Many of the humanities and social sciences have ignored sexual selection. Most theories of human behavior and culture may be inadequate because they may have vastly underestimated the role of sexual competition, courtship, and mate choice in human affairs (Miller, 1998). Sexual selection has two components, intrasexual competition and intersexual selection. Intrasexual competition consists of contests between members of one sex, where the winner of the contest gains preferential access to members of the opposite sex (Buss, 1996). The prototypical example of intrasexual competition is buck deer locking horns in combat during the mating season. The winner of the bout gains the territory and access to the females within it. The loser goes home with a broken horn. This process continues year after year (Darwin, 1871).
Intersexual selection involves both sexes. If members of one sex have a consensus about the desirable characteristics of the opposite sex, then those individuals with the desired characteristics will have a mating advantage. One example is the size and decorative coloration of feathers seen on many male birds. It is hypothesized that the underlying reason why female birds find a male peacock's tail feathers attractive is their indication of the survival ability of male birds. The feathers would not only be noticeable to females, but also to predators and a hindrance in movement, etc. It would seem easier to survive without this costly display, the females of the species usually possess coloration that blends in well with the features of the habitat. The birds that are able to survive with this handicapping display must also have some characteristics that would be valuable for the female's potential offspring to acquire (Darwin, 1871).
There are many characteristics that both males and females value in a potential mate. Kindness, understanding and intelligence topped a recent list (Kenrick & Simpson, 1997). Humans also have gender specific criteria for sexual selection. In a study of 37 cultures on six continents and five islands, Buss (1989) found that women value prospective male suitors on a cluster of characteristics related to resource potential: good financial prospects, ambition, industriousness, older age, and emotional maturity. On the other hand, men value potential female partners in terms of fecundity, the ability to produce and care for children. This is expressed in a preference for youth and physical attractiveness (Tesser & Martin, 1996). Cunningham (1986) found that "baby face" features, i.e. large eyes and a small nose, were consistently positively correlated with attractiveness in women, perceived fertility, and perceptions of few medical problems.
Recently, the renowned social psychologist Alice Eagly has criticized the evolutionary explanation for gender specific mate selection criteria. Eagly (1997) proposed a cultural explanation for human mate selection criteria, namely that males exhibit social dominance in the societies under study. If females were the dominant gender, the favored attributes could be expected to exhibit a reversal in gender assignments. Unfortunately, this proximate explanation cannot be tested since there are no presently known human societies where females socially dominate the males.
In other animal species, this reversal in gender assignment for sexual selection criteria does occur. For example, in seahorses, females compete for the attention of males. The females of this species typically have brightly colored ornamentation, in contrast to the dull males. Another "unique" characteristic of this species is that the males have a special pouch that holds infant seahorses while they mature. This would indicate that the relative level of parental investment by each gender is at the root of sexually dimorphic characteristics valued in potential reproductive partners. The competition will be for the sex with the greater parental investment. Similar phenomena occurs in bird species where males account for the majority of egg-hatching time in the nests.
Another salient characteristic in humans is the ratio of the circumference of the hips to the waist. Before puberty, men and women have a similar distribution of fatty tissues, but afterwards women have a greater hip-to-waist ratio. Singh (1993) noted that a larger hip-to-waist ratio was associated with better health status and greater reproductive capacity, and used archival data to examine the measurements of Miss America Pageant winners and Playboy centerfolds over the past fifty years. Singh found that a small waist set against full hips was a consistent feature of female attractiveness, while bustline, overall body weight, and physique varied over the years.
A female's choice of mates is generally more important than the male's choice in driving sexual dimorphism within a species (Kendrick, Trost, & Sheets, 1996). Since women have to make a greater obligatory investment in offspring though gestation and infant care, they are more choosy about whom they are going to mate with (Buss & Malamuth, 1996). This universal preference has selected for men who are able to provide for potential offspring through the resources they are able to devote. Men then have to be more competitive with each other for access to women. This has created a strong demand for men who are able to provide valuable resources, resulting in the characteristics of assertiveness, aggressiveness, and sensitivity to hierarchy found in men.
In a series of four studies, Sadalla, Kenrick & Vershure (1987) found that dominant behavior in males increased female's sexual attraction to them. Dominant behavior in females was not related to sexual attraction in males. Although male dominance enhanced their sexual attractiveness, it did not improve their general likability. Graziano, Jensen-Campbell, Todd, & Finch (1997) used structural equations modeling to show that men who were not agreeable were not attractive regardless of dominance status. For men who were high in agreeableness, dominance enhanced their physical attractiveness significantly. The authors cautioned that overt mate choices made in one context may reflect social coercion. Under some conditions, a woman who chose a dominant aggressor for a sexual partner may have chosen to stay alive, rather than expressed a personal preference for dominant men. It was also suggested that prosocial tendencies may increase in importance as a criterion when the anticipated length of the relationship with a man increases.