Little research has compared human males and females in their investment in unrelated same-sex individuals. Part of the difficulty lies in defining investment. The current study utilized durability of 6- to 11-year-old children's same-sex friendships as a measure of investment. Results demonstrated that males' friendships were more durable than those of females. Analyses comparing conflicts in current same-sex friendships of males and females did not yield any proximate explanations for sex differences in the durability of same-sex bonds. An evolutionary account is proposed for the greater durability of males' versus females' same-sex relationships.

Although many studies of male facial attractiveness assume that facial masculinity is related to circulating testosterone levels in adult males, there is little empirical evidence in support of this assumption. Here, we used salivary testosterone assays to investigate the relationship between circulating testosterone and both masculinity and attractiveness of facial appearance by (1) constructing digital composites from the faces of men with high and low testosterone, which were presented using a forced-choice task to subjects and (2) using a forced-choice task in which participants judged the masculinity of pairs of original photographs. Composites from high-testosterone men were judged to be more masculine than those from low-testosterone men. Evidence that high-testosterone composites are considered more attractive than low-testosterone composites was equivocal. The forced-choice task using the original face images indicated that participants identified faces associated with relatively high circulating testosterone as being more masculine than faces of men with lower circulating testosterone. This effect was more pronounced when the faces in the pair were from men who differed greatly in testosterone levels. These preliminary findings provide support for the underlying assumptions of much attractiveness research, particularly studies that have identified systematic variation in female preferences for masculine faces.

We attempted to clarify prior reported discrepancies between males judging females and females judging males in the attraction value of pupil size. Our hypothesis was that attraction values of pupil size will be described by an interaction effect, such that males will be most attracted by large pupils in females and females by medium size pupils in males. The rationale for the hypothesis was that the reproductive strategies of males are best served by unequivocal female sexual interest and arousal, whereas the strategies of females will predispose them to favor more moderate sexual attentions. As expected, the relationship of attraction to pupil size was positive and linear for males viewing females. Females, however, rather than showing the predicted inverted U function, showed consistent preferences for either medium or large pupils in males. Further investigation revealed that females attracted by large pupils also reported preferences for proverbial bad boys as dating partners. Analogous findings in the literature on female romantic partner preferences are discussed.

The benefits of a good reputation can help explain why some individuals are willing to be altruistic in situations where they will not receive direct benefits. Recent experiments on indirect reciprocity have shown that when people stand to benefit from having a good reputation, they are more altruistic towards groups and charities. However, it is unknown whether indirect reciprocity is the only thing that can cause such an effect. Individuals may be altruistic because it will make them more trustworthy. In this study, I show that participants in a cooperative group game contribute more to their group when they expect to play a dyadic trust game afterwards, and that participants do tend to trust altruistic individuals more than nonaltruistic individuals. I also included a condition where participants had to choose only one person to trust (instead of being able to trust all players) in the dyadic trust game that followed the cooperative group game, and contributions towards the group were maintained best in this condition. This provides some evidence that competition for scarce reputational benefits can help maintain cooperative behaviour because of competitive altruism.

We tested the hypothesis that men would favor routine paternity testing more than women by administering surveys. Indeed, more men (50%) than women (32%) favored paternity testing. We found no support for two secondary hypotheses about ways in which one's attitude would vary with income and marital status. We also present preliminary evidence that many males prefer preserving the ambiguity of paternity.

We report effects of affective recollections on performance in two logically identical, content-rich reasoning problems. The between-groups manipulation entailed requesting a written autobiographical recollection of an event in which research volunteers had been cheated, had been the beneficiary of another's altruism, had been happy, or had been angry. The reasoning problems were two published variants of the Wason selection task: “cheater detection” and “altruist detection”. Subjects who recalled having been cheated or angry performed significantly better on the cheater detection problem than those who recalled being happy or benefited, and those who recalled being benefited did significantly worse on the altruism detection problem than the other groups. We discuss the relevance of these results to the proposition that emotions are functional operating systems that facilitate situation-appropriate cognition.

We tested the prediction derived from the evolutionary view of jealousy that men preferentially recall cues to sexual infidelity, whereas women preferentially recall cues to emotional infidelity. This preferential recall was predicted to be more pronounced in a personally more threatening than in a personally less threatening context condition. In the personally less threatening context condition, the participants listened to a story about an anonymous couple spending an evening together; in the personally more threatening context condition, the same story referred to one's own romantic relationship. Integrated in this story were five ambiguous cues each to sexual and emotional infidelity. As predicted, in a surprise recall test, men preferentially recalled cues to sexual infidelity, whereas women preferentially recalled cues to emotional infidelity. This preferential recall was significant for both men and women only in the personally more threatening context condition.

