Throughout sub-Saharan Africa, there is resistance to changing sexual behavior despite survey data indicating high levels of knowledge about HIV transmission patterns and high-risk behavior. Previous explanations for this paradox emphasize indigenous cultural models. An alternative explanation is that, due to a strong natural selection for sexual gratification, individuals evoke the evolved trait of self-deception to continue practicing high-risk sexual behavior. This alternative is tested using survey data from an Ariaal community in Marsabit District, northern Kenya. Results indicate that respondents make highly accurate self-assessments of HIV risk, negating the concept of self-deception in this study. These results are discussed within the larger context of the applicability of evolutionary theory to the AIDS pandemic.

Humorous interaction is a ubiquitous aspect of human social behavior, yet the function of humor has rarely been studied from a Darwinian perspective. One exception is Miller's theory that one's ability to produce high-quality humor functioned as a fitness indicator, and hence, humor production and appreciation have evolved as a result of sexual selection. In this study, we examined whether there are sex differences in attraction to humorous individuals, and whether using humor influences perceptions of humorists' personality traits. We experimentally manipulated how humorous two-stimulus persons were perceived to be by presenting them with autobiographical statements that were either funny or not. Participants chose which person was a more desirable partner for a romantic relationship, and which individual was more likely to have several personality traits. Only women evaluating men chose humorous people as preferred relationship partners. For both sexes, humorous individuals were seen as less intelligent and trustworthy than their nonhumorous counterparts, but as more socially adept. These results are discussed in light of sexual selection theory.

Recent models of altruism point out the success of a strategy called 'Raise-The-Stakes' (RTS) in situations allowing variability in cooperation. In theory, RTS is difficult to exploit because it begins with a small investment in an iterated Prisoner's Dilemma Game (PDG). When its cooperation is reciprocated, RTS increases its generosity, thereby taking advantage of cooperative opportunities. Previous research has shown that human participants indeed adopt RTS but start out moderately cooperative rather than with a minimal investment. This raises the question how robust RTS is against exploitation, especially in a noisy situation. We investigate whether human participants vary their cooperation in interaction with reciprocators and cheaters in an iterated nondiscrete version of a PDG. When confronted with a strategy that matches the investment of the participant on the previous round, we find that participants are likely to increase cooperation. However, cooperation gradually breaks down in interaction with a strategy that undercuts the level of cooperation of the participants, indicating the robustness of RTS. In line with RTS modeling studies, but in contrast with the cheater detection literature, we find that human participants are less willing to increase cooperation when the perceived likelihood of mistakes increases.

In humans and several other species, face and body symmetry have been found to enhance physical attractiveness. A proposed explanation is that symmetry is a phenotypic indicator of biological fitness. Throughout the world, symmetrical designs also are a common feature in face and body painting and the decorative arts. The implication is that symmetrical designs might provide an additional way to enhance physical attractiveness. To find out, we conducted three experiments, two with human faces and one with abstract or nonrepresentational designs. In Experiments 1 and 2, we showed undergraduate students photographs of pairs of faces and instructed them to choose the more attractive face in each pair. The photographs were of physically symmetrical and asymmetrical faces (as indexed by facial features) that had been decorated with either symmetrical or asymmetrical designs of the kind used in many preindustrial societies. As indexed by the number of times they were chosen, symmetrical faces were judged to be more attractive than asymmetrical faces; adding asymmetrical designs to symmetrical faces decreased their attractiveness; and adding symmetrical designs to asymmetrical faces increased their attractiveness. In Experiment 3, undergraduates made similar choices from pairs of abstract designs taken from several cultures and modified in shape, coloration, and orientation of design features. Symmetrical designs again were judged to be more attractive, with shape and coloration playing the more important roles. We interpret the results as suggesting that the same mechanisms underlying the judgment of physical attractiveness also underlie cultural practices of face painting and abstract art.

The second-to-fourth digit ratio (2D:4D) and mental rotation (MR) ability are sexually dimorphic traits that appear early in development and have been correlated with exposure to prenatal androgens (Grimshaw, Sitarenios, & Finegan, 1995; Lutchmaya, Baron-Cohen, Raggatt, Knickmeyer, & Manning, 2004). The current study examined how 2D:4D and MR differences among women of European descent (N=41) were related to their (a) preferences for male faces: attractive (ATM), short-term (STM), and long-term mate (LTM), and (b) psychological femininity and masculinity, as measured by the Bem Sex-Role Inventory (BSRI) (Bem, 1981). To examine potential changes in facial preferences over their menstrual cycle, participants' preferences were measured during two experimental sessions separated by 2 weeks. The results indicated that (a) femininity scores decreased with decreasing 2D:4D, (b) masculinity scores increased with faster MR, (c) women preferred a more masculine male face for an STM than for an LTM, and (d) preference changes over the menstrual cycle varied systematically with 2D:4D. When compared with women with high 2D:4D ratios, low 2D:4D women (e) preferred a more masculine LTM, (f) recalled less parental bonding, (g) had shorter intimate relationships, and (h) reported more menstrual irregularity. The results are interpreted as support for an interactive hormonal theory of physical attraction.

Cultural evolution is driven, in part, by the strategies that individuals employ to acquire behavior from others. These strategies themselves are partly products of natural selection, making the study of social learning an inherently Darwinian project. Formal models of the evolution of social learning suggest that reliance on social learning should increase with task difficulty and decrease with the probability of environmental change. These models also make predictions about how individuals integrate information from multiple peers. We present the results of microsociety experiments designed to evaluate these predictions. The first experiment measures baseline individual learning strategy in a two-armed bandit environment with variation in task difficulty and temporal fluctuation in the payoffs of the options. Our second experiment addresses how people in the same environment use minimal social information from a single peer. Our third experiment expands on the second by allowing access to the behavior of several other individuals, permitting frequency-dependent strategies like conformity. In each of these experiments, we vary task difficulty and environmental fluctuation. We present several candidate strategies and compute the expected payoffs to each in our experimental environment. We then fit to the data the different models of the use of social information and identify the best-fitting model via model comparison techniques. We find substantial evidence of both conformist and nonconformist social learning and compare our results to theoretical expectations.

