this phenomenon in humans. Models that turn on individuals' cultivating reputations as moralistic punishers clearly predict that psychological systems should be designed to increase punishment in response to information that one's decisions to punish will be known by others. We report two experiments in which we induce participants to commit moral violations and then present third parties with the opportunity to pay to punish wrongdoers. Varying conditions of anonymity, we find that the presence of an audience—even if only the experimenter—causes an increase in moralistic punishment.

Work on adult humans has revealed a limit on the size of freely forming conversational groups that has been attributed to the mechanical constraints on human speech production. However, it is also possible that cognitive constraints limit the number of individuals with which it is possible to interact. Data from South African and British children were used to test this hypothesis. A significant developmental trend in both clique and group sizes was found, which mapped onto the developmental trend for metacognitive skills. Notably, children with high levels of metarepresentational skill for their age were not found in significantly larger groups. This may be because group size is set by the average level of metarepresentation within a particular population and/or that extensive experience of within-group peer interaction is needed to function within larger groups.

Recent research employing a disease-threat model of the psychology of intergroup attitudes has provided preliminary support for a link between subjectively disease-salient emotional states and ethnocentric attitudes. Because the first trimester of pregnancy is a period of particular vulnerability to infection, pregnant women offer an opportunity to further test this association. We explored the expression of intergroup attitudes in a sample of pregnant women from the United States. Consistent with the predictions of the disease-threat model, results from our cross-sectional study indicate that favoritism toward the ingroup peaks during the first trimester of pregnancy and decreases during the second and third trimesters. We discuss this finding in light of the possible contributions of cultural and biological factors affecting ethnocentrism.

We investigated differences between firstborn and secondborn siblings on major dimensions of personality, in the context of the proposal of Sulloway [Sulloway, F. J. (1996). Born to rebel: Birth order, family dynamics and creative lies. New York: Pantheon] that personality is influenced by the specialized niches siblings adopt in the quest for access to parental resources. Using a within-family methodology, we tested two predictions from Sulloway's model: that firstborns are more achieving and conscientious than secondborns and that secondborns are more rebellious and open to new experiences than firstborns. To test an alternative prenatal hypomasculinization theory proposed by Beer and Horn [Beer, J. M., & Horn, J. M. (2000). The influence of rearing order on personality development within two adoption cohorts. Journal of Personality, 68, 769–819], we also examined the size of birth-order effects in sister–sister versus brother–brother pairs. The hypothesized effects of birth order on personality were found in both Study 1 (n=161 sibling pairs) and Study 2 (n=174 sibling pairs) and provided support for Sulloway's family-niche model. No support was found for Beer and Horn's hypomasculinization model.

We present an explanation about the origins of monetary income inequality when an economically self-sufficient society opens to a market economy. The chain of associations runs from patience, to the accumulation of different forms of human capital, to self-selection into different occupations, and to the division of labor, which contributes to monetary income inequality. In a self-sufficient society, patience is exogenously determined and people rely on folk knowledge as the only form of human capital. With the establishment of schools, patient and impatient people sort themselves out by the type of human capital they begin to accumulate. Impatient people do not acquire folk knowledge because return to schooling takes many years to bear fruit. Schooling opens opportunities in occupations outside the village, whereas folk knowledge enhances employment opportunities that draw on farming or foraging. Self-selection into different occupations with different earnings potential spawns monetary income inequality. To test the explanation, we draw on data from a foraging–farming society in the Bolivian Amazon, the Tsimane'. We collected data during four consecutive quarters in 1999–2000 and a follow-up interview (2004). Data came from 151 adults (age, 16 years or more) from all households (n=48) in two villages with different levels of market exposure. During 1999–2000, impatience was associated with (a) greater folk knowledge and fewer years of schooling, (b) lower likelihood of working in wage labor, and (c) greater likelihood of working in rural subsistence occupations. People who had been patient in 1999–2000 had greater wage earnings and more modern physical assets in 2004.

