Randy Thornhill

The evolution of human sexuality

The study of human sexuality from the darwinian perspective is in an explosive phase. Recent research is diverse; for instance, the dynamics of heterosexual relationships, the role of honest advertisement in attractiveness, the role of fluctuating asymmetry in sexual competition, and sexual conflict over fertilization, seen in sperm competition adaptations of men and possible cryptic sire-choice adaptation of women. Also, recent research reveals that the sexual selection that designed human secondary sexual traits was functional rather than strictly fisherian.

Human (Homo sapiens) facial attractiveness and sexual selection: the role of symmetry and averageness.

We hypothesized from the parasite theory of sexual selection that men (Homo sapiens) would prefer averageness and symmetry in womens faces, that women would prefer averageness and symmetry in mens faces, and that women would prefer largeness (not averageness) of the secondary sexual traits of mens faces. We generated computer images of mens and womens faces and of composites of the faces of each sex, and then had men and women rate opposite-sex faces for 4 variables (attractive, dominant, sexy, and healthy). Symmetry, averageness, and the sizes of facial features were measured on the computerized faces. The hypotheses were supported, with the exception of the hypothesized effects of averageness of female and male faces on attractiveness ratings. This is the first study to show that facial symmetry has a positive influence on facial attractiveness ratings.

CRYPTIC FEMALE CHOICE AND ITS IMPLICATIONS IN THE SCORPIONFLY HARPOBITTACUS NIGRICEPS

Female choice is a poorly understood area of evolutionary biology. Mate choice theory and the generally greater control of the process of reproduction by females than by males imply that female choice is prevalent in organisms and that it may be subtle in its nature, occurring even after mating and at any time until the females control of gametes or zygotes terminates. Female choice in the Australian scorpionfly Harpobittacus nigriceps, a species in which males provide females with a nuptial arthropod gift during mating, was studied in the field and laboratory. Female choice before mating occurs in H. nigriceps and there is considerable evidence of its occurrence during and after mating in this species. Females apparently choose during mating by controlling mating duration and thus the number of sperm received from different mates. Postmating choice in H. nigriceps appears to include a females ability to regulate egg laying in relation to characteristics of her mate. Female choice during and after mating in H. nigriceps almost certainly leads to stronger sexual selection on males than premating choice. All forms of female choice in H. nigriceps would result in directional selection of the criteria important in choice (male body size and its correlate, ability to obtain large prey). Female choice on the basis of body size in H. nigriceps may enhance female fitness if body size is heritable: Large individuals obtain more matings (males) and are more fecund (females) than small individuals, and, in general, survival and size are positively related in both sexes. Fluctuating natural selection pressures on body size within generations may generate enough genetic variation in male body size to maintain adaptive female choice in H. nigriceps.

Human facial beauty

It is hypothesized that human faces judged to be attractive by people possess two features—averageness and symmetry—that promoted adaptive mate selection in human evolutionary history by way of production of offspring with parasite resistance. Facial composites made by combining individual faces are judged to be attractive, and more attractive than the majority of individual faces. The composites possess both symmetry and averageness of features. Facial averageness may reflect high individual protein heterozygosity and thus an array of proteins to which parasites must adapt. Heterozygosity may be an important defense of long-lived hosts against parasites when it occurs in portions of the genome that do not code for the essential features of complex adaptations. In this case heterozygosity can create a hostile microenvironment for parasites without disrupting adaptation. Facial bilateral symmetry is hypothesized to affect positive beauty judgments because symmetry is a certification of overall phenotypic quality and developmental health, which may be importantly influenced by parasites. Certain secondary sexual traits are influenced by testosterone, a hormone that reduces immunocompetence. Symmetry and size of the secondary sexual traits of the face (e.g., cheek bones) are expected to correlate positively and advertise immunocompetence honestly and therefore to affect positive beauty judgments. Facial attractiveness is predicted to correlate with attractive, nonfacial secondary sexual traits; other predictions from the view that parasite-driven selection led to the evolution of psychological adaptations of human beauty perception are discussed. The view that human physical attractiveness and judgments about human physical attractiveness evolved in the context of parasite-driven selection leads to the hypothesis that both adults and children have a species-typical adaptation to the problem of identifying and favoring healthy individuals and avoiding parasite-susceptible individuals. It is proposed that this adaptation guides human decisions about nepotism and reciprocity in relation to physical attractiveness.

