Genetic dissimilarity,genetic diversity,and mate preferences in humans

It is clear that genes at the major histocompatibility complex (MHC) are involved in mate preferences in a range of species, including humans. However, many questions remain regarding the MHC's exact influence on mate preference in humans. Some research suggests that genetic dissimilarity and individual genetic diversity (heterozygosity) at the MHC influence mate preferences, but the evidence is often inconsistent across studies. In addition, it is not known whether apparent preferences for MHC dissimilarity are specific to the MHC or reflect a more general preference for genome-wide dissimilarity, and whether MHC-related preferences are dependent on the context of mate choice (e.g., when choosing a short-term and long-term partner). Here, we investigated whether preferences for genetic dissimilarity are specific to the MHC and also whether preferences for genetic dissimilarity and diversity are context dependent. Genetic dissimilarity (number of alleles shared) influenced male, but not female, partner preferences, with males showing a preference for the faces of MHC-dissimilar females in both mating contexts. Genetic diversity [heterozygosity (H) and standardized mean (d²)] influenced both male and female preferences, regardless of mating context. Females preferred males with greater diversity at MHC loci (H) and males preferred females with greater diversity at non-MHC loci (d²) in both contexts. Importantly, these findings provide further support for a special role of the MHC in human sexual selection and suggest that male and female mate preferences may work together to potentially enhance both male and female reproductive success by increasing genetic diversity in offspring.     

More than skin deep: Major histocompatibility complex (MHC)-based attraction among Asian American speed-daters

What might explain our instinctual attraction to certain individuals, aside from visible factors such as appearance? We examined possible biologically-driven selection for immunology genes, specifically preferences for Major Histocompatibility Complex (MHC)-dissimilarity, through the ecologically-valid method of speed-dating. Two-hundred-and-sixty-two single Asian Americans went on speed-dates (N observations?=?2215) with participants of the other sex, making second date offers and rating each other on measures of mate desirability, facial attractiveness, and body scent attractiveness. Using a single nucleotide polymorphism (SNP) analysis, women, but not men, showed preferences for speed-dating partners based on MHC-complementarity. The direction of findings varied by single nucleotide polymorphism (SNP), such that SNPs closer to the major HLA (Human Leukocyte Antigen) genes supported dissimilarity preferences, whereas those farther away supported similarity preferences. The relative effects of MHC-based measures in comparison to an array of behavioral predictors were examined via random forests. Results indicated that for both men and women, the importance of MHC-based indices was comparable to that of a partner's self-reported personality attributes in predicting second date offers.     

Common HLA alleles associated with health, but not with facial attractiveness

Three adaptive hypotheses have been proposed to explain the link between the human leucocyte antigen (hla) genes, health measures and facial attractiveness: inbreeding avoidance, heterozygote advantage and frequency-dependent selection. This paper reports findings that support a new hypothesis relating HLA to health. We suggest a new method to quantify the level of heterozygosity. HLA heterozygosity did not significantly predict health measures in women, but allele frequency did. Women with more common HLA alleles reported fewer cold and flu bouts per year, fewer illnesses in the previous year and rated themselves healthier than women with rare alleles. To our knowledge, this is the first study to show a positive correlation between HLA allele frequency and general health measures. We propose that certain common HLA alleles confer resistance to prevalent pathogens. Nevertheless, neither HLA heterozygosity nor allele frequency significantly predicted how healthy or attractive men rated the female volunteers. Three non-mutually exclusive explanations are put forward to explain this finding.     

The Scent of Symmetry: A Human Sex Pheromone that Signals Fitness?

A previous study by the authors showed that the body scent of men who have greater body bilateral symmetry is rated as more attractive by normally ovulating (non-pill-using) women during the period of highest fertility based on day within the menstrual cycle. Women in low-fertility phases of the cycle and women using hormone-based contraceptives do not show this pattern. The current study replicated these findings with a larger sample and statistically controlled for men's hygiene and other factors that were not controlled in the first study. The current study also examined women's scent attractiveness to men and found no evidence that men prefer the scent of symmetric women. We propose that the scent of symmetry is an honest signal of phenotypic and genetic quality in the human male, and chemical candidates are discussed. In both sexes, facial attractiveness (as judged from photos) appears to predict body scent attractiveness to the opposite sex. Women's preference for the scent associated with men's facial attractiveness is greatest when their fertility is highest across the menstrual cycle. The results overall suggest that women have an evolved preference for sires with good genes.     