Much recent research has focused on the ratio of the lengths of the second to fourth manual digits (2D:4D) as a predictor of the degree of expression of sexually dimorphic and other sex-hormone-mediated traits. However, published findings are often contradictory or subject to various methodological problems. In the present study, we reassessed the relationships among three measures of 2D:4D (left hand, right hand, and mean) and several variables previously claimed to be related to 2D:4D, including sexual orientation, spatial ability, status, physical prowess, and components of reproductive success. In addition, we examined the relationship between 2D:4D measures and several other traits whose expression is thought to be related to sex hormones, including voice pitch, sociosexuality, mating success, and fluctuating asymmetry. 2D:4D measures showed highly significant sex differences, as did spatial ability, sociosexuality, components of reproductive and mating success, and fluctuating asymmetry. However, out of 57 correlations, 2D:4D correlated significantly in the predicted direction only with sexual orientation (for both sexes) and only for left hand 2D:4D. We discuss the recent 2D:4D literature in light of these findings and consider their implications for understanding the timing of developmental events.

We analyzed sex differences in spatial and recall abilities of college students while also examining the relationship to 2D:4D ratio. The 2D:4D ratio is thought to be an indirect measure of testosterone and estrogen exposure in fetal development. Participants completed a mental rotation (MR) test, free recall tests, and placement recall tests. It was predicted and confirmed that males have lower 2D:4D ratio and higher MR scores. Additionally, females outscored males on picture free recall and picture placement. Higher scoring females on these two measures had higher 2D:4D ratios, that is, more “feminine” looking hands. The results from our study are consistent with the hypothesis that fetal hormones affect 2D:4D ratio while also directly or indirectly influencing visual recall abilities in females. The tendency of sexually dimorphic cognitive skills to correlate with 2D:4D ratio in only one sex may be typical of traits that were evolutionarily adaptive in one sex, but not maladaptive in the opposite sex.

Platek et al. [Evol. Hum. Behav. 23 (2002) 159; 24 (2003) 81] reported that facial resemblance between self and a child increases professed willingness to invest in that child, and does so much more for men than for women. Because facial resemblance is a possible cue of kinship, and men, unlike women, can be mistaken about parenthood, Platek et al. predicted and interpreted this sex difference as an adaptation whereby men allocate parental investment in proportion to cues of the likelihood of paternity. Extending their approach using a more realistic technique for manipulating facial resemblance and eliminating some of the confounds in their methodology, In the current study, I found that facial resemblance increased attractiveness judgments and hypothetical investment decisions, although the published sex difference was not found. This could not be explained by differences in resemblance between the participants and the morphed images because a separate group of participants could match the original adult images to the new morphs with slightly (but not significantly) greater accuracy than to morphs made using Platek et al.'s method. In addition, composite scores indicating positive regard toward an image were correlated with resemblance as judged by independent observers.

A preference for negative reciprocity is an important part of the human emotional repertoire. We model its role in sustaining cooperative behavior but highlight an intrinsic free-rider problem: the fitness benefits of negative reciprocity are dispersed throughout the entire group, while the fitness costs are borne personally. Evolutionary forces tend to unravel people's willingness to bear the personal cost of punishing culprits. In our model, the countervailing force that sustains negative reciprocity is a meme consisting of a group norm together with low-powered (and low-cost) group enforcement of the norm. The main result is that such memes coevolve with personal tastes and capacities so as to produce the optimal level of negative reciprocity.

Do infants look more like their fathers or their mothers? The available data are contradictory, but were collected through different procedures: either by asking judges to identify the parent in a triplet of adults (straight guess) or by asking them to rate resemblance on a scale and then recoding highest ratings to parents as correct guesses (guess from rating). Here, we used both procedures and compared their results. Eighty judges were asked, first, to estimate the facial resemblance of 40 one-year-olds to each of three adults, and then to guess which adult in each triplet was the parent. Accuracy was better than chance with both methods, but performance in the guessing task was significantly higher. Judgments of parental resemblance were distributed normally, with a few infants preferentially resembling one parent (father or mother with equal probability), and most resembling father and mother to approximately the same degree.

This research examined the relationship between disordered eating attitudes in women and perceptions of parental readiness with perceptions of support from different sources. A total of 100 heterosexual women currently involved in a romantic relationship responded to three subscales of the Eating Disorders Inventory (EDI; Body Dissatisfaction, Drive for Thinness, and Maturity Fears), questions measuring perceived parental readiness, and perceptions of social support from their partners, family, and friends. Women who perceived relatively low levels of support, particularly from romantic partners and family, reported increased dieting symptomatology and lower perceptions of parental readiness. For women, disordered eating attitudes may reflect an ancestral reproductive suppression mechanism that is activated by contemporary cues that indicated poor reproductive outcomes in ancestral environments. The potential implications of these results, as well as alternative interpretations of the findings, are discussed.