Substantial evidence suggests that physical attractiveness plays an important role in shaping overt mating preferences, judgments, and choices. Relatively few studies, however, have investigated the hypothesis that perceivers are attuned to signs of attractiveness at early, lower-order stages of social perception. In the current research, a visual cueing task was used to assess biases in attentional disengagement—the extent to which people's attention becomes “stuck” on particular social stimuli. Findings indicate that, consistent with some evolutionary theories, perceivers of both sexes exhibited attentional attunement to attractive women, but not attractive men. Additional findings suggest that this bias was pronounced in sexually unrestricted men and in women who felt insecure about a current romantic relationship. This research provides novel evidence for adaptive, lower-order perceptual attunements in the domain of human mating.

Human groups are unusual among primates in that our leaders are often democratically selected. Faces affect hiring decisions and could influence voting behavior. Here, we show that facial appearance has important effects on choice of leader. We show that differences in facial shape alone between candidates can predict who wins or loses in an election (Study 1) and that changing context from war time to peace time can affect which face receives the most votes (Study 2). Our studies highlight the role of face shape in voting behavior and the role of personal attributions in face perception. We also show that there may be no general characteristics of faces that can win votes, demonstrating that face traits and information about the environment interact in choice of leader.

The theory of cultural transmission distinguishes between biased and unbiased social learning. Biases simply mean that social learning is not completely random. The distinction is critical because biases produce effects at the aggregate level that then feed back to influence individual behavior. This study presents an economic experiment designed specifically to see if players use social information in a biased way. The experiment was conducted among a group of subsistence pastoralists in southern Bolivia. Treatments were designed to test for two widely discussed forms of biased social learning: a tendency to imitate success and a tendency to follow the majority. The analysis, based primarily on fitting specific evolutionary models to the data using maximum likelihood, found neither a clear tendency to imitate success nor conformity. Players instead seemed to rely largely on private feedback about their own personal histories of choices and payoffs. Nonetheless, improved performance in one treatment provides evidence for some important but currently unspecified social effect. Given existing experimental work on cultural transmission from other societies, the current study suggests that social learning is potentially conditional and culturally specific.

Using a sample of men living in Albuquerque, NM, we examined the relationship between paternity confidence and men's investment in children. In humans, men may reduce their investment in a child in two ways: indirectly, by ending their relationship with the child's mother and ceasing to cohabit with the child (e.g., divorce), and directly, by allocating less time and fewer resources to the child. In this article, we tested two hypotheses regarding the effect of paternity confidence on investment in children: (1) men will be more likely to divorce women if they suspect or are sure that they are not the father of their wife's child, and (2) controlling for divorce, men will reduce direct investments in low paternity confidence children relative to high paternity confidence children. The first hypothesis was supported by the data. The second hypothesis was supported for two out of three measures of paternal investment we examined; low paternity confidence reduces the time men spend with a child in a group with other children or adults, and it reduces extensive involvement with the child's educational progress; there was no effect of paternity confidence on the amount of time men spend with children in one-on-one interactions. We also examined the effects of unstated paternity confidence (e.g., when men decline to answer the question) on divorce and paternal investment. Overall, the results suggested that paternity confidence plays an important role in shaping men's relationships with women and with their putative genetic children.

Left- and right-handers in humans coexist at least since the Paleolithic, and this variation in hand preference has a heritable basis. Because there is extensive evidence of an association between left-handedness and several fitness costs, the persistence of the polymorphism requires an explanation. It is not known whether the frequency of left-handedness in Western societies is stable or not. If the polymorphism is at equilibrium and maintained by frequency dependence, it implies that the fitness of left-handers equals that of right-handers. On the contrary, if left- and right-handers have a different fitness, the polymorphism will evolve. Using two large cohorts of French adults (men and women), we investigated the relations between handedness and several estimators of the reproductive value: marital status, number of sexual partners (of the opposite sex), number of children, and number of grandchildren. Left-handers seem to have disadvantages for some life-history traits, such as marital status (for women) and number of children. For other traits, we observed sex-dependent interactions with socioeconomic status: for high-income categories, left-handed women report less sex partners and left-handed men have more grandchildren. These kinds of interactions are to be expected under the hypothesis that the polymorphism of handedness is stable.

