Evolutionary sexual psychology posits that sexual preferences evolved in response to recurring adaptive problems faced by men and women in regard to reproduction and mating. Accordingly, asymmetries in the mating-related problems faced by the sexes should result in sex-differentiated preferences. Some asymmetries which could be expected to result in sex-differentiated preferences include: 1) the length of time during which one is able to produce offspring (much longer for men as compared to women, which is posited to result in men showing a preference for partners who display cues to fertility and reproductive viability); 2) minimum investment needed to produce offspring (much greater for women as compared to men, which is posited to result in men showing a greater preference for short-term mating relative to women); and 3) certainty of maternity/paternity of offspring (much greater for women as compared to men, which is posited to result in men showing preferences which mitigate partner infidelity and sperm competition). Consistent with the predictions of evolutionary sexual psychology, many of the physical characteristics which men find to be attractive in women are associated with fecundity (e.g., a low waist-to-hip ratio, youthfulness). Men do appear to display a greater interest in engaging in short-term mating relative to women. Men self-report more permissive attitudes toward casual sex, desire a greater number of sexual partners across various time periods, and report being more motivated by casual sex when dating or using dating apps. Large representative surveys frequently find a sizable sex difference in the number of sexual partners reported over the lifespan, although the degree to which this may reflect factors like differences in the way that men and women respond to such questions (e.g., estimating versus counting) is debated. Field experiments indicate that men are more inclined to accept offers of casual sex from opposite-sex strangers, and men appear to be more likely to pay for sex. The content of sexual fantasies and pornography also offer insights into the nature of men’s sexual preferences. Men’s sexual fantasies more frequently involve elements of sexual variety and nonmonogamy (e.g., casual sex with multiple partners). Men also appear to consume pornography more frequently than women, which may reflect pornography providing vicarious access to excellent short-term mating opportunities in the form of a myriad of virtual partners who are youthful, attractive, and display unusually high levels of sexual accessibility. The contents of pornography, and themes common to men’s sexual fantasies, also demonstrate a preoccupation with partner infidelity.

Much of evolutionary psychology has focused on species-typical behavior or sex differences. However, recent attention has turned to considering the role of individual differences in shaping human behavior, in particular in the realm of sexual strategies. While some research still focused broadly on sex differences, there is a growing body of work that examines within-sex differences in sexual strategies. This work includes genetic and environmental influences on individual differences factors including flexible responses to ecological contingencies such as sex ratio and mortality rates. These within-sex individual differences can be divided into directly selected individual differences in mating strategies (genetically influenced biases toward developing longer-term or shorter-term mating relationships), indirectly selected individual differences in mating strategies (variations in mating strategy that might be influenced developmentally by individual differences in strategically relevant traits), indirectly selected individual differences in mating strategies that are influenced developmentally by external environmental contingencies (which could include environmental unpredictability or resource scarcity), and combinations of all three acting concurrently in one individual. This chapter will examine all three options and some of the evidence collected to date on their role.

The historical, and perhaps still stereotypical, image of women as coy and passive in mate selection is not correct. While intrasexual competition among men has been studied extensively, theoretical and empirical work focusing on how women engage in intrasexual competition and intersexual mate choice has received less attention. More recently, there has been a growing literature which has shown that women’s intrasexual competition can be fierce, and that it is not limited to self-promotion in order to attract a partner. The evolutionary psychology theoretical aspects guiding this research are parental investment theory, sexual conflict theory, and sexual strategies theory, which we present briefly at the start of this chapter. Several individual differences, such as personality traits, sociosexuality, dark triad traits, age and mate value, influence what attraction tactics work for whom. Further, we present various tactics women use to show sexual interest, how tactics are linked to motivation for sexual intercourse, and how women’s attraction tactics work with new technology, such as dating apps. In addition, we have reviewed the literature on women’s intrasexual competition, with specific focus on how women derogate rivals, self-promotion their traits and how they use various tactics to disrupt rivals’ relationships through mate poaching.

For folivores, socio-ecological models predict scramble competition and egalitarian dominance relationships with female dispersal, but female Asian colobines do not all nicely fit these predictions. Alternative explanations concern an absence of competition, and the Folivore Paradox, where group size is limited by social factors such as infanticide. Relevant data are scarce, but our review shows that colobine foraging costs increase with group size, yet female fitness may increase or decrease. Dominance relationships vary from despotic to egalitarian, and are individualistic. The lack of female nepotism in despotic species (or populations) still requires explanation. Female dispersal is found in egalitarian and some despotic species, yet costs may be low if females migrate into a group with familiar females. Asian colobine social organization seems to follow one of two patterns. First, in seasonal species, food limits group size, while infanticide does not; these groups may experience contest competition. Second, in a-seasonal species groups are uni