Kin selection and parental investment theories state that, in highly social species, such as humans, individuals can increase their inclusive fitness by extending support to their relatives. Here, we document patterns of kin support in a rural Ethiopian community, where postmarital residence practices provide differential access to relatives. Using demographic, anthropometric, and behavioral data collected from four villages we are able to (a) identify the effects of the presence of kin on child mortality and growth patterns and (b) provide detailed information on the role of relatives within the household. Mortality analyses indicate that grandmothers had a positive effect on child survival. Anthropometric data reveal that maternal grandmothers had a particularly beneficial effect on child height, but paternal grandmothers less so. Time allocation data suggest that grandmothers continued to visit their daughters' households, irrespective of postmarital residence, where they relieved their daughters of heavy domestic tasks rather than helping with direct grandchild care. Matrilocal postmarital residence was associated with improved child survival, although children in matrilocal households were actually smaller. This may be due to wealth effects, increased competition between siblings, or higher survival of smaller infants in matrilocal households.

Women's waist-to-hip ratio (WHR) varies with age, and a lower WHR is associated with a higher estrogen-to-androgen ratio and possibly higher fecundity, at least in some populations. Consequently, it has been argued that selection has favored a universal male preference for a low female WHR. In previous studies using frontal pictures, men in the United States preferred a low WHR of 0.7, but men among Hadza hunter–gatherers and a few other small-scale societies preferred higher ratios. Unlike the actual WHR of women, measured with a tape around the waist and the hips and buttocks, the WHR in frontal pictures excludes the buttocks. Because frontal WHR gives only a partial picture, we used profile views of women to measure men's preferences for the profile WHR. Hadza men preferred a lower profile WHR (more protruding buttocks) than American men. Since Hadza men preferred higher frontal WHR but lower profile WHR, and since both contribute to the actual WHR, these results imply there is less disparity between American and Hadza preferences for the actual WHR of real women. We suggest men's preferences vary with the geographic variation in the shape of women who have wider hips in some populations and more protruding buttocks in others.

Physical condition (e.g., health, fertility) influences female mate preferences in many species, with females in good condition preferring "higher quality" (e.g., healthier) mates. In humans, condition may comprise both physical (e.g., health and fertility) and psychological factors (e.g., stress, anxiety, and depression). We found that women with low waist-to-hip ratios (indicating health and fertility) or who scored low on anxiety, depression, and stress measures expressed greater attraction to composite male (but not female) faces with color and texture cues associated with apparent health than did women with relatively high waist-to-hip ratios or who scored relatively high on the anxiety, depression, and stress measures. These effects of physical and psychological condition were independent and were not mediated by women's perceptions of their own attractiveness. Our findings indicate that women's physical and psychological conditions both contribute to individual differences in face preferences.

Kinship is a key element in the recognition of human genetic relatedness. People use phenotype matching, especially resemblance with respect to facial features, to recognize kin. Bressan and Dal Martello (2002) assessed the effects of actual and assumed genetic relatedness on resemblance ratings made by Italian adults. We replicated their experiment using Japanese individuals as our stimuli and raters. To further assess the processes underlying resemblance ratings, we also compared reaction times. As in previous research, belief in genetic relatedness strongly affected resemblance ratings, but the direction of the cognitive bias was different. Analysis of reaction times indicated that participants took shorter time to assess the resemblance of unrelated pair members than related ones.

There is a lively literature on sex differences in mate preferences primarily in Western cultures. The few cross-cultural studies that exist are limited, as they ignore the role of religion on mate preferences. In this article, we randomly selected 500 personal advertisements placed by Muslims who live in the United States. Content analysis revealed that there were no significant sex differences in seeking a physically attractive mate. However, women mentioned their physical attractiveness more than men did, while men, more than women, sought mates younger than themselves. Women preferred financially secure partners and placed a higher value on finding partners who were emotionally sensitive and sincere than did the men. Furthermore, women significantly, more than men, advertised their religiosity and sought religious partners.

Variation in women's preferences for male facial masculinity may reflect variation in attraction to immunocompetence or to maturity. This paper reports two studies on (a) the interrelationships between women's preferences for masculinity, apparent health, and age in male faces and (b) the extent to which manipulating each of these characteristics affects women's attributions of the remaining characteristics. Both studies were carried out with a large sample of the general public (Studies 1a and 2a) and independently in a laboratory environment with smaller undergraduate samples (Studies 1b and 2b). In both samples, masculinity and age preferences were positively related, and masculinity preferences were not associated with preferences for apparent health. There was also a positive relationship between perceived age and perceived masculinity in both samples, but evidence for a link between perceptions of masculinity and health was equivocal. Collectively, these findings suggest that variation in women's preferences for masculine proportions in male faces reflect variation in attraction to male age and do not support a strict immunocompetence explanation of preferences for facial masculinity.

The male sex hormone testosterone is an immunosuppressant. This has led evolutionary theorists to speculate that masculine facial structure in humans is a Zahavian handicap trait: an honest signal of genetic quality, as males with masculine faces are displaying survival ability despite maintaining high testosterone levels. If this theory is correct, females should show preference for masculine facial architecture. Empirical tests of female preference for traits likely to be testosterone-related have, however, failed to show consistent results. In the present research, facial photographs were taken of 45 men, and measurements of jaw size and eyebrow ridge development were taken from these. Sixty women were presented with 10 randomly chosen pairs of male faces and asked to select the more attractive. No overall preference for facial masculinity emerged, but women with high sociosexuality scores, indicating preference for short-term mating, were more likely to prefer faces with masculine features. A theoretical implication of these and previous findings is that the handicap principle alone may not explain male testosterone expression. Instead, the results supported the hypothesis that preference for facial masculinity is condition dependent: Women attempting to secure a mate who will provide long-term parental investment are likely to avoid men with masculine facial structure, while women seeking short-term sexual relationships show a preference for it.