It has been suggested that certain physical cues can be used to predict mate quality and that sensitivity to these cues would therefore be adaptive. From this, it follows that in environments where the optimal values for these features differ, the attractiveness preferences should also be different. In this study, we show that there are striking differences in attractiveness preferences for female bodies between United Kingdom (UK) Caucasian and South African Zulu observers. These differences can be explained by different local optima for survival and reproduction in the two environments. In the UK, a high body mass is correlated with low health and low fertility, and the converse is true in rural South Africa. We also report significant changes in the attractiveness preferences of Zulus who have moved to the UK. This suggests that these preferences are malleable and can change with exposure to different environments and conditions. Additionally, we show that Britons of African origin, who were born and who grew up in the UK, have exactly the same preferences as our UK Caucasian observers. These results suggest that humans have mechanisms for acquiring norms of attractiveness that are highly plastic, which allow them to track different ecological conditions through learning.

Evolutionary psychologists have proposed that preferences for facial characteristics, such as symmetry, averageness, and sexual dimorphism, may reflect adaptations for mate choice because they signal aspects of mate quality. Here, we show that facial skin color distribution significantly influences the perception of age and attractiveness of female faces, independent of facial form and skin surface topography. A set of three-dimensional shape-standardized stimulus faces—varying only in terms of skin color distribution due to variation in biological age and cumulative photodamage—was rated by a panel of naive judges for a variety of perceptual endpoints relating to age, health, and beauty. Shape- and topography-standardized stimulus faces with the homogeneous skin color distribution of young people were perceived as younger and received significantly higher ratings for attractiveness and health than analogous stimuli with the relatively inhomogeneous skin color distribution of more elderly people. Thus, skin color distribution, independent of facial form and skin surface topography, seems to have a major influence on the perception of female facial age and judgments of attractiveness and health as they may signal aspects of underlying physiological condition of an individual relevant for mate choice. We suggest that studies on human physical attractiveness and its perception need to consider the influence of visible skin condition driven by color distribution and differentiate between such effects and beauty-related traits due to facial shape and skin topography.

The ability to detect cheaters has been proposed as an adaptive design feature of psychological adaptations for cooperation. This proposal has been tested with studies on the Wason selection task, which purportedly demonstrate that humans possess a specific competence for detecting cheaters in cooperative interactions. An alternative set of theories suggests that people are not looking for cheaters per se, but are looking for losses in an effort to maximize their utility. In previous investigations of cheater detection, cheating has been confounded with someone suffering a loss. We sought to test rival accounts of cheater detection by devising versions of the selection task in which cheating is unconfounded with losses. The results suggest that people are competent at detecting cheaters even when no losses are involved, lending support to the view that cheater detection is a specific design feature of psychological adaptations for cooperation.

A set of computerized tasks was used to investigate sex differences in the speed and accuracy of emotion recognition in 62 men and women of reproductive age. Evolutionary theories have posited that female superiority in the perception of emotion might arise from women's near-universal responsibility for child-rearing. Two variants of the child-rearing hypothesis predict either across-the-board female superiority in the discrimination of emotional expressions (“attachment promotion” hypothesis) or a female superiority that is restricted to expressions of negative emotion (“fitness threat” hypothesis). Therefore, we sought to evaluate whether the expression of the sex difference is influenced by the valence of the emotional signal (Positive or Negative). The results showed that women were faster than men at recognizing both positive and negative emotions from facial cues, supporting the attachment promotion hypothesis. Support for the fitness threat hypothesis also was found, in that the sex difference was accentuated for negative emotions. There was no evidence that the female superiority was learned through previous childcare experience or that it was derived from a sex difference in simple perceptual speed. The results suggest that evolved mechanisms, not domain-general learning, underlie the sex difference in recognition of facial emotions.

Fluctuating asymmetry (FA) is believed to reflect developmental stability and thus may serve as a marker of the biologic quality of individuals. To test the hypothesis that degree of FA is related to a woman's potential fertility, we measured finger length together with levels of estradiol in saliva samples collected daily for an entire menstrual cycle in 171 Polish urban and rural women. We show that women who are more symmetrical, as assessed by the degree of inequality in the fourth-digit length of their right and left hands, have 13% higher average levels of estradiol over the menstrual cycle than less symmetrical women (19.4 and 17.1 pmol/l, respectively; p=.0001). Among urban women, mid-cycle levels of estradiol were 28% higher in the symmetrical group than in the asymmetrical group. Because higher hormone levels in women may lead to a substantial rise in the probability of conception, low FA may therefore be associated with increased fertility.

