Darwin originally proposed his theory of sexual selection to explain exaggerated male characteristics (Darwin, 1871). His theory of sexual selection is based on the asymmetry in reproductive costs between the sexes (Trivers, 1972). The sex that invests more in reproductive costs tends to be choosy and discriminating. In most cases, females invest more energy in reproduction than males, through eggs, gestation, and lactation; therefore, female choice occurs. In response, males tend to signal their genetic quality through costly displays (c.f. Penn & Számadó, 2020). Peacocks with long and heavy trains, for example, are easy targets for predators because the tail slows them down, making them conspicuous (Petrie et al., 1991). Evidence suggests that peahens focus on conspicuous features because they signal the inherent quality of peacocks. In another type of sexual selection, males compete for females by using force or threat of force to exclude other males. Such intrasexual selection results in sexual size dimorphism, because males with better fighting ability or larger body size have an advantage in accessing females or securing resources that females need. Thus, according to the ‘classical’ Darwinian sexual selection theory, sexual selection pressure is stronger on males than on females. The dichotomous stereotype that males are easily aroused and proactive and that females are passive and discriminating is based on this logic.

However, there are a few species with conspicuous traits in females (Clutton-Brock, 2007; Clutton-Brock, 2009; Isaac, 2005; Kraaijeveld et al., 2007), although the extent of the costly trait may be lower in females than in males (Hare & Simmons, 2019). There are even females with secondary sexual characteristics that are absent in males; in other cases, both males and females have conspicuous traits. Even in species where sex roles are not reversed, there are quite a few cases where a positive coefficient of reproductive success regressed on mating success in females (i.e. Bateman gradient) has been reported where females have conspicuous traits (e.g. two-spotted gobies, Gobiusculus flavescens (Amundsen & Forsgren, 2001)). This indicates that in some species, mate choice works both ways. Mutual ornamentation in fish and birds has been studied extensively in recent decades (Amundsen, 2000; Amundsen, 2018). Moreover, Darwin singled out humans as an exception to sexual selection without male choice, which exemplifies mutual choice (Darwin, 1871).

Conspicuous female traits may be explained by parental investment theory (Trivers, 1972). According to parental investment theory, in species with high paternal investment, the direction of mate choice may be reversed because males may incur higher reproductive costs. Females may compete with each other for caregiving males, or males may choose conspicuous females. In some species with substantial paternal investment, females are larger than males (e.g. spotted sandpipers and pipefishes).

Recently, behavioral ecologists have found several cases in which multiple female mating increases reproductive success (Jennions & Petrie, 2000; Taylor et al., 2014). Multiple female mating is widespread in birds and invertebrates, although the maximum number of mates is usually lower than for males (Hare & Simmons, 2019). In some human societies, extra-pair mating by women is common and tolerated (Beckerman et al., 1998; Scelza, 2013; Scelza et al., 2020). For instance, Himba women with extra-pair partners may enjoy better genes, reduced risk of infanticide, more resources, and protection.

Classical sexual selection theory attributes conspicuous female traits to genetic correlations (Amundsen, 2000; Fisher, 1930; Lande, 1980, Lande, 1987). However, subsequent studies have shown that the effect of genetic correlation is usually weak and that attractive ornamentation is rarely inherited via sex chromosomes (Amundsen, 2000; Wiens, 2001). Plausible explanations for conspicuous traits in females are either a different trait expression by sex (e.g., body fat on the breasts, hips and buttocks vs. muscle mass, beards, and deep voices), a signal to males (i.e. male choice), a byproduct of social interaction between females (Clutton-Brock, 2009; Hare & Simmons, 2019; Johnstone et al., 1996; Kraaijeveld et al., 2007) and/or sex-dependent expression of autosomal genes by sexually antagonistic selection (Rowe et al., 2018). Thus, female ornamentation may indicate the inherent quality of an individual to potential mates.

However, except for birds and fish, empirical research on mutual mate choice is lacking. Furthermore, few empirical studies have been conducted on mutual choice in humans. Mate choice has been a major area of research in evolutionary psychology, and most research has focused on male-male competition and female choice (Etcoff, 1999; Hooper & Miller, 2008 are a few exceptions). Most of the necessary conditions for mutual choice are present in humans: males provide paternal care as one of the major caregivers in cooperative breeding (Hrdy, 2011), and there is variation in male earning potential that can be translated into fitness in contemporary societies. Additionally, the relatively long duration of pair-bonding compared to other animals favours mutual choice. All these factors vary across societies, which means that well-designed cross-cultural research can shed light on the factors that influence mutual mate choice in humans.

According to the ‘revised’ sexual selection theory that assumes mutual mate choice, the direction and degree of competitiveness may differ within and between societies although the essential logic is the same. In other words, the direction and strength of sexual selection depends on relative investment and availability to the opposite sex. The sex that spends more energy on reproduction and has less availability to the other sex will be more discriminating.