The attractiveness of women's faces, voices, bodies, and odors appear to be interrelated, suggesting that they reflect a common trait such as femininity. We invoked novel approaches to test the interrelationships between female vocal and facial attractiveness and femininity. In Study 1, we examined the relationship between facial-metric femininity and voice pitch in two female populations. In both populations, facial-metric femininity correlated positively with pitch of voice. In Study 2, we constructed facial averages from two populations of women with low- and high-pitched voices and determined men's preferences for resulting prototypes. Men preferred averaged faces of women from both populations with higher pitched voices to those with lower pitched voices. In Study 3, we tested whether the findings from Study 2 also extended to the natural faces that made up the prototypes. Indeed, men and women preferred real faces of women with high-pitched voices to those with low-pitched voices. Because multiple cues to femininity are related, and feminine women may have greater reproductive fitness than do relatively masculine women, male preferences for multiple cues to femininity are potentially adaptive.

Fundamental frequency (F0) is the vocal acoustic parameter closest to what we perceive as pitch. Men speak at a lower F0 than do women, even controlling for body size. Although the developmental and anatomical reasons for this sex difference are known, the evolutionary reasons are not. By examining fertility-related variation in women's preferences for men's voices, the present study tests the hypothesis that female choice for good genes influenced the evolution of male voice pitch (VP). Unlike previous correlational studies that did not consider the effects of menstrual phase and mating context on women's preferences for male VP, the present study includes these variables and utilizes experimental pitch (P) manipulations. Results indicate that low VP is preferred mainly in short-term mating contexts rather than in long-term, committed ones, and this mating context effect is greatest when women are in the fertile phase of their ovulatory cycles. Moreover, lower male F0 correlated with higher self-reported mating success. These findings are consistent with the hypothesis that an association between low male VP and heritable fitness led to the evolution of the observed patterns in women's P preferences and men's mating success and that these patterns influenced the evolution of low VP in men. However, alternative explanations are considered.

Altruism is behaviorally defined as an act that benefits others at the expense of the actor. Altruism is usually associated with helping others in need, but it can also take place in the context of punishment. People who help to maintain cooperation by punishing cheaters are benefiting others at their own expense as surely as if they performed acts of overt helping. The proximate psychological mechanisms that motivate altruistic helping and altruistic punishment are almost certainly different from each other (e.g., empathy vs. moralistic anger). We present two studies suggesting that the impulse to altruistically help and altruistically punish differ in their sensitivity to information regarding genetic relatedness and probability of future interactions. This interesting empirical result is relevant to the interpretation of altruistic punishment as an evolved adaptation versus a byproduct of modern environments, and to the evolution of psychological traits associated with morality.

Heritable individual differences in personality have not been fully accounted for within the framework of evolutionary psychology. This paper argues that personality axes such as extraversion can usefully be seen as dimensions of trade-off of different fitness costs and benefits. It is hypothesized that increasing extraversion will be associated with increasing mating success, but at the cost of either increased physical risk or decreased parenting effort. In a sample of 545 British adults, extraversion was a strong predictor of lifetime number of sexual partners. Male extraverts were likely to have extra-pair matings, whilst female extraverts were likely to leave existing relationships for new ones. On the cost side, increasing extraversion increased the likelihood of hospitalization for accident or illness. There was no direct evidence of reduced parenting effort, but extravert women had an increased likelihood of exposing their children to stepparenting. The study demonstrates that extraversion has fitness costs as well as benefits. Population variation related in the trait is unlikely to be eliminated by selection due to its polygenic nature, likely spatiotemporal variability in the optimal value, and possible status- and frequency-dependent selection.

The present study was designed to delineate further the female advantage in location memory by testing memory for faces that varied in their emotional expressions. A female advantage in face location was found, consistent with the implicit assumption that a female advantage in location memory is not dependent on the nature of the stimulus. However, exposure to threatening facial expressions abolished (in the instance of mental rotation) or reversed (in the instance of location memory) the characteristic sex differences in task performance. In both males and females, a brief exposure to sad facial expressions impaired subsequent performance on a targeting task. These findings suggest that sex differences in male and female spatial ability are not fixed or absolute but may be influenced by a sex-dimorphic affective system that is responsive to the immediate demands of a social context.

By motivating avoidance of contaminants, the experience of disgust guards against disease. Because behavioral prophylaxis entails time, energy, and opportunity costs, Fessler and Navarrete [Evol. Hum. Behav. 24 (2003) 406–417] hypothesized that disgust sensitivity is adjusted as a function of immunocompetence. Changes in immune functioning over the course of pregnancy offer an opportunity to test this notion. Relative to later stages, the first trimester of pregnancy involves substantial suppression of the maternal immune response, and both maternal and fetal vulnerability to pathogens are greatest during this phase; food-borne illnesses, in particular, pose a threat during the first trimester. Using a Web-based survey of 496 pregnant women, we compared participants in the first trimester with those in later stages of pregnancy. Results reveal heightened disgust sensitivity in the first trimester, notably including disgust sensitivity in the food domain. This pattern is not simply a consequence of elevated nausea during the first trimester, as, although disgust sensitivity and current level of nausea are correlated, first trimester women remain more easily disgusted in the food domain even after controlling for the greater incidence of nausea. These results provide preliminary support for the hypothesis that disgust sensitivity varies during pregnancy in a manner that compensates for maternal and fetal vulnerability to disease.

