Copyright © 1999 Massachusetts Institute of Technology
Evolutionary psychology is evolutionary biology applied to the brain's adaptations. An adaptation is a phenotypic feature, psychological or otherwise, whose ultimate cause is some type of historical Darwinian selection (Thornhill 1997). Genes, physiology, development, and environment are proximate causes of each adaptation. Because adaptations are the products of past selection, they exhibit functional or purposeful design. Evolutionary psychology's focus is on identifying and characterizing psychological adaptations, which are functionally designed for processing information about survival and reproductive success in human evolutionary history.
Humans make aesthetic judgments in numerous domains, and for theoretical and empirical reasons, these judgments are viewed as reflecting domain-specific psychological adaptations, not general-purpose ones (Thornhill 1998). Thus, it is proposed that there is special-purpose adaptation for assessing sexual attractiveness, and further, that in this domain are numerous psychological adaptations, each functionally designed for assessing a component of attractiveness that corresponded to a marker of the reproductive value of an individual to the mate chooser during human evolutionary history. Aesthetic judgments in the domain of sexual attraction are sometimes sex specific because males and females, consistently throughout human evolutionary history, faced sex-specific adaptive problems in mate selection, and therefore have sex-specific psychological adaptation. For example, women value resources and status of potential mates more, and physical attractiveness and youth in a potential mate less, than men do (Symons 1979; Grammer 1993; Buss 1994).
Cross-cultural research has shown that although men place more value on physical attractiveness of a mate than women do, both sexes value it highly (Grammer 1993; Buss 1994; cf. HUMAN UNIVERSALS). Physical beauty is a health certification (Thornhill and Gangestad 1993; Symons 1995). In human evolutionary history, individuals who saw as sexually attractive body traits of health outreproduced other individuals because the formers' preferred mates survived better, had genetic health, and were better able to provide investment. Two important categories of physical beauty traits identified in recent research are the hormone markers and developmental stability/ bilateral symmetry (Thornhill and Gangestad 1996), which are the focus of the rest of this article.
The adult human form is an array of sex-specific, sex hormone-mediated secondary sexual traits that signal health. These traits apparently evolved for the reason that health signalers outreproduced less healthy individuals, because the most healthy were preferred mates. Men's and women's faces differ in the size of the lower face. Largeness in the lower face is attractive in men's faces, whereas smallness is attractive in women's faces (Symons 1995; Thornhill and Gangestad 1996). High estrogen at puberty caps the growth of the adult female face. Testosterone facilitates the sex difference in muscle mass, body size, and athleticism, and largeness and athleticism are attractive in men. Estrogen facilitates the redistribution and increased deposition of fat on the bodies of females at adolescence, giving rise to the relatively small waist and large hips in women. Low waist-to-hip ratio in women connotes hormonal health, fertility, youth and relative freedom from a diversity of diseases (Singh 1993).
The hormone-facilitated facial features and the other hormone markers mentioned not only display information about an individual's hormonal health, but apparently also his/her immunocompetence, because both sex hormones are immunosuppressors (Thornhill and Gangestad 1993). Attractive expressions of secondary sexual traits require high hormone titers during their construction, and thus can be afforded only by immunocompetent individuals. Disease organisms are important in human attractiveness (reviewed in Thornhill and Møller 1997). For example, the value placed on physical attractiveness in choice of a long-term mate in each sex correlates positively with the prevalence of parasitic diseases across human societies (Gangestad and Buss 1993).
Developmental stability connotes developmental health in humans (Thornhill and Møller 1997). Developmental stability occurs when the adaptive developmental trajectory is achieved despite environmental and genetic perturbations during development. Developmental instability is most often measured as fluctuating asymmetry because fluctuating asymmetry is a highly sensitive measure of developmental disturbance. Fluctuating asymmetry is deviation from perfect bilateral symmetry in normally bilaterally symmetrical traits. These deviations are random in direction, typically small in any one trait, and found in every individual to some degree. Fluctuating asymmetry may be the best measure available of phenotypic and genetic quality of the individual. It reflects the individual's ability to resist environmental (e.g., parasites, low food quantity and quality, environmental toxins) and genetic perturbations during development, or an absence of genes that disrupt development (Møller and Swaddle 1997). Accordingly, low fluctuating asymmetry is associated in a wide range of species with rapid growth rate, reduced parasitism, longevity, fecundity, and sexual attractiveness (Møller and Swaddle 1997). It also shows significant heritability across species (Møller and Thornhill 1997).
Symmetry is a component of sexual attractiveness in humans. In both sexes, faces with high bilateral symmetry are more attractive than less symmetrical faces (e.g., Grammer and Thornhill 1994; review of studies in Møller and Thornhill 1998). Also, symmetry in body features such as fingers, elbows, ankles, and feet correlates with facial symmetry and attractiveness (Gangestad and Thornhill 1997). Interestingly, facial symmetry in each sex correlates with the sex-specific attractive expression of the facial secondary sex traits (e.g., symmetry positively correlates with lower face size in men and negatively with lower face size in women; Gangestad and Thornhill 1997). Moreover, women with symmetrical breasts report higher age-independent fertility (number of children) and earlier onset of child bearing, and breast-size symmetry positively affects women's attractiveness (Møller, Soler, and Thornhill 1995; Singh 1995; Manning et al. 1997).
Numerous studies show that nonfacial body symmetry of men positively correlates with their mating success. This pattern is not seen in women. Compared to men with high body fluctuating asymmetry, men with low asymmetry report having had more sex partners and more extra-pair-bond sex partners, are chosen as extra-pair partners more often by women in committed relationships, and begin sex earlier in their life history and in their romantic relationships (Thornhill and Gangestad 1994; Gangestad and Thornhill 1997). Also, the mates of symmetrical men show the most reported copulatory orgasms (Thornhill et al. 1995). The female copulatory orgasm seems to be a female choice adaptation. It increases sperm retention and may be involved in selective sperm retention (Baker and Bellis 1995). Also, it may affect selective bonding as a result of associated oxytocin (Thornhill et al. 1995). Finally, there is evidence that secondary sexual traits of the body in men such as musculature, athleticism, and body size correlate positively with their body symmetry (Gangestad and Thornhill 1997).
Social psychology had demonstrated earlier in research beginning in the 1960s the profound importance of physical attractiveness in human everyday life (e.g. Jackson 1992). Only relatively recently have evolutionary psychologists asked why the importance in the first place and what kinds of body features are expected to play central roles given the nature of the evolutionary process. Physical attractiveness signals the individual's ability to cope with stresses from reproductive hormones during puberty and adolescence, from disease organisms, and from the many environmental and genetic perturbations that throw off the development of bilateral symmetry. This is attractiveness described in terms of evolved human preferences. How these preferences interact with other criteria of sexual and romantic interest (e.g., status, age, need of investment) and through compromises or trade-off generate actual romantic choices is under investigation (Cunningham, Druen, and Barbee 1997; Gangestad and Thornhill 1997; Graziano et al. 1997).
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