Sexual selection can cause evolution in traits that affect mating success, and it has thus been implicated in the evolution of human physical and behavioural traits that influence attractiveness. We use a large sample of identical and nonidentical female twins to test the prediction from mate choice models that a trait under sexual selection will be positively genetically correlated with preference for that trait. Six of the eight preferences we investigated, i.e. height, hair colour, intelligence, creativity, exciting personality, and religiosity, exhibited significant positive genetic correlations with the corresponding traits, while the personality measures ‘easy going’ and ‘kind and understanding’ exhibited no phenotypic or genetic correlation between preference and trait. The positive results provide important evidence consistent with the involvement of sexual selection in the evolution of these human traits.
Sexual selection is an evolutionary process generated by individual differences in number and identity of mates (Andersson, 1994). These differences can result from interactions between individuals of the same-sex (intrasexual selection; e.g. male–male competition) or opposite-sex (intersexual selection; i.e. mate choice). Here we focus on the latter, in which heritable traits in one sex are subject to selection pressure from mate preferences of the opposite sex.
Humans are highly selective when choosing a partner, and mate choices are based on a variety of traits providing cues to quality or compatibility (Buss & Barnes, 1986). These attractive traits can potentially affect reproduction, and it has been suggested that the consequent sexual selection has played an important role in shaping our physical and behavioural characteristics (Darwin, 1859, Darwin, 1871 and Miller, 2000). Sexual selection tends to result in traits that differ greatly between even closely related species (Andersson, 1994 and Darwin, 1871) and also show wide heritable variation within species (Pomiankowski & Moller, 1995). Many defining human characteristics are of known importance in mate selection, differ greatly from those of other apes, and exhibit large heritable variation between individuals, begging investigation into the possible role of sexual selection in their evolution.
Such traits include intelligence, creativity, personality characteristics, and morphological characteristics including body shape and size, and hair colour. As well as differing distinctly from those of apes, these traits exhibit substantial heritable variation within humans (Bouchard and McGue, 2003 and Zietsch et al., 2011), and recent evidence indicates substantial heritable variation in unconstrained mate preferences for these traits (Verweij et al., 2012 and Zietsch et al., 2012). Despite these clues, formulating tests of the historical influence of sexual selection on the traits has proven difficult. However, evolutionary genetics does provide a testable prediction for traits under sexual selection. Given heritable variation in traits and trait preferences, individuals with stronger-than-average preference for a certain trait will tend to choose a mate with above-average values of that trait, with the resulting offspring tending to inherit alleles predisposing to both stronger-than-average trait and stronger-than-average preference. This co-inheritance leads to linkage disequilibrium (i.e. correlated allelic values across loci, and therefore genetic correlation) between a trait and the preference for it. As such, a prediction from mate choice models is that a trait under sexual selection will be positively genetically correlated with preference for that trait (Fuller et al., 2005 and Lande, 1981).
In many animals, apparent sexual displays are only present (or highly exaggerated) in the male, and female preference for these displays drives sexual selection. As such, animal studies have generally tested for a genetic correlation between male display and female preference, since while genes for preference and display are expected to be present in each individual, phenotypic expression of the genes will only be observed in one or the other sex. Using designs such as artificial selection and full-sib/half-sib breeding, these studies have yielded positive genetic correlations between female preferences and corresponding male traits in some studies (of insects and fish; Bakker, 1993, Blows, 1999, Houde, 1994, Simmons and Kotiaho, 2007 and Wilkinson and Reillo, 1994), but not in other studies (of insects, caterpillars, fish, and birds; Allison et al., 2008, Breden and Hornaday, 1994, Lofstedt et al., 1989, Morris et al., 1996, Muhlhauser and Blanckenhorn, 2004, Qvarnstrom et al., 2006, Ritchie et al., 2005 and Zhou et al., 2011). In humans, many of the traits that most contribute strongly to attractiveness are exhibited in both sexes, and the vast majority of traits that can be measured in both sexes (e.g. height, body mass index, personality traits, intelligence) do not exhibit marked genetic sex-limitation—that is, for most human traits, underlying genes express similar effects in males and females (Vink et al., 2012). As such, genetic correlation between trait and preference should be observable within-sex. Only one human study has tested this, using a relatively small sample of female twins to find significant genetic correlation between altruism and preference for altruism in a mate (Phillips, Ferguson, & Rijsdijk, 2010).
In the present study we investigate a range of mate preferences for traits of known salience in human mate selection, along with the corresponding traits themselves, in a large sample of identical and nonidentical female twins. Using bivariate genetic modelling, we test the prediction from sexual selection that each trait preference will be positively genetically correlated with the trait itself.
2. Methods