Homicide is predominantly committed by and against men, especially young men, all over the world. This has been documented in many societies across various times and cultures and has been considered to be one of the universal patterns of homicide. However, the homicide rate in Japan has decreased drastically since the end of the Second World War, owing to a huge decrease in the homicide rate by men in their 20s, and as a result, the usual peak in the age-related homicide curve completely disappeared in recent Japan. When homicide rates are calculated for each cohort of men, however, age effects remain clear, regardless of the actual homicide rates. I investigated the relevance of sociocultural factors hypothesized to affect the risk proneness of men in relation to age.

One of our most fundamental cognitive adaptations is the ability to infer the intentions of others. Whole-body motion is a reliable, valid, easily perceived source of information about intentions because different kinds of intentional action have different motion signatures. In this study, we report four experiments that examined the ability of German adults, German children, and Shuar adults from Amazonian Ecuador to distinguish, on the basis of motion cues alone, between six categories of intentional interaction: chasing, fighting, courting, following, guarding, and playing. Naturalistic motion trajectories were elicited from untutored participants in a game-like situation with performance-based monetary payoffs and were categorized by other participants in a forced-choice design. On a six-category task, German adults correctly categorized intention 75% of the time (where 17% represents chance performance). On a four-category judgment task, children's performance was above chance by age 4, with a mean of 64% correct. A final study compared the judgments of German adults with those of Shuar hunter-horticulturalists. Performance was identical and well above chance in both populations, suggesting that cognitive adaptations for inferring intention from motion deserve further research as possible universal components of human psychology.

Most mammals stop drinking milk at weaning, which is also the time when they cease producing lactase, the digestive enzyme that hydrolyzes lactose. Cessation of lactase production and milk drinking also characterize most human populations, especially those of African and Asian descent. However, a genetic mutation that maintains the functionality of lactase production into adulthood occurs commonly among populations from northern Europe, where dairying is practiced routinely. Indeed, the ability to absorb lactose is nutritionally beneficial for adults only if milk consistently is available. What determines the distribution of dairying? We hypothesized that specific environmental circumstances affect where milk-producing ungulates can be raised safely and economically, thus influencing the geographical occurrence of dairying and lactase persistence. To evaluate this hypothesis, we compiled data on adult lactose absorption (LA) and malabsorption (LM) frequencies in 270 indigenous African and Eurasian populations (Appendix A). Partial correlation analyses revealed that, as predicted, adult LM is associated with extreme climates (at high and low latitudes) and, more significantly, with the historical (pre-1900) geographical occurrence of nine deadly, communicable diseases of cattle. These results suggest that areas where adult LM predominates are those where it is impossible or dangerous to maintain dairy herds.

The hypothesis derived from the evolutionary view of jealousy that men's jealousy mechanism (JM) preferentially processes cues signaling a mate's sexual infidelity, whereas women's JM preferentially processes cues signaling a mate's emotional infidelity was tested. Depending on the condition, the participants were successively presented with a series of cues signaling either a mate's sexual or emotional infidelity in ascending order of cue diagnosticity. The participants had to determine two thresholds of the jealousy feeling. The first threshold dealt with the cue to infidelity that elicits a first pang of jealousy. The second threshold concerned that cue to infidelity where the intensity of the jealousy feeling becomes intolerable. No sex-specific differences were found with respect to the number of cues to sexual or emotional infidelity until the first threshold. However, after the first feeling of jealousy had been elicited, men needed significantly fewer cues to sexual infidelity and women needed significantly fewer cues to emotional infidelity until the second threshold. Moreover, men were significantly faster in determining the two thresholds for cues to sexual infidelity, whereas women were significantly faster for cues to emotional infidelity. The implications of the present findings are discussed.

Cross-cultural diversity in economic game behavior has been cited as evidence that humans do not possess psychological adaptations specialized for cooperation in collective actions (CAs). In this paper, it is argued that such adaptations may, in fact, exist and that their design may be illuminated by the appropriate kinds of cross-cultural data. To exemplify an aspect of cooperation that may not vary cross culturally, data are provided suggesting that, in the CAs of Shuar hunter–horticulturists, punitive sentiment towards free riders takes a form similar to that which it takes in industrialized societies: It is experienced mainly by high contributors and directed mainly at CA beneficiaries who could have contributed highly but chose not to. If anti-free-rider punitive sentiment is essentially similar cross culturally, then it may be the product of a species-typical psychological mechanism specialized for such sentiment. How such a mechanism may have evolved is discussed.

Buss and Schmitt [Psychol. Rev. 100 (1993) 204–232] found that men report preferring more sex partners than women do. Pedersen, Miller, Putcha-Bhagavatula, and Yang et al. [Psychol. Sci. 13 (2002) 157–161] reanalyzed the data of Buss and Schmitt and also collected their own. They found no sex differences in either data set. We show that, when appropriate graphical and statistical methods are used, men clearly report preferring more partners. At all comparable locations on curves showing cumulative number of desired partners, men report preferring more than three times as many partners as women do. Furthermore, the data show that men and women who desire between one and about five desired partners over their lifetime come from a normal distribution, with a standard deviation of about 5. Thus it is incorrect to conclude that men and women who desire more that one lifetime sexual partner come from a different population than those who want only one. We discuss the implication of the data for sexual strategies theory [Psychol. Rev. 100 (1993) 204–232] and attachment fertility theory (AFT; Miller & Fishkin, 1997).