Men's facial masculinity predicts changes in their female partners' sexual interests across the ovulatory cycle,whereas men's intelligence does not

A substantial body of work demonstrates that women's mate preferences change across the ovulatory cycle. When fertile in their cycles, women are especially attracted to masculine features (e.g., faces, voices, bodies), socially dominant behavior, and male scents associated with body symmetry and social dominance. Women may also find intelligent men particularly attractive when fertile, though findings are mixed. Related research shows that, on average, romantically-involved women report stronger sexual attraction to men other than their pair-bond partners, but not partners, when fertile, and especially when their partners lack features fertile women prefer (e.g., symmetry). In the current study, we examined whether women's patterns of sexual interests across the cycle are similarly moderated by partners' facial masculinity, facial attractiveness, and intelligence. Results revealed predicted effects of male partners’ facial masculinity but none for partners’ intelligence. Facial attractiveness may have effects, but we find no evidence that it does so independently of facial masculinity.     

MHC-assortative facial preferences in humans

Individuals tend to choose mates who are sufficiently genetically dissimilar to avoid inbreeding. As facial attractiveness is a key factor in human mate preference, we investigated whether facial preferences were related to genetic dissimilarity. We asked female volunteers to rate the attractiveness of men from photographs and compared these results with individual genotypes at the major histocompatibility complex (MHC). In contrast to previously reported preferences based on odour, we found a non-significant tendency for women to rate MHC-similar faces as more attractive, suggesting a preference for cues to a self-similar MHC in faces. Further analysis revealed that male faces received higher attractiveness scores when rated by women who were MHC-similar than by MHC-dissimilar women. Although unexpected, this MHC-similar facial preference is consistent with other studies documenting assortative preferences in humans, including for facial phenotype.     

Fertility in the cycle predicts women's interest in sexual opportunism

Research over the past decade has documented clear, robust changes in women's sexual preferences and interests across the ovarian cycle. When fertile, women are particularly attracted to a number of masculine male features (e.g., masculine faces, voices, scents and bodies) and other traits, and especially when they evaluate men's “sexiness” rather than their attractiveness as long-term partners. The current research extended this line of research by examining changes in women's self-reported sexual interests across the cycle. We asked 68 normally ovulating women in committed romantic relationships to fill out questionnaires about their sexual preferences and interests (at that time, not in general) twice across their cycles: once when fertile and once during the luteal phase. Relative to during the luteal phase, fertile women expressed (a) greater emphasis on the physical attractiveness of a partner; (b) greater arousal at the sight or thought of attractive male bodily features; (c) greater willingness to engage in and interest in sex with attractive men, even ones who they do not know well (interest in sexual opportunism). These findings importantly extend our understanding of women's fertile-phase sexuality.     

Male steroid hormones and female preference for male body odor

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.     

Head Tilt and Fertility Contribute to Different Aspects of Female Facial Attractiveness

Subjective attractiveness ratings of facial portraits of women taken at the fertile phase of the menstrual cycle are higher than those of portraits of the same women taken during non‐fertile periods. As female faces tilted downward are rated as more attractive and female courtship behaviours change across the menstrual cycle, we investigated whether systematic downward tilt of women's faces during the fertile phase might be responsible for increased attractiveness ratings. In the original study (Proc. R. Soc. Lond. B, 271, 2004, S272), the fertile‐phase portrait of each woman was deemed more attractive in 56–62% of cases. When the portraits were reclassified by head pitch, the more downward‐tilted portrait was preferred in 64–73% of cases. The fertile‐phase portrait was no more likely to be the downward‐tilted one, however, suggesting that effects of fertility on attractiveness are not simply due to changes in head position. We also had these portraits rated (N = 130) for physical attractiveness and behavioural allure. Fertile‐phase portraits were rated as more physically attractive than non‐fertile portraits, while more downward‐tilted portraits were rated as more behaviourally alluring than less downward‐tilted ones. These data not only confirm reported effects of head tilt and fertility on perceived female attractiveness, but also suggest that these factors influence different components of the attractiveness percept.     