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

Evidence suggests that female sexual preferences change across the menstrual cycle. Womens 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 womens olfactory preferences for mens scent would tend to favour the scent of more symmetrical men, most notably during the womens 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 mens scent. A separate analysis revealed that, within the set of normally cycling women, individual womens 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.

Pathogen prevalence predicts human cross-cultural variability in individualism/collectivism

Pathogenic diseases impose selection pressures on the social behaviour of host populations. In humans (Homo sapiens), many psychological phenomena appear to serve an antipathogen defence function. One broad implication is the existence of cross-cultural differences in human cognition and behaviour contingent upon the relative presence of pathogens in the local ecology. We focus specifically on one fundamental cultural variable: differences in individualistic versus collectivist values. We suggest that specific behavioural manifestations of collectivism (e.g. ethnocentrism, conformity) can inhibit the transmission of pathogens; and so we hypothesize that collectivism (compared with individualism) will more often characterize cultures in regions that have historically had higher prevalence of pathogens. Drawing on epidemiological data and the findings of worldwide cross-national surveys of individualism/collectivism, our results support this hypothesis: the regional prevalence of pathogens has a strong positive correlation with cultural indicators of collectivism and a strong negative correlation with individualism. The correlations remain significant even when controlling for potential confounding variables. These results help to explain the origin of a paradigmatic cross-cultural difference, and reveal previously undocumented consequences of pathogenic diseases on the variable nature of human societies.

FACIAL ATTRACTIVENESS, SYMMETRY AND CUES OF GOOD GENES

Cues of phenotypic condition should be among those used by women in their choice of mates. One marker of better phenotypic condition is thought to be symmetrical bilateral body and facial features. However, it is not clear whether women use symmetry as the primary cue in assessing the phenotypic quality of potential mates or whether symmetry is correlated with other facial markers affecting physical attractiveness. Using photographs of mens faces, for which facial symmetry had been measured, we found a relationship between womens attractiveness ratings of these faces and symmetry, but the subjects could not rate facial symmetry accurately. Moreover, the relationship between facial attractiveness and symmetry was still observed, even when symmetry cues were removed by presenting only the left or right half of faces. These results suggest that attractive features other than symmetry can be used to assess phenotypic condition. We identified one such cue, facial masculinity (cheek–bone prominence and a relatively longer lower face), which was related to both symmetry and full– and half–face attractiveness.

Bilateral Symmetry and Sexual Selection: A Meta-Analysis

A considerable body of primary research has accumulated over the last 10 yr testing the relationship between developmental instability in the form of fluctuating asymmetry and performance of individuals in mating success itself or sexual attractiveness. This research comprises 146 samples from 65 studies of 42 species of four major taxa. We present the results of a meta‐analysis of these studies, which demonstrates that there is indeed an overall significant, moderate negative relationship: for studies, the overall mean Pearsons r or effect size = −.42, P <.0005; for species, the overall mean r = −.34, .01 < P < .025. Based on calculated fail‐safe numbers, the effect‐size estimates are highly robust against any publication or reporting bias that may exist. There is considerable evidence that the magnitude of the negative correlation between fluctuating asymmetry and success related to sexual selection is greater for males than for females, when a secondary sexual trait rather than an ordinary trait is studied, with experimentation compared with observation, and for traits not involved with mobility compared with traits affecting mobility. There is also limited evidence that higher taxa may differ in effect size and that intensity of sexual selection negatively correlates with effect size.

Darwinian aesthetics: sexual selection and the biology of beauty

Current theoretical and empirical findings suggest that mate preferences are mainly cued on visual, vocal and chemical cues that reveal health including developmental health. Beautiful and irresistible features have evolved numerous times in plants and animals due to sexual selection, and such preferences and beauty standards provide evidence for the claim that human beauty and obsession with bodily beauty are mirrored in analogous traits and tendencies throughout the plant and animal kingdoms. Human beauty standards reflect our evolutionary distant and recent past and emphasize the role of health assessment in mate choice as reflected by analyses of the attractiveness of visual characters of the face and the body, but also of vocal and olfactory signals. Although beauty standards may vary between cultures and between times, we show in this review that the underlying selection pressures, which shaped the standards, are the same. Moreover we show that it is not the content of the standards that show evidence of convergence ‐ it is the rules or how we construct beauty ideals that have universalities across cultures. These findings have implications for medical, social and biological sciences.

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

Abstract 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 mens hygiene and other factors that were not controlled in the first study. The current study also examined womens 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. Womens preference for the scent associated with mens 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.