Brown and Konner [Brown, P. J., & Konner M. (1987). Ann. N.Y. Acad. Sci., 499, 29–46] proposed that plumpness or moderate fatness is valued in most preindustrial societies because of fat's adaptive value during periods of resource scarcity. Using three measures of resource scarcity, we tested the hypothesis that societies with little or no such scarcity value thinness in women, whereas those with high scarcity value plumpness. In one cross-cultural sample, the evidence was significantly opposed to this hypothesis, and in a second, resource scarcity and valuation of fatness were unrelated. We explore possible reasons for the contradiction between these results and those of Anderson, Crawford, Nadeau, and Lindberg [Anderson, J. L., Crawford, C. B., Nadeau J., & Lindberg T. (1992), Ethol. Sociobiol., 13, 197–227], who reported a positive relationship between resource scarcity and plumpness being beautiful and conclude that their measure of scarcity was, in fact, a measure of food storage, which modulates the relationship: Resource scarcity and valuing fatness in women are negatively associated when there is little or no food storage and unrelated when there is moderate or high storage. Finally, we retest the possible effects of climate and male dominance suggested by Anderson et al., finding that some measures of male dominance indeed predict valuing fatness in women, but we suggest that considering these to be measures of “protest masculinity” rather than male dominance may better account for the results.

Models indicate that opportunities for reputation formation can play an important role in sustaining cooperation and prosocial behavior. Results from experimental economic games support this conclusion, as manipulating reputational opportunities affects prosocial behavior. Noting that some prosocial behavior remains even in anonymous noniterated games, some investigators argue that humans possess a propensity for prosociality independent of reputation management. However, decision-making processes often employ both explicit propositional knowledge and intuitive or affective judgments elicited by tacit cues. Manipulating game parameters alters explicit information employed in overt strategizing but leaves intact cues that may affect intuitive judgments relevant to reputation formation. To explore how subtle cues of observability impact prosocial behavior, we conducted five dictator games, manipulating both auditory cues of the presence of others (via the use of sound-deadening earmuffs) and visual cues (via the presentation of stylized eyespots). Although earmuffs appeared to reduce generosity, this effect was not significant. However, as predicted, eyespots substantially increased generosity, despite no differences in actual anonymity; when using a computer displaying eyespots, almost twice as many participants gave money to their partners compared with the controls. Investigations of prosocial behavior must consider both overt information about game parameters and subtle cues influencing intuitive judgments.

To investigate the choices that people make in dating partners, we analyzed data provided by HurryDate, a commercial dating service aimed at adult singles living in major metropolitan areas. Here, we report data from 10,526 participants in HurryDate sessions, in which roughly 25 men and 25 women interacted with each other for three minutes and subsequently indicated which of the people they met they would be interested in having contact with in the future. We had general survey information collected by HurryDate for all the participants and additional survey information for 2,650 participants. Our main findings are that (1) HurryDate interactions are driven primarily by generally agreed-upon mate values and less by niche-based or assortative patterns, (2) the agreed-upon mate values for both men and women derive almost exclusively from physically observable attributes like attractiveness, BMI, height, and age and are not substantially related to harder-to-observe attributes such as education, religion, sociosexuality, having children, or desiring future children, and (3) small positive assortative trends arise in the areas of race and height. Our results provide rare behavioral evidence regarding people's preferences in dating partners.

Females gain direct or indirect fitness benefits by choosing between males with traits indicating “good genes,” but we usually know very little about the nature of these genes. However, it has been suggested that genetic quality may often be defined as heterozygosity at certain loci. Here, we show that heterozygosity at three key loci in the major histocompatibility complex (MHC) is associated with facial attractiveness: Faces of men who are heterozygous at all three loci are judged more attractive by women than faces of men who are homozygous at one or more of these loci. MHC genes code for proteins involved in immune response. Consistent with this function, faces of MHC heterozygotes are also perceived to be healthier. In a separate test, in the absence of any other cues, patches of skin from the cheeks of heterozygotes are judged healthier than skin of homozygotes, and these ratings correlate with attractiveness judgements for the whole face. Because levels of MHC similarity can influence mate preferences in animals and humans, we conducted a second experiment with genotyped women raters, finding that preferences for heterozygosity are independent of the degree of MHC similarity between the men and the female raters. Our results are the first to directly link facial attractiveness and a measure of genetic quality and suggest a mechanism to help explain common consensus concerning individual attractiveness. In a relatively monogamous species like humans, evolutionary benefits from choosing heterozygous mates could include prolonged parental care and reduced risk of contracting disease for females and their offspring.

In this paper, I review hypotheses about why hunters share meat, and I use quantitative data on meat transfers between households of Achuar, Quichua, and Zapara speakers in Conambo, an indigenous community of horticultural foragers in the Ecuadorian Amazon, to test them. I show that meat is distributed to political allies in Conambo and argue that meat is strategically transferred to recruit and maintain coalitional support in a political landscape where loyalties are shifting, crosscutting, and consequential. Additionally, I find clear evidence of kinship and reciprocity influences on meat transfers, with mixed support for tolerated theft, costly signaling, and showing-off influences. Postmarital residence is matrilocal, hunters have control over meat distribution, game is abundant, and coalitional membership and loyalties are unstable. These environmental and social variables may explain different patterns in meat sharing in Conambo than those found in previous studies.

Using methods of experimental cognitive psychology, we tested the hypothesis that attitude similarity serves as a heuristic cue signaling kinship, which may motivate kin-recognition responses (e.g., prosocial behavior) even to unrelated individuals. The experiment employed a reaction-time methodology to assess cognitive associations between specific target individuals and kinship cognitions. Results revealed that, relative to targets with dissimilar attitudes, attitudinally similar targets were automatically linked to kinship cognitions. This effect was especially strong among perceivers who more strongly trusted their intuitions, indicating that the similarity–kinship connection is based on heuristic impressions, not rational decision making. Additional results showed that the activation of kinship cognitions was correlated with perceivers' willingness to help similar others. These findings add to our understanding of proximate mechanisms linked to kin selection processes and implicate the role of kinship processes in prosocial behavior toward unrelated strangers as well.