How conception risk affects competition and cooperation with attractive women and men

We investigated competition and cooperation for resources across the menstrual cycle in the context of bargaining games. Although bargaining has been studied within an evolutionary framework, little attention has been paid specifically to the role of mating motives in economic behavior. To investigate how motives related to reproductive success affect bargaining, participants at high or low risk for conception or who were on oral contraceptives played ultimatum and dictator games with partners who varied in sex and facial attractiveness. In ultimatum games, women in the fertile phase were more competitive over resources with attractive women than with less attractive women. Intrasexual competition was not observed in dictator games. Women were more cooperative with attractive men than with less attractive men in both games, regardless of fertility status. Low fertility women were more cooperative with attractive members than with less attractive members of both sexes in both games. Results support the view that, during periods of high fertility, when women are most intrasexually competitive for mates, withholding resources from potential rivals would enable women to gain the means to enhance their attractiveness and weaken competitors' abilities to do the same at a time when relative advantages in appearance are most crucial to reproductive success. The lack of a fertility effect for cooperation with potential mates supports the view that displays of generosity accrue benefits for women across the cycle in their efforts to attract men who will invest in relationships.     

Women's evaluations of other women's natural body odor depend on target's fertility status

A large research literature indicates that men perceive women as more attractive when they are at high fertility than at low fertility within the ovulatory cycle. However, it remains unclear whether women also perceive women as more attractive at high fertility. This study examined women's ratings of samples of natural body odor collected from naturally-cycling women at high and low fertility within the cycle and from hormonal contraceptive-using women at mid-cycle. Like men, women rated naturally-cycling women's high-fertility scent samples as more attractive than their low-fertility samples. Women rated hormonal contraceptive (HC) users' scent samples as more attractive than naturally-cycling women's high- and low-fertility samples, though the difference between HC and high-fertility samples was statistically significant only when raters were treated as the unit of analysis. These findings reveal a potentially important role for scent communication in women's perceptions of other women and are consistent with the notion that the ovulatory cycle could influence women's interactions with one another. The findings also highlight the need for rigorous investigations of the possible impacts of hormonal contraception on women's attractiveness and social relationships with other women.     

Menstrual cycle, trait estrogen level, and masculinity preferences in the human voice

Men with low testosterone (feminine men) invest in relationships and offspring more than men with high testosterone (masculine men). Women's attraction to testosterone dependent traits (e.g. masculine face shape) is enhanced during the late-follicular, fertile phase of the menstrual cycle. Attractive, feminine women have stronger preferences for masculine men as possible long-term partners than less attractive, masculine women. We manipulated 2 testosterone related vocal traits (voice pitch and apparent vocal-tract length) in voices to test if women prefer masculinized men's voices to feminized men's voices; masculinity preferences are enhanced at the fertile (late-follicular) menstrual cycle phase; the amount that masculinity preferences shift cyclically relates to average estrone-3-glucuronide concentration (the primary urinary metabolite of estrone, E3G). We found women displayed general masculinity preferences for men's voices; masculinity preferences were greater in the fertile (late-follicular) phase of the cycle than the non-fertile (early-follicular and luteal) phase; and this effect was most pronounced for women with low average E3G concentration. As feminine women (i.e. those with high average E3G levels) are most able to obtain investment even from masculine men, these women may not need to change their mating preference or strategy during the menstrual cycle as much as masculine women.     

Menstrual cycle variation in women's preferences for the scent of symmetrical men.

Evidence suggests that female sexual preferences change across the menstrual cycle. Women''s extra-pair copulations tend to occur in their most fertile period, whereas their intra-pair copulations tend to be more evenly spread out across the cycle. This pattern is consistent with women preferentially seeking men who evidence phenotypic markers of genetic benefits just before and during ovulation. This study examined whether women''s olfactory preferences for men''s scent would tend to favour the scent of more symmetrical men, most notably during the women''s fertile period. College women sniffed and rated the attractiveness of the scent of 41 T-shirts worn over a period of two nights by different men. Results indicated that normally cycling (non-pill using) women near the peak fertility of their cycle tended to prefer the scent of shirts worn by symmetrical men. Normally ovulating women at low fertility within their cycle, and women using a contraceptive pill, showed no significant preference for either symmetrical or asymmetrical men''s scent. A separate analysis revealed that, within the set of normally cycling women, individual women''s preference for symmetry correlated with their probability of conception, given the actuarial value associated with the day of the cycle they reported at the time they smelled the shirts. Potential sexual selection processes and proximate mechanisms accounting for these findings are discussed.     