Although previous studies of individual differences in preferences for masculinity in male faces have typically emphasized the importance of factors such as changes in levels of sex hormones during the menstrual cycle, other research has demonstrated that recent visual experience with faces also influences preferences for sexual dimorphism in faces. Adaptation to either masculine or feminine faces increases preferences for novel faces that are similar to those that were recently seen. Here, we replicate this effect and demonstrate that adaptation to masculine or feminine faces also influences the extent to which masculine faces are perceived as trustworthy. These adaptation effects may reflect a proximate mechanism that contributes to the development of face preferences within individuals, underpins phenomena such as imprinting-like effects and condition-dependent face preferences, and shapes personality attributions to faces that play an important role in romantic partner and associate choices. Furthermore, our findings also support the proposal that visual exposure alone cannot explain the context specificity of attitudes to self-resemblance in faces.

Subjects performed four deontic Wason selection tasks in three experiments to investigate possible commonalities in people's performance in making logical inferences in social contexts. These tasks tested sensitivity to another party being an altruist, cheater or willing to share a resource and following a precaution rule in the context of danger. The results indicated no significant association between performance on the altruist detection and the cheater detection tasks. This result suggests that whatever the nature of the altruist-detection algorithm, it functions independently of the cheater-detection algorithm. The results also indicated significant associations between the cheater-detection task and the resource-sharing task. Possible mechanisms and functions of social intelligence suggested by these results are discussed.

Disgust is a powerful behavioral adaptation, which confers the advantage of reducing the risk of pathogen infection. However, there are situations in which disgust at core elicitors (e.g., feces) must be modulated in the service of other goals (e.g., caring for a close kin). In Study 1, mothers of infants completed a self-report questionnaire about their reactions to changing their baby's feces-soiled diaper compared with the diaper of someone else's baby. In Study 2, mothers of infants were presented with a series of trials in which they smelled concealed samples of their own baby's feces-soiled diaper and those of someone else's baby. In addition, labels were used to identify the source of the sample (correctly labeled, mislabeled, or no label). Both studies provide evidence suggesting that mothers regard their own baby's fecal smell as less disgusting than that from someone else's baby. Furthermore, labeling had relatively little influence on this effect, and the effect persisted when social desirability was controlled.

There is now compelling evidence that psychosocial stress is a cause of reproductive suppression in humans. However, women continue to conceive in the harshest conditions of war, poverty, or famine, suggesting that suppression can be bypassed. The reproductive suppression model (RSM) proposes that natural selection should favor factors that reliably predict conditions for reproduction. In this study, we examine two such factors, age and social position, in women undergoing fertility treatment. We hypothesized that stress-related reproductive suppression would be more likely in younger compared to older women and in women in lower compared to higher social positions. The final sample consisted of 818 women undergoing fertility treatment. Psychosocial stress and sociodemographic data were collected prior to the start of treatment (Time 1), whereas fertility, as indexed by pregnancy or live birth, was assessed 12 months later (Time 2). The results showed that younger women were four times more likely to suppress than older women, and that unskilled and manual workers were more likely to suppress than those in middle social positions (e.g., white collar workers). However, significant associations between stress and fertility were also observed for women in higher social positions (e.g., professionals and executives). The findings provide support for the RSM.

Many studies show that people act cooperatively and are willing to punish free riders (i.e., people who are less cooperative than others). However, nonpunishers benefit when free riders are punished, making punishment a group-beneficial act. Presented here are four studies investigating whether punishers gain social benefits from punishing. Undergraduate participants played public goods games (PGGs) (cooperative group games involving money) in which there were free riders, and in which they were given the opportunity to impose monetary penalties on free riders. Participants rated punishers as being more trustworthy, group focused, and worthy of respect than nonpunishers. In dyadic trust games following PGGs, punishers did not receive monetary benefits from punishing free riders in a single-round PGG, but did benefit monetarily from punishing free riders in iterated PGGs. Punishment that was not directed at free riders brought no monetary benefits, suggesting that people distinguish between justified and unjustified punishment and only respond to punishment with enhanced trust when the punishment is justified.