If attractiveness is an important cue for mate choice, as proposed by evolutionary psychologists, then attractive individuals should have greater mating success than their peers. We tested this hypothesis in a large sample of adults. Facial attractiveness correlated with the number of short-term, but not long-term, sexual partners, for males, and with the number of long-term, but not short-term, sexual partners and age of first sex, for females. Body attractiveness also correlated significantly with the number of short-term, but not long-term, sexual partners, for males, and attractive males became sexually active earlier than their peers. Body attractiveness did not correlate with any sexual behavior variable for females. To determine which aspects of attractiveness were important, we examined associations between sexual behaviors and three components of attractiveness: sexual dimorphism, averageness, and symmetry. Sexual dimorphism showed the clearest associations with sexual behaviors. Masculine males (bodies, similar trend for faces) had more short-term sexual partners, and feminine females (faces) had more long-term sexual partners than their peers. Feminine females (faces) also became sexually active earlier than their peers. Average males (faces and bodies) had more short-term sexual partners and more extra-pair copulations (EPC) than their peers. Symmetric women (faces) became sexually active earlier than their peers. Given that male reproductive success depends more on short-term mating opportunities than does female reproductive success, these findings suggest that individuals of high phenotypic quality have higher mating success than their lower quality counterparts.

Several hypotheses about attitudes toward risk takers, derived from costly signaling theory (CST), were tested. Male and female participants evaluated the attractiveness of risk takers compared with risk avoiders as potential mates, and as potential same-sex friends, in 21 different scenarios. Both females and males preferred heroic physical risk takers as mates, with the preference being stronger for females. Contrary to predictions, for nonheroic physical risks (such as risky sports), both males and females preferred risk avoiders over risk takers as mates. However, for same-sex friends, males significantly preferred nonheroic physical risk takers, whereas females preferred risk avoiders. It was concluded that insofar as nonheroic risk taking by males is a costly signal, the signal is directed more toward fellow males than toward females. Preferences for risk takers were positively correlated with reported self risk-taking tendencies, but the correlation was significantly higher for friends than for mates for both heroic and nonheroic physical risks. In a second study, both males and females accurately predicted the opposite sex's preferences for heroic risk takers as mates. However, males failed to predict females' preferences for nonheroic physical risk avoiders. Both males and females underestimated the opposite sex's preferences for drug risk avoiders.

Around the world and across time, women's lives and opportunities vary—but this is patterned variation, produced by the interplay of natural selection (life history theory) and ecological and social constraints. Our evolutionary background (e.g., evolution of anisogamy) and phylogenetic constraints (female mammals' specialization for postnatal care) create different costs and benefits for males and females. These interact with environmental conditions to produce patterned variation in mating and marriage systems, degree of male parental investment, for example. Here, I review how, in response to these conditions, women's strategies (reproductive, resource, coalitional, and political) vary.

Moral systems require individuals to act in service to their social groups. Despite the human tendency to view moral norms as invariant and constantly deserving of adherence, we vary not only in the moral norms that we espouse but also in the degree to which our behavior reflects those norms. Nevertheless, moral systems exhibit patterns and complexity that suggest the action of natural selection. We propose that much observed variation in commitment to the group can be explained by a rule of stability-dependent cooperation, where the adaptive level of individual commitment varies inversely with the stability of the social group. This hypothesis is rooted in the understanding that humans are caught in an evolutionary trade-off between two methods of increasing reproductive success: competing with fellow group members, and increasing the stability of the group relative to other groups. If cooperation is stability dependent, however, human groups in times of high stability and low cooperation may be susceptible to fast-acting extrinsic threats, as well as self-destructive competitive races to the bottom. In light of this, we hypothesize that the absolutism and unchangeableness commonly attributed to moral norms serve a group stability insurance function and we present predictions from this hypothesis.

Richard Alexander has been a major contributor to the development of theory concerning the evolution of human sociality. His most important contributions include (1) a theory of the evolution of morality as a form of indirect reciprocity that aids in intergroup competition, (2) a theory of the relationship between biological evolution and culture, (3) an elaboration of Humphrey's theory of the human intellect as a social tool, (4) theories about human parental investment and nepotism, and (5) theories about scenario building, consciousness, and human communication. He also has offered a hypotheses on a large range of other human traits. He is a biologist and has also made major contributions to theories of speciation, communication, eusociality, and social organization in nonhuman animals and has contributed extensively to the study of a number of specific taxa other than the human species: crickets, katydids, cicadas, naked mole rats, and horses. His contribution to the study of nonhuman animals and evolutionary theory, in general, are sufficient to earn him a reputation as an outstanding leader in biology without reference to his work on humans. The same can be said for his contribution to the understanding of human sociality taken alone.

In this paper, I investigate the relevance of recent work in reproductive skew theory to explanations of the evolution of social stratification in human societies. I briefly review human social evolution and corresponding increases in stratification, as well as recent developments in skew theory. I then attempt to integrate basic factors that have been a focus of interest by skew theorists to previous work on human social stratification from an evolutionary ecological perspective. I also discuss factors peculiar to human social systems that could profitably be incorporated into future models of reproductive skew.

Human cognitive abilities are extraordinary. Our large brains are significantly modified from those of our closest relatives, suggesting a history of intense natural selection. The conditions favoring the evolution of human cognitive adaptations, however, remain an enigma. Hypotheses based on traditional ecological demands, such as hunting or climatic variability, have not provided satisfying explanations. Recent models based on social problem solving linked with ecological conditions offer more convincing scenarios. But it has proven difficult to identify a set of selective pressures that would have been sufficiently unique to the hominin lineage. What was so special about the evolutionary environments of our ancestors that caused them, and them alone, to diverge in such astonishing ways from their close relatives and all other life forms? Richard Alexander proposed a comprehensive integrated explanation. He argued that as our hominin ancestors became increasing able to master the traditional “hostile forces of nature,” selective pressures resulting from competition among conspecifics became increasingly important, particularly in regard to social competencies. Given the precondition of competition among kin- and reciprocity-based coalitions (shared with chimpanzees), an autocatalytic social arms race was initiated, which eventually resulted in the unusual collection of traits characteristic of the human species, such as concealed ovulation, extensive biparental care, complex sociality, and an extraordinary collection of cognitive abilities. We term this scenario the “ecological dominance–social competition” (EDSC) model and assess the feasibility of this model in light of recent developments in paleoanthropology, cognitive psychology, and neurobiology. We conclude that although strong or direct tests are difficult with current data, Alexander's model provides a far-reaching and integrative explanation for the evolution of human cognitive abilities that is consistent with evidence from a wide range of disciplines.