Mate choice trade-offs and women’s preference for physically attractive men

Researchers studying human sexuality have repeatedly concluded that men place more emphasis on the physical attractiveness of potential mates than women do, particularly in long-term sexual relationships. Evolutionary theorists have suggested that this is the case because male mate value (the total value of the characteristics that an individual possesses in terms of the potential contribution to his or her mate’s reproductive success) is better predicted by social status and economic resources, whereas women’s mate value hinges on signals conveyed by their physical appearance. This pattern may imply that women trade off attractiveness for resources in mate choice. Here I test whether a trade-off between resources and attractiveness seems to be occurring in the mate choice decisions of women in the United States. In addition, the possibility that the risk of mate desertion drives women to choose less attractive men as long-term mates is tested. The results were that women rated physically attractive men as more likely to cheat or desert a long-term relationship, whereas men did not consider attractive women to be more likely to cheat. However, women showed no aversion to the idea of forming long-term relationships with attractive men. Evidence for a trade-off between resources and attractiveness was found for women, who traded off attractiveness, but not other traits, for resources. The potential meaning of these findings, as well as how they relate to broader issues in the study of sex differences in the evolution of human mate choice for physical traits, is discussed.     

MHC,mate choice and heterozygote advantage in a wild social primate

Preferences for mates carrying dissimilar genes at the major histocompatibility complex (MHC) may help animals increase offspring pathogen resistance or avoid inbreeding. Such preferences have been reported across a range of vertebrates, but have rarely been investigated in social species other than humans. We investigated mate choice and MHC dynamics in wild baboons (Papio ursinus). MHC Class II DRB genes and 16 microsatellite loci were genotyped across six groups (199 individuals). Based on the survey of a key segment of the gene‐rich MHC, we found no evidence of mate choice for MHC dissimilarity, diversity or rare MHC genotypes. First, MHC dissimilarity did not differ from random expectation either between parents of the same offspring or between immigrant males and females from the same troop. Second, female reproductive success was not influenced by MHC diversity or genotype frequency. Third, population genetic structure analysis revealed equally high genotypic differentiation among troops, and comparable excess heterozygosity within troops for juveniles, at both Mhc‐DRB and neutral loci. Nevertheless, the age structure of Mhc‐DRB heterozygosity suggested higher longevity for heterozygotes, which should favour preferences for MHC dissimilarity. We propose that high levels of within‐group outbreeding, resulting from group‐living and sex‐biased dispersal, might weaken selection for MHC‐disassortative mate choice.     

Are there vocal cues to human developmental stability? Relationships between facial fluctuating asymmetry and voice attractiveness

Fluctuating asymmetry (FA), deviation from perfect bilateral symmetry, is thought to reflect an organism's relative inability to maintain stable morphological development in the face of environmental and genetic stressors. Previous research has documented negative relationships between FA and attractiveness judgments in humans, but scant research has explored relationships between the human voice and this putative marker of genetic quality in either sex. Only one study (and in women only) has explored relationships between vocal attractiveness and asymmetry of the face, a feature-rich trait space central in prior work on human genetic quality and mate choice. We therefore examined this relationship in three studies comprising 231 men and 240 women from two Western samples as well as Hadza hunter–gatherers of Tanzania. Voice recordings were collected and rated for attractiveness, and FA was computed from two-dimensional facial images as well as, for a subset of men, three-dimensional facial scans. Through meta-analysis of our results and those of prior studies, we found a negative association between FA and vocal attractiveness that was highly robust and statistically significant whether we included effect sizes from previously published work, or only those from the present research, and regardless of the inclusion of any individual sample or method of assessing FA (e.g., facial or limb FA). Weighted mean correlations between FA and vocal attractiveness across studies were ?.23 for men and ?.29 for women. This research thus offers strong support for the hypothesis that voices provide cues to genetic quality in humans.     

Complexity and context of MHC-correlated mating preferences in wild populations

There is now substantial and growing evidence for a role of the major histocompatibility complex (MHC) in shaping individual mate preferences. In view of both its codominant expression and its function in immune response, it is often expected that females aim to avoid inbreeding or maximize offspring MHC-heterozygosity by selecting as mates those males which share fewest or no MHC alleles with themselves. However, it is becoming increasingly clear that this view is over-simplistic: not only is MHC dissimilarity just one of several (perhaps many) criteria important in mate choice decision-making, extremely MHC-dissimilar males may be avoided, and furthermore, specific alleles or combinations might be preferred if they bestow particular advantages. These points are raised in two papers in this Molecular Ecology issue, in which patterns of reproductive success in tiger salamanders ( Ambystoma tigrinum ) and three-spined sticklebacks ( Gasterosteus aculeatus ) are each inconsistent with a generalized preference for MHC dissimilarity. Together, these studies demonstrate several adaptive reasons for decision rules that do not necessarily result in maximizing mate dissimilarity.     