From the history of mathematics, it is clear that some numerical concepts are far more pervasive than others. In a densely multimodular mind, evolved cognitive abilities lie at the basis of human culture and cognition. One possible way to explain the differential spread and survival of cultural concepts based on this assumption is the epidemiology of culture. This approach explains the relative success of cultural concepts as a function of their fit with intuitions provided by conceptual modules. A wealth of recent evidence from animal, infant, and neuroimaging studies suggests that human numerical competence is rooted in an evolved number module. In this study, I adopted an epidemiological perspective to examine the cultural transmission of numerical concepts in the history of mathematics. Drawing on historical and anthropological data on number concepts, I will demonstrate that positive integers, zero, and negative numbers have divergent cultural evolutionary histories owing to a distinct relationship with the number module. These case studies provide evidence for the claim that science can be explained in terms of evolved cognitive abilities that are universal in Homo sapiens.

Several studies have suggested that women may prefer to engage in extra-pair copulations with males who appear dominant and to do so near ovulation. While there is some evidence that males are more jealous of dominant rivals and more proprietary when their partners are near ovulation, there is none that suggests the existence of counterstrategic perceptual shifts that mirror those seen in women. We provide such evidence here. Composites of male faces that were either high or low in rated dominance were presented to male participants who provided ratings of dominance. A three-way interaction between stimulus-face dominance, partner conception risk phase, and partner oral contraceptive use was found; men whose partners did not use an oral contraceptive and were in the high conception risk phase of their cycle displayed increased dominance ratings of high-dominance male faces. We conclude that males have evolved counterstrategies to deal with female infidelity that include an overattribution of dominance to those rivals most likely to present a threat at times when that threat is greatest. This overattribution is likely to lead to increases in jealousy and mate-retention behaviors.

The developmental and anatomical causes of human voice sexual dimorphisms are known, but the evolutionary causes are not. Some evidence suggests a role of intersexual selection via female mate choice, but other evidence implicates male dominance competition. In this study, we examine the relationships among voice pitch, dominance, and male mating success. Males were audio recorded while participating in an unscripted dating-game scenario. Recordings were subsequently manipulated in voice pitch using computer software and then rated by groups of males for dominance. Results indicate that (1) a masculine, low-pitch voice increases ratings of men's physical and social dominance, augmenting the former more than the latter; and (2) men who believe they are physically dominant to their competitor lower their voice pitch when addressing him, whereas men who believe they are less dominant raise it. We also found a nonsignificant trend for men who speak at a lower pitch to report more sexual partners in the past year. These results are consistent with the hypothesis that male intrasexual competition was a salient selection pressure on the voices of ancestral males and contributed to human voice sexual dimorphism.

Extending a model relating xenophobia to disease avoidance [Faulkner, J., Schaller, M., Park, J. H., & Duncan, L. A. (2004). Evolved disease-avoidance mechanisms and contemporary xenophobic attitudes. Group Processes & Intergroup Relations, 7(4), 333–353.], we argue that both inter- and intragroup attitudes can be understood in terms of the costs and benefits of interacting with the in-group versus out-groups. In ancestral environments, interaction with members of the in-group will generally have posed less risk of disease transmission than interaction with members of an out-group, as individuals will have possessed antibodies to many of the pathogens present in the former, in contrast to those prevalent among the latter. Moreover, because coalitions are more likely among in-group members, the in-group would have been a potential source of aid in the event of debilitating illness. We conducted two online studies exploring the relationship between disease threat and intergroup attitudes. Study 1 found that ethnocentric attitudes increase as a function of perceived disease vulnerability. Study 2 found that in-group attraction increases as a function of disgust sensitivity, both when measured as an individual difference variable and when experimentally primed. We discuss these results with attention to the relationships among disease salience, out-group negativity, and in-group attraction.

It has been suggested that human scent works as a signal in mate selection, but the empirical evidence is scarce. Here, we examined whether women's olfactory preferences for a man's scent could be correlated with his testosterone, estradiol, or cortisol concentrations, and whether these preferences change along with the menstrual cycle. In line with previous studies, women in their most fertile period gave the highest attractiveness ratings to all men. However, the intensity ratings by women at different menstrual phases did not significantly differ statistically. Interestingly, we found that cortisol concentration in saliva correlated positively with the attractiveness but not with the intensity ratings of male T-shirt odor by all women's groups. However, neither testosterone nor estradiol was significantly associated with the ratings of attractiveness or intensity. Thus, our study suggests that there could be a novel mechanism for odor-based selection in humans.