Many of the body's adaptive responses, such as pain, fever, and fear, are defenses that remain latent until they are aroused by cues that indicate the presence of a threat. Natural selection should shape regulation mechanisms that express defenses only in situations where their benefits exceed their costs, but defenses are often expressed in situations where they seem unnecessary, with much resulting useless suffering. An explanation emerges from a signal detection analysis of the costs and benefits that shaped defense regulation mechanisms. Quantitative modeling of optimal regulation for all-or-none defenses and for continuously variable defenses leads to several conclusions. First, an optimal system for regulating inexpensive all-or-none defenses against the uncertain presence of large threats will express many false alarms. Second, the optimum level of expression for graded defenses is not at the point where the costs of the defense and the danger are equal, but is instead where the marginal cost of additional defense exceeds the marginal benefit. Third, in the face of uncertainty and skewed payoff functions, the optimal response threshold may not be the point with the lowest cost. Finally, repeated exposures to certain kinds of danger may adaptively lower response thresholds, making systems vulnerable to runaway positive feedback. While we await quantitative data that can refine such models, a general theoretical perspective on the evolution of defense regulation can help to guide research and assist clinical decision making.

The detection of genetic relatedness (i.e., kinship) affects the social, parental, and sexual behavior of many species. In humans, self-referent phenotype matching based on facial resemblance may indicate kinship, and it has been demonstrated that facial resemblance increases perceptions of trustworthiness and attractiveness [Proc. R. Soc. Lond., B Biol. Sci. 269 (2002) 1307–1312; Proc. R. Soc. Lond., B Biol. Sci. (in press)]. However, investigations of sex differences in reaction to facial resemblance have produced mixed results [Evol. Hum. Behav. 25 (2004) 142–154; Evol. Hum. Behav. 23 (2002) 159–166; Evol. Hum. Behav. 24 (2003) 81–87]. Here, we replicate the effects of Platek et al. [Evol. Hum. Behav. 23 (2002) 159–166] using high-resolution color morphing. We also extend these findings using functional magnetic resonance imaging (fMRI) to demonstrate a possible neural mechanism that may account for the observed sex difference. These data support the hypothesis that human males may use and favor facial resemblance as a paternity cue.

Females apparently are the choosier sex in courtship contexts, but there still is limited information about female selection criteria in real courtship settings. Given that a female knows little about a heretofore unacquainted male, upon what dimensions can (and do) females base their initial courtship decisions? Here, we report findings from observational studies that investigated male nonverbal behavior in a bar context. Study 1 documented the body movements of males prior to making contact with a female. It was found that males who successfully made “contact” courtship initiation with females exhibited different body language in this precontact phase than did males who did not make contact with females, including significantly more glancing behaviors, space-maximization movements, intrasexual touching, and less closed-body movements. The findings from a second within-subject study comparing the behavior of men in a bar when women were present or not present supported the initial study's findings and showed that males' emphasis on these behaviors increases in a mate-relevant context. We suggest that certain aspects of male nonverbal behavior in courtship contexts can serve as self-presentation and mate-value signals.

The present study argues that pressures associated with the threat of paternal uncertainty shaped a male mate-preference for relatively subordinate partners in the context of long- (high investment) but not short-term (low investment) relationships. Using a hypothetical scenario depicting a workplace setting, 120 male and 208 female undergraduates were randomly assigned to one of six experimental conditions in which they were asked to judge a male or female target, described as either their supervisor (higher dominance), coworker (equal dominance), or their assistant (lower dominance). Participants exposed to an opposite-sex target rated their attraction to the target for varying types of investment (e.g., desire for a one-night stand, desire to affiliate, or desire for a long-term relationship), and participants exposed to a same-sex target rated their desire to affiliate with the target. Results supported predictions and indicated that males preferred the subordinate over the dominant target for affiliation and high-investment items, and females were unaffected by the target's dominance status. The results of same-sex ratings indicated that dominance status did not influence females' ratings of the female target, and that dominance did not influence males' ratings of the male target. These results are discussed in terms of the role of relational dominance in shaping male mate-preferences.

One of the most critical features of human society is the pervasiveness of cooperation in social and economic exchanges. Moreover, social scientists have found overwhelming evidence that such cooperative behavior is likely to be directed toward in-group members. We propose that the group-based nature of cooperation includes punishment behavior. Punishment behavior is used to maintain cooperation within systems of social exchange and, thus, is directed towards members of an exchange system. Because social exchanges often take place within groups, we predict that punishment behavior is used to maintain cooperation in the punisher's group. Specifically, punishment behavior is directed toward in-group members who are found to be noncooperators. To examine this, we conducted a gift-giving game experiment with third-party punishment. The results of the experiment (N=90) support the following hypothesis: Participants who are cooperative in a gift-giving game punish noncooperative in-group members more severely than they punish noncooperative out-group members.

Here, we investigate whether variation in male parental investment can be explained in terms of (1) men's perception of the degree of resemblance between themselves and their offspring and (2) men's perception of their mates' fidelity. In a sample of men from London's Heathrow airport, both variables were found to predict reported investment. We also examined whether the predictors of investment varied when men were no longer in a relationship with the mother of their children and are therefore no longer investing in mating effort with them. Among men no longer in a relationship with the mother of their children, resemblance became a stronger predictor of investment, while fidelity was no longer a significant predictor. Overall, men provided less investment to their children if they were no longer in a relationship with the mother of their children.