From an evolutionary perspective, beauty is regarded as an assessment of fitness value. The fitness value of a social partner can be influenced by both physical and nonphysical traits. It follows that the perceived beauty of a social partner can be influenced by nonphysical traits such as liking, respect, familiarity, and contribution to shared goals in addition to physical traits such as youth, waist-to-hip ratio, and bilateral symmetry. We present three studies involving the evaluation of known social partners showing that judgments of physical attractiveness are strongly influenced by nonphysical factors. Females are more strongly influenced by nonphysical factors than males and there are large individual differences within each sex. In general, research on physical attractiveness based on the evaluation of purely physical traits of strangers might miss some of the most important factors influencing the perception of physical attractiveness among known associates.

This article reports results of a multiple-coder content analysis of the mate preferences of characters in two different sources of literary data: 658 traditional folktales from 48 different culture areas and plot and character summaries from 240 works taken to be representative of Western literature. This study represents an attempt to extend some of the mate preference findings of Buss [Behav. Brain. Sci. 12 (1989a) 1] in a large data set of non-Western, preindustrial populations. A prominent criticism of Buss was that a preponderance of data was gathered from Western or westernized societies and, as a result, it was not possible to rule out the conclusion that regularities in mate preferences resulted from cultural transfer rather than from evolved sexual psychology. By including information derived from band, tribal, and preindustrial state populations, the current study attempts to address concerns based on the possibility of cross-cultural transfer.

We examine the characteristics and relative strength of third-party sanctions in a series of experiments. We hypothesize that egalitarian distribution norms and cooperation norms apply in our experiments, and that third parties, whose economic payoff is unaffected by the norm violation, may be willing to enforce these norms although the enforcement is costly for them. Almost two-thirds of the third parties indeed punished the violation of the distribution norm and their punishment increased the more the norm was violated. Likewise, up to roughly 60% of the third parties punished violations of the cooperation norm. Thus, our results show that the notion of strong reciprocity extends to the sanctioning behavior of “unaffected” third parties. In addition, these experiments suggest that third-party punishment games are powerful tools for studying the characteristics and the content of social norms. Further experiments indicate that second parties, whose economic payoff is reduced by the norm violation, punish the violation much more strongly than do third parties.

Women vary with respect to monogamous/polyandrous inclinations, as indexed by the Sociosexual Orientation Inventory (SOI). Possible sources of SOI variation include variation in perceptions relating to the utility of different mating tactics and variation in one's degree of masculinity/femininity, among other things. In three studies with undergraduate participants SOI, an index of self-perceived attractiveness and two measures of masculinization, namely scores on the Vandenberg Mental Rotation test (V-MRT) and 2D:4D digit ratios, were measured. Self-perceived attractiveness predicted SOI in the first study, but not in the second study. Right 2D:4D did predict SOI in the second study. In the third study, both self-perceived attractiveness and right 2D:4D predicted SOI, and so did V-MRT scores. However, the strongest single predictor of SOI in Study 3 was the reported amount spent on alcohol during the average month.

Several studies have reported a positive association between degree of facial symmetry and attractiveness ratings, although the actual causes of the development of facial asymmetries remains to be confirmed. The current study hypothesizes that early hormone levels may play a crucial role in the development of facial asymmetries. Recent evidence suggests that the relative length of the second to fourth finger (2D:4D) is negatively related to prenatal testosterone and positively related to prenatal estrogen and may thus serve as a window to the prenatal hormonal environment. We measured 2D:4D in a sample of male and female college students and analysed their faces for horizontal asymmetries. 2D:4D was significantly negatively related to facial asymmetry in males, whereas in females facial asymmetry was significantly positively related to 2D:4D. We suggest that digit ratio may thus be considered as a pointer to an individual's developmental instability and stress through its association with prenatal sexual steroids.

The proposition that universal standards of female beauty reflect adaptations for reproductive value assessment does not preclude cross-cultural variation that is contingent on local environmental variation. Cross-cultural tests of the hypothesis that men have adaptations generating preference for low female waist-to-hip ratios (WHR) have used stimuli that were not scaled to local conditions, and have confounded WHR with level of body fat. I present a reassessment of the WHR hypothesis, showing that when effects of WHR and body weight are less confounded, and local environmental context is taken into account, it appears that Shiwiar forager–horticulturist men of Ecuadorian Amazonia may use both WHR and body weight in assessments of female sexual attractiveness in a manner consistent with the prediction of a context-sensitive preference psychology.