Because ancestral women faced trade-offs in choosing mates, they may have evolved to pursue a dual-mating strategy in which they secured investment through one partner and obtained good genes through others. The dual-mating theory predicts that women will display greater interest in extra-pair sex near ovulation, especially if they are mated to a primary male partner who is low in sexual attractiveness. Forty-three normally ovulating women rated their partner's sexual attractiveness and separately reported their own desires and their partner's mate retention behaviors at high and low fertility (confirmed using luteinizing hormone tests). In the high-fertility session relative to the low, women who assessed their partners as being lower in sexual attractiveness reported greater extra-pair desires and more expressed love and attention from their male partners. Women's desire for their own partners did not differ significantly between high and low-fertility sessions.

Many studies have found differences in the types of aggression used by males and females, at least in children and adolescents. Boys tend to use direct physical or verbal aggression, whereas girls tend to use more indirect forms of aggression that prominently feature gossip. Evolutionary theories of sex differences in indirect aggression propose selection pressures that would have acted on older teenagers and adults. Evidence for sex differences in indirect aggression in adults, however, is equivocal. Virtually all studies of adults have found a sex difference in physical aggression, but most have failed to find sex differences in the use of the more indirect forms of aggression. Almost all of these studies have measured indirect aggression using self-reports of aggressive behavior. We investigated sex differences in the psychology of indirect aggression by exposing young adult women and men to the same aggression-evoking stimulus. As evolutionary models predict, we found that women had a stronger desire than men to aggress indirectly, even after controlling for perceptions of social norms and approval. Future work on both evolutionary and social norm models of indirect aggression is warranted.

We investigated whether the repeatedly demonstrated increase in risk of child abuse and infanticide associated with living with a step parent generalized to cases of unintentional childhood fatal injury, the most common cause of death in children across the developed world. Reports were drawn from the Australian National Coroners' Information System (NCIS) on all cases of intentionally (n=32) and unintentionally (n=319) produced fatal injury in children aged under 5 years between 2000 and 2003. Even when using the most conservative possible analytic approach, in which all cases in which family type was unclear were classified as being from an ‘intact biological family’, step children under 5 years of age were found to be at significantly increased risk of unintentional fatal injury of any type, and of drowning in particular. Children from single-parented families were generally not found to be at significantly increased risk of intentional or unintentional fatal injury, while children who lived with neither of their biological parents were at greatest risk overall for fatal injury of any type.

With respect to autosomal genes, a grandparent is equally related to male and female grandchildren. Because males are heterozygous for sex chromosomes, however, grandparents are asymmetrically related to male and female grandchildren via the sex chromosomes. For example, the Y chromosome from the paternal grandfather passes directly down to grandsons. This asymmetry leads to a prediction that genes on the sex chromosomes could drive differential grandparental care. Alternatively, the paternity uncertainty hypothesis for differential grandparent care brings about a different set of predictions. A grandfather, for example, has two degrees of uncertainty to his son's children but only one to his daughter's children. Thus, under high extra-pair paternity rates, paternity uncertainty predicts that a grandfather will favor his daughter's children over his son's children. A paternity uncertainty vs. a genetic relatedness hypothesis was tested using data from questionnaires asking adult grandchildren to rate the amount and quality of care of their various grandparents. We found no support for preferential care based on expected sex chromosome similarities. Instead, our data were in general accord with the predictions of the paternity uncertainty hypothesis of grandparental care. A model is presented to predict the rates of extra-pair paternity required in a population to have the effects of paternity uncertainty outweigh sex chromosome effects.

A number of sex differences in mate preferences have been reported across cultures. Women prefer partners who are older than them whereas men prefer partners who are younger than them. Women have stronger preferences for resource-acquisition characteristics whereas men have stronger preferences for physical attractiveness. Recently, studies have reported shifts in female preferences with increasing female empowerment and associated female attitudes. Other studies, however, report opposite effects of female wealth and income. In this study, we investigated the effects of female control of the resources necessary to raise offspring successfully on mate preferences. We developed measures of resource control at the level of the individual and investigated relationships between these and mate preferences using an internet survey. Resource control was associated with preferences for physical attractiveness over good financial prospects and greater maximum partner age tolerated. Resource control, however, was also associated with tolerance of younger partners. The results implicate the role of constraints on female access to and control of resources in sex-differentiated mate preferences and highlight differences between resource control and wealth.

Recent studies suggest that both the form and the content of persecutory delusional beliefs may reflect pathological exaggerations of evolved psychological mechanisms for dealing with social threat recognition. Here, we tested the hypothesis first put forward by Walston et al. [Evolution and Human Behavior 19 (1998) 257–260] that sex differences in the content of persecutory delusions reflect divergent ancestral hostile social threats, in a prospective study of two samples of German and Russian patients with delusions of persecution. Deluded men and women differed significantly in their attributions of perceived threats. The majority of men felt persecuted by groups of hostile strange males, whereas women projected their paranoid fears onto familiar people of their social environment, largely irrespective of psychiatric diagnosis or cultural background. In contrast to our predictions, however, both men and women with persecutory delusions were most frightened of physical violence. Fear of sexual coercion was only present in a small number of patients. In sum, this study is largely supportive of the hypothesis that the content of persecutory delusions reflects ancestral hostile threats.

It has been claimed that blending processes such as trade and exchange have always been more important in the evolution of cultural similarities and differences among human populations than the branching process of population fissioning. In this paper, we report the results of a novel comparative study designed to shed light on this claim. We fitted the bifurcating tree model that biologists use to represent the relationships of species to 21 biological data sets that have been used to reconstruct the relationships of species and/or higher level taxa and to 21 cultural data sets. We then compared the average fit between the biological data sets and the model with the average fit between the cultural data sets and the model. Given that the biological data sets can be confidently assumed to have been structured by speciation, which is a branching process, our assumption was that, if cultural evolution is dominated by blending processes, the fit between the bifurcating tree model and the cultural data sets should be significantly worse than the fit between the bifurcating tree model and the biological data sets. Conversely, if cultural evolution is dominated by branching processes, the fit between the bifurcating tree model and the cultural data sets should be no worse than the fit between the bifurcating tree model and the biological data sets. We found that the average fit between the cultural data sets and the bifurcating tree model was not significantly different from the fit between the biological data sets and the bifurcating tree model. This indicates that the cultural data sets are not less tree-like than are the biological data sets. As such, our analysis does not support the suggestion that blending processes have always been more important than branching processes in cultural evolution. We conclude from this that, rather than deciding how cultural evolution has proceeded a priori, researchers need to ascertain which model or combination of models is relevant in a particular case and why.

A sexual asymmetry has been recently found on semantic memory tasks: after brain damage, a disproportionate deficit for information about biological categories has been reported more frequently for male patients. A review of cases shows that the fine-grained pattern is more complicated in that there is a strong interaction with sex: Disproportionate plant-knowledge deficits are restricted to males, whereas disproportionate animal-knowledge deficits are rare and show no sex bias. These clinical data are consistent with semantic-knowledge data from normal subjects indicating a task-invariant female advantage with plant categories. In this study, we seek an explanation for this sex-by-semantic category interaction and discuss the possible roles of a greater female experience with plant items, both ontogenetically and over evolutionary time.

One prediction derived from parental investment theory is that women will be more attentive than men are to cues of a prospective mate's dispositions to invest in children. Research with 1793 Internet participants, representing a diverse population sample, found that (a) women tend to be generally more critical than men are in their evaluations of potential mates, but not potential friends or neighbors, and (b) cues of a positive disposition towards parental investment (DPI) have a positive influence on female evaluations of the attractiveness of males. This latter effect, however, is less domain-specific than previous research [La Cerra, M. M. (1995). Evolved mate preferences in women: Psychological adaptations for assessing a man's willingness to invest in offspring (doctoral dissertation, University of California, Santa Barbara). Dissertation Abstracts International: Section B: the Sciences & Engineering Mar, 55(9-B), 4149] indicated; it is not limited to mating contexts and to cues focusing on parental investment. In fact, much of the sex difference appears to be due to indifference by males towards cues of female DPI. A second study further clarified that the previous findings were not due solely to the Internet methodology or the immediate accessibility of images being evaluated.

