Introduction

Substantial evidence supports a general rule of greater emphasis on female physical attractiveness in Homo sapiens. This has been exhaustively documented in Western societies by an intellectual nexus of academic feminists, social psychologists, historians, and sociologists (e.g., Brownmiller, 1984; Bordo, 1993; Travis, Meginnis, and Bardari, 2000; Wolf, 1991). Moreover, cross-cultural studies find that men consistently express stronger preferences for attractive mates than women do (Buss, 1989; Gottschall, Martin, Quish, and Rhea, 2004; for literature reviews see for Buss, 2003; Geary, Vigil, and Byrd-Craven, 2004), and that women feel more anxiety about their physical appearances, spend more time and money enhancing them, and are more likely to suffer from eating disorders (see Etcoff, 1999). The few large-scale cross-cultural studies that directly address gendered attractiveness emphasis coincide with studies of Western populations. For instance, Ford and Beach's (1951) trail-breaking study of sexuality in 190 traditional populations concluded that, “in most societies the physical beauty of the female receives more explicit consideration than does the handsomeness of the male” (p.86). (While Ford and Beach's vague wording, “most societies,” implies cultures where “explicit consideration” was equal or male-skewed, they do not identify any exceptional societies). Similarly, a meta-analysis of several content-analytic studies suggests that print advertising featuring attractive young women in “decorative roles” is longitudinally stable and cross-culturally pervasive (Saad, 2004). Finally, in two distinct, large-scale content analyses of reasonably representative samples of traditional world folk tales, Gottschall and colleagues (2005; in press) reported that information on character attractiveness was much more likely to be conveyed, and repeatedly stressed, if the character was female.

Of course, modern academics were hardly first to sense this phenomenon and to recognize it as a riddle in need of a solution. For example, over-emphasis on female attractiveness has inspired feminist indignation and resistance from the movement's earliest beginnings (see Wollstonecraft, 1792) up to the present (see Wolf, 1991). Similarly, the riddle has consistently excited the interest of evolutionists, from Darwin (1871; for other early thinking see Ellis, 1926; Westermarck, 1921) up to the present (see Dawkins, 1976; Symons, 1979; Buss, 1989; Sugiyama, 2005). In The Descent of Man and Selection in Relation to Sex (1871), Darwin identified a common pattern of sexual selection in which male animals are more competitive, more eager sexually and—most importantly for our purposes–more conspicuous in courtship display, ornamentation, and coloration (also see Andersson, 1994). He noted that humans generally fit this pattern except for one thing: in humans it is the female, not the male, who uses physical attractiveness to “charm,” “excite,” “fascinate,” and “allure” the opposite sex (1871, p.618). In contrast to his treatment of female attractiveness salience in certain bird species (1871, chap. 16), Darwin did not connect the peculiarity of greater emphasis on women's physical attractiveness to men's higher than average parental investment. Rather, Darwin suggested that human females were more attractive, in an absolute sense, because male choice, not female, came to dominate the course of human sexual selection (p.619). And throughout that time, males were selecting, perhaps above all, for female beauty (see chap. 20, esp. p.619).

Modern evolutionists have also been drawn to the riddle. In the wake of Trivers (1972), evolutionists began thinking of sexual selection mainly in terms of parental investment (but see Clutton-Brock and Parker, 1992). Parental investment theory predicts that members of the sex investing less—usually males–will compete for access to the other sex's larger reproductive investment. Therefore, given women's much larger minimal and average parental investment, keener physical attractiveness competition among women is initially problematic. Dawkins's neatly summarized the problem in The Selfish Gene:

As we have seen, it is strongly to be expected on evolutionary grounds that, where the sexes differ, it should be the males who advertise and the females who are drab…. [B]ut, on average, there can be no doubt that in our society the equivalent of the peacock's tail is exhibited by the female, not by the male…. Faced with these facts, a biologist would be forced to suspect that he was looking at a society in which females compete for males, rather than vice versa…. Has the male really become the sought-after sex, the one that is in demand, the sex that can afford to be choosy? If so, why? (1976, pp.164–165; see also Jones, 1996, pp.16–17)

Symons (1979) provided an answer to Dawkins's question, arguing that greater emphasis on female attractiveness—not just in the West but worldwide—reflects the unusually high variability and detectability of women's reproductive value, especially aspects of reproductive value that are can be accurately assessed on the basis of age. Greater emphasis on women's attractiveness reflects the fact that “a female's reproductive value can be assessed more accurately from her physical appearance than a male's reproductive value can” (p. 201).