Skin pigment may be related to mate choice, marriage systems, resistance to microorganisms, and photoprotection. Here we use second to fourth digit ratio (2D:4D) to disentangle the relationships among these variables. There is evidence that 2D:4D is negatively associated with prenatal testosterone and positively with prenatal oestrogen. We show (i) a negative association between skin colour and 2D:4D in Caucasian women, but not in men, suggesting that skin colour in women is partly dependent on prenatal oestrogen; and (ii) Caucasian subjects with low 2D:4D reported higher susceptibility to sun-burn, athlete's foot and eczema than subjects with high 2D:4D, suggesting that prenatal testosterone increases susceptibility to sunburn and skin diseases. Frost [Hum. Evol. 9 (1994) 141] has reported that with latitude controlled, highly polygynous peoples have relatively dark and monogamous peoples relatively light skin. We suggest that polygynous populations incur selection for high prenatal testosterone and low prenatal oestrogen because of competition among men for wives. Such groups have low 2D:4D, and high susceptibility to sunburn and skin infections which may result from the immunosuppressive effects of prenatal testosterone. Where polygynous groups are found at low latitudes, they have evolved dark skin for protection against UV and microorganisms. More monogamous peoples experience selection for low prenatal testosterone and high prenatal oestrogen as a result of mate choice for light-skinned oestrogenised women. Such groups have high 2D:4D, resistance to sunburn and skin infections, and light skin. The association between very dark skin and low latitude exists only when polygynous societies are found at low latitudes, as is common in sub-Saharan Africa, but not in the New World.

Fluctuating asymmetry (FA), a pattern of bilateral variation that is normally distributed around a mean of zero, appears to correlate inversely with fitness and health. In this study, we compared the FA of asymptomatic control subjects (n=51) and patients with low back pain (n=44). We measured eight traits, from the upper and lower limbs, and used them to obtain asymmetry indices for each subject. We also measured pelvic asymmetry in standing subjects. The low back pain (LBP) group showed significantly higher asymmetry in the pelvis, and in ulnar length and bistyloid breadth. Our results demonstrate a link between LBP and asymmetry not only in a weight-bearing trait (i.e., pelvic configuration), but in two traits that are not functionally related to the back (i.e., ulnar length and bistyloid breadth). We can now consider LBP as another health and fitness measure correlated with FA.

Studies have shown that male faces high in symmetry are judged more attractive than faces low in symmetry even in images where visual cues to facial symmetry are reduced. These findings suggest that there are correlates of facial symmetry that influence male facial attractiveness independently of symmetry itself. Apparent healthiness of facial skin is one factor that may influence male facial attractiveness and covary with facial symmetry. Here, using real and composite male faces, we found that males with symmetric faces were perceived as having healthier facial skin than males with relatively asymmetric faces (Study 1), and that facial colour and texture cues were sufficient to maintain an attractiveness–symmetry relationship when the influence of facial shape was minimised (Study 2). These findings suggest that colour and texture cues contribute to the relationship between attractiveness and symmetry in real faces.

Societies in which women have substantial control of resources and hold powerful political positions are relatively rare. Among the many circumstances in which women are likely to have resource control and/or political authority, polygyny is not an obvious candidate. However, women's lives are highly variable across polygynous societies. We hypothesized that within polygynous societies, women will have greater resource control and political activity when they have female allies; furthermore, that ecological factors influence women's access to such allies. We examined statistical associations among measures of ecological factors, the presence of female allies, and female power. The results of multiple regression analyses of ethnographic materials demonstrate that, cross culturally, ecological and sociocultural factors interact so that polygynously married women have more resource control and power when they are geographically close to their kin and have sisters as cowives. Additional statistical associations reveal how ecological factors moderate women's access to potential allies, which in turn are associated with resource control, female power/authority, and prevalence of negative attitudes about appropriate female behavior.

Physical competition is widespread in human societies. Because performance in competitive sports can signal phenotypic quality and fighting ability, high level performance, especially on the part of men, is likely to be attractive to the opposite sex. We investigated the relationship between involvement in competitive sport and self-reported numbers of sexual partners. Both male and female students who compete in sports reported significantly higher numbers of partners than other students, and within the athletes, higher levels of performance predicted more partners. Among men, body mass index (BMI) and educational level also had significant effects. We discuss possible implications for the evolution of competitive sport, ritual fighting behavior, and the persistence of left-handedness.