Body odor attractiveness as a cue of impending ovulation in women: evidence from a study using hormone-confirmed ovulation

Scent communication plays a central role in the mating behavior of many nonhuman mammals but has often been overlooked in the study of human mating. However, a growing body of evidence suggests that men may perceive women's high-fertility body scents (collected near ovulation) as more attractive than their low-fertility body scents. The present study provides a methodologically rigorous replication of this finding, while also examining several novel questions. Women collected samples of their natural body scent twice--once on a low-fertility day and once on a high-fertility day of the ovulatory cycle. Tests of luteinizing hormone confirmed that women experienced ovulation within two days of their high-fertility session. Men smelled each woman's high- and low-fertility scent samples and completed discrimination and preference tasks. At above-chance levels, men accurately discriminated between women's high- and low-fertility scent samples (61%) and chose women's high-fertility scent samples as more attractive than their low-fertility scent samples (56%). Men also rated each scent sample on sexiness, pleasantness, and intensity, and estimated the physical attractiveness of the woman who had provided the sample. Multilevel modeling revealed that, when high- and low-fertility scent samples were easier to discriminate from each other, high-fertility scent samples received even more favorable ratings compared with low-fertility scent samples. This study builds on a growing body of evidence indicating that men are attracted to cues of impending ovulation in women and raises the intriguing question of whether women's cycling hormones influence men's attraction and sexual approach behavior.     

The role of facial hair in women's perceptions of men's attractiveness,health, masculinity and parenting abilities

Facial hair strongly influences people's judgments of men's socio-sexual attributes. However, the nature of these judgments is often contradictory. The levels of intermediate facial hair growth presented to raters and the stage of female raters' menstrual cycles might have influenced past findings. We quantified men's and women's judgments of attractiveness, health, masculinity and parenting abilities for photographs of men who were clean-shaven, lightly or heavily stubbled and fully bearded. We also tested the effect of the menstrual cycle and hormonal contraceptive use on women's ratings. Women judged faces with heavy stubble as most attractive and heavy beards, light stubble and clean-shaven faces as similarly less attractive. In contrast, men rated full beards and heavy stubble as most attractive, followed closely by clean-shaven and light stubble as least attractive. Men and women rated full beards highest for parenting ability and healthiness. Masculinity ratings increased linearly as facial hair increased, and this effect was more pronounced in women in the fertile phase of the menstrual cycle, although attractiveness ratings did not differ according to fertility. Our findings confirm that beardedness affects judgments of male socio-sexual attributes and suggest that an intermediate level of beardedness is most attractive while full-bearded men may be perceived as better fathers who could protect and invest in offspring.     

Results of an explorative empirical study on human mating in Germany: handsome men, not high-status men, succeed in courtship

Recent research on human mating depicts men as searching for physical attractiveness (PA) and women as searching for status. To identify the mechanisms which lead to universal, biologically interpretable structures in social processes, we focused on the proximate causes for inter- and intrasexual differences in human mating preferences, attraction, and tactics. We collected data on 180 young singles (mean age 26.9 years) without a steady relationship. A questionnaire and a video sequence (20-30 seconds) of each subject was taken. Next, each video sequence was rated by approximately 20 individuals of the opposite sex, who also participated in this study. Surprisingly, the answers given by male and female subjects regarding sociosexual behaviour and mating preferences are predominantly congruent. Sex differences among preferences for good looking and high-status partners were small or even insignificant. Lower educated subjects had considerably higher status preferences than higher educated individuals. In both sexes, PA was much more preferred in a potential partner than status. For both sexes, physical appearance was decisive for the subject's dating attractiveness. Male, but not female dating attractiveness also correlates with a kind and charismatic appearance. Furthermore, there was a positive linear relationship between men's PA and their number of sexual partners within the last year. Men with more than four sexual partners were all above-average in PA, while the most attractive women had a medium number of sexual partners. However, in this respect, status had no influence. The results show that sex differences in mating are more complex than hitherto assumed.