Symons's solution was accurate but not complete. This paper seeks to more efficiently isolate the fundamental variables differentiating human patterns of male-female mating preferences from those of most other mammals—patterns which, it is argued, result in more intense emphasis on women's attractiveness. Developing broad trends in the theoretical and empirical literature of animal mate choice (e.g., Kokko, Brooks, Jennions, and Moreley, 2003; Iwasa and Pomiankowski, 1999), it is argued that a species' mate preferences can be simply and reliably predicted on the basis of the relative variance and detectability of reproductively significant traits in the opposite sex. This argument is supported with evidence of correspondences between human mate preference patterns and those of other species with similar patterns of male-female variance and detectability of reproductively significant traits. More specifically, I suggest that the evolutionary riddle of greater emphasis on human female attractiveness is resolved when it is realized that Homo sapiens is a partially sex-role reversed species. While greater emphasis on women's attractiveness is usually approached as a comparative anomaly, male preferences for physically attractive mates (i.e., mates with phenotypes signaling fecundity and/or high genetic quality) are quite common in other partially sex-role reversed species that share, with humans, similar patterns of variance and detectability of reproductively significant traits. In short, this contribution hinges on the argument that greater emphasis on human female attractiveness has been seen as an anomaly mainly because humans have been held up to the wrong zoological comparison groups.

Homo sapiens: A Partially Sex-Role Reversed Species

Over the last twenty-five years a “basic model” of animal mate choice has emerged (for an overview see Bonduriansky, 2001; for the seminal paper see Parker, 1983). The basic model helps identify the factors which, in addition to parental investment, determine degree of choosiness in both sexes as well as patterns of mate preference. The basic model suggests that mating choosiness is influenced by three primary factors: parental investment, mate quality variance, and costliness of choice. In the majority of animal species, especially among mammals, female default investment in reproduction dwarfs male investment, and the familiar pattern of female choice and male competition emerges. Exceptions occur 1) when males invest heavily in reproduction, 2) when variance in female mate quality is high, and/or 3) when the costs of choice for males are atypically low (the costs of choice can include increased opportunity, search, and competition costs). These exceptional species vary along a continuum from partially sex-role reversed species (males both choosy and competitive) to the inverted species featured in Darwin's (1871) and Trivers's (1972) models of sexual selection (for sex-role reversal see also Wallace, 1867; Williams, 1966; Bonduriansky, 2001; Gwynne, 1991; Parker, 1983; Johnstone, Reynolds, and Deutsch, 1996).

Degree of sex-role reversal is an increasing function of male mating choosiness. And, according to comparative studies, variance in female quality may influence male choosiness as much as relative parental investment (see reviews in Bonduriansky, 2001; Johnstone et al., 1996; Andersson, 1994). Moreover, the same studies show that choosy males most commonly discriminate on the basis of phenotypic indicators of female fecundity (see reviews in Bonduriansky, 2001; Andersson, 1994, pp.132–142, 186). This tendency for selective males to prefer the most fecund mates is most pronounced in more polygynous systems. In more monogamous systems, where male reproductive success is more closely linked to the genetic quality of single females, male preferences for fecund mates may be balanced with preferences for mates exhibiting indicators of “good genes.”

Thus when viewed in comparative context, the puzzle of greater emphasis on human female physical attractiveness may not be so puzzling after all. As Darwin seemed to sense (1871, p. 619), humans—with exceptionally high male parental investment (see Alexander and Noonan, 1979) and high variances in female mate quality (more on this below)–represent a relatively straightforward example of a partially sex-role reversed species. The males of partially-reversed species are still competitive, but they are also more sexually discriminating (see Bonduriansky, 2001). Further, in dozens of partially reversed species for which we have data—mainly fishes, insects, and other invertebrates–males discriminate mainly on the basis of “physical attractiveness.” That is, the choosy males of partially sex-role reversed species prefer females who exhibit phenotypic indicators of fecundity and/or–depending where they sit on the monogamy-polygyny continuum–good genes. The apparent correspondence of human patterns of male choice with those relatively commonly encountered in remote taxa, but rarely encountered in mammals, is an evolutionary puzzle in its own right—one that will be addressed in this paper's last section.