We investigated aspects of self-reported health history–the number and duration of respiratory and stomach or intestinal infections and the number of uses of antibiotics over the last 3 years–in relation to measured facial masculinity, developmental instability [facial asymmetry and body fluctuating asymmetry (FA)] and facial attractiveness in a sample of 203 men and 203 women. As predicted from the hypothesis that the degree of facial masculinity is an honest signal of individual quality, men's facial masculinity correlated negatively and women's positively with respiratory disease number and duration. Stomach illness, however, was not associated significantly with facial masculinity and antibiotic use correlated significantly (negatively) only with men's facial masculinity. For both facial asymmetry and body FA, significant, positive associations were seen with the number of respiratory infections. In addition, facial asymmetry was associated positively with the number of days infected and marginally, in the same direction, with antibiotic use. Facial attractiveness showed no significant relationships with any of our health-history measures. This study provides some evidence that facial masculinity in both sexes may signal disease resistance and that developmental stability covaries positively with disease resistance. The validity of our health measures is discussed.

Both men and women prefer someone with a “good sense of humor” as a relationship partner. However, two recent studies have shown that men are not attracted to funny women, suggesting the sexes use the phrase good sense of humor differently. To investigate this question, we measured the importance participants placed on a partner's production of humor vs. receptivity to their own humor. Men emphasized the importance of their partners' receptivity to their own humor, whereas women valued humor production and receptivity equally. In a second task, participants chose whether they preferred a person who only produced humor or a person who only appreciated their own humor for several types of relationships. Women preferred those who produced humor for all types of relationships, whereas men preferred those who were receptive to their own humor, particularly for sexual relationships. Our results suggest that sexual selection may have operated on men's and women's preferences during humorous interaction in dramatically different ways.

This paper reexamines the relationship between status and reproductive success (at the ultimate and proximate levels) using data on sex frequency and number of biological children from representative samples of the U.S. population. An ordered probit analysis of data from the 1989–2000 General Social Survey (GSS) shows that high-income men report greater frequency of sex than all others do. An OLS regression of data from the 1994 GSS shows that high-income men have more biological children than do low-income men and high-income women. Furthermore, more educated men have more biological children than do more educated women. Results also show that intelligence decreases the number of offspring and frequency of sex for both men and women.

Human hair and eye color is unusually diverse in northern and eastern Europe. The many alleles involved (at least seven for hair color) and their independent origin over a short span of evolutionary time indicate some kind of selection. Sexual selection is particularly indicated because it is known to favor color traits and color polymorphisms. In addition, hair and eye color is most diverse in what used to be, when first peopled by hunter-gatherers, a unique ecozone of low-latitude continental tundra. This type of environment skews the operational sex ratio (OSR) of hunter-gatherers toward a male shortage in two ways: (1) men have to hunt highly mobile and spatially concentrated herbivores over longer distances, with no alternate food sources in case of failure, the result being more deaths among young men; (2) women have fewer opportunities for food gathering and thus require more male provisioning, the result being less polygyny. These two factors combine to leave more women than men unmated at any one time. Such an OSR imbalance would have increased the pressures of sexual selection on early European women, one possible outcome being an unusual complex of color traits: hair- and eye-color diversity and, possibly, extreme skin depigmentation.

Sex differences in competitiveness are well established, but it is unknown if they originate from sociocultural conditions or evolved predispositions. Testing these hypotheses requires a quantifiable sex difference in competitiveness and the application of a powerful sociocultural manipulation to eliminate it. Study 1 reviews previous work showing that more male distance runners are motivated by competition and maintain large training volumes, suggesting that more males should run fast relative to sex-specific world-class standards. I then use two independent statistical approaches to demonstrate that, in matched populations of male and female U.S. runners, two to four times as many males as females ran relatively fast in 2003. Study 2 investigates whether the growth in opportunities and incentives for female athletes in the past 30 years is eliminating this sex difference. I first show that there was a marked increase in the number of fast female runners in the 1970s and early 1980s, a period during which female participation increased dramatically. However, I found no indication of an absolute or relative increase in the number of fast female distance runners since the mid-1980s. These findings therefore support the hypothesis that sex differences in competitiveness partly reflect evolved predispositions.