In the UK and Japan, both men and women prefer somewhat feminised opposite-sex faces, especially when choosing a long-term partner. Such faces are perceived as more honest, caring, and sensitive; traits that may be associated with successful male parental investment. By contrast, women prefer less feminised faces for short-term relationships and when they are near ovulation. As genetic quality may be associated with facial masculinity, women may ‘trade-off’ cues between genetic quality and paternal investment in potential partners. No analogous trade-off has been suggested to influence men's preferences, as both attributions of prosociality and potential cues to biological quality are associated with facial femininity in female faces. Ecological and cultural factors may influence the balance of trade-offs leading to populational differences in preferences. We predicted that Jamaican women would prefer more masculine faces than British women do because parasite load is higher in Jamaica, medical care less common (historically and currently), and male parental investment less pronounced. Male preferences, however, were predicted to vary less cross-culturally, as no trade-off has been identified in female facial characteristics. We constructed masculinised and feminised digital male and female face stimuli of three populations (Jamaican, Japanese, and British) and presented them to men and women in Jamaica and in Britain. The results demonstrated that Jamaican women preferred more masculine male faces than their British counterparts did. Jamaican men tended to prefer more masculine female faces than did British men did, but this effect was complicated by an interaction suggesting that more feminised faces were preferred within culture.

Despite huge interest in human mate choice in the last two decades, intraspecific variation in human mate preferences has received relatively little attention. We investigated individual variation in mate preferences in a group of university students (n=292) relative to perceptions of equality and autonomy. If the constraints of societal role occupancy strongly influence sex differences in the ranking of mate preferences, then we predict that these sex differences should diminish with increasing endorsement of gender equality and autonomy. Women's mate preferences did not emulate men's with increasing endorsement of gender equality. The importance placed on “earning potential” in a potential mate decreased with increasing “feminist” attitude score, however, feminist attitude was not related to the importance of “physical attractiveness.” Findings reflect the variation in women's mate preferences and are discussed in terms of evolved conditional strategies.

We investigated the relationship between ratings of voice attractiveness and sexually dimorphic differences in shoulder-to-hip ratios (SHR) and waist-to-hip ratios (WHR), as well as different features of sexual behavior. Opposite-sex voice attractiveness ratings were positively correlated with SHR in males and negatively correlated with WHR in females. For both sexes, ratings of opposite-sex voice attractiveness also predicted reported age of first sexual intercourse, number of sexual partners, number of extra-pair copulation (EPC) partners, and number of partners that they had intercourse with that were involved in another relationship (i.e., were themselves chosen as an EPC partner). Coupled with previous findings showing a relationship between voice attractiveness and bilateral symmetry, these results provide additional evidence that the sound of a person's voice may serve as an important multidimensional fitness indicator.

Experiments may contribute to understanding the basic processes of cultural evolution. We drew features from previous laboratory research with small groups in which traditions arose during several generations. Groups of four participants chose by consensus between solving anagrams printed on red cards and on blue cards. Payoffs for the choices differed. After 12 min, the participant who had been in the experiment the longest was removed and replaced with a naı̈ve person. These replacements, each of which marked the end of a generation, continued for 10–15 generations, at which time the day's session ended. Time-out duration, which determined whether the group earned more by choosing red or blue, and which was fixed for a day's session, was varied across three conditions to equal 1, 2, or 3 min. The groups developed choice traditions that tended toward maximizing earnings. The stronger the dependence between choice and earnings, the stronger was the tradition. Once a choice tradition evolved, groups passed it on by instructing newcomers, using some combination of accurate information, mythology, and coercion. Among verbal traditions, frequency of mythology varied directly with strength of the choice tradition. These methods may be applied to a variety of research questions.

Culture of honor (COH) theory [Nisbett, R. E., & Cohen, D. (1996). Culture of honor: The psychology of violence in the south. Boulder, CO: Westview Press] predicts that the importance of upholding one's reputation is cross-culturally variable: Revenge should be more prevalent in herding societies than in farming societies, and should be entirely absent in foraging societies. This study was designed to replicate the effects that they found among herding and farming societies and to either support or refute the claim regarding foraging societies. Using a 32-item questionnaire measuring the constructs of Reciprocity and Revenge, this study cross-culturally validates Nisbett and Cohen's COH theory and extends it to fishers, a special kind of forager. Researchers sampled two herding communities (Hermosillo, Sonora, Mexico, and Liberia, Guanacaste, Costa Rica), two farming communities (Mexico City, Mexico, and San Jose, Costa Rica), and two fishing communities (La Paz, Baja California Sur, Mexico, and Puntarenas, Costa Rica.) The differences between the herding and farming samples replicated previous findings in that herders were higher on the Revenge scale than farmers. The fisher samples approximate the herder samples on the Revenge scale more than the farmer samples, but were significantly different from each other. Discrepancies between the fisher samples called for the investigation of alternative theories.

While substantial evidence supports the existence of Westermarck's [Westermarck, E. (1891). The history of human marriage. London: Macmillan & Co.] hypothesized inbreeding avoidance mechanism, questions remain. We examined the Westermarck hypothesis using third-party reactions to a fictional case of sibling incest, a method paralleling that of Lieberman et al. [Proc. R. Soc. Lond., B Biol. Sci. 273 (2003) 819]. Controlling for cultural attitudes, a history of cosocialization with an opposite-sex individual was associated with increased disgust at, and decreased tolerance of, others' incestuous behavior. Consistent with parental investment theory, this effect was stronger in females than in males. In males, each additional cosocialized sibling increased the strength of the response; in females, there was a nonsignificant trend in the same direction. Indirect measures failed to reveal a time-limited sensitive period during which cosocialization has maximal effect. These results both bolster the evidence in favor of Westermarck's inbreeding avoidance hypothesis and support his neglected explanation of the origin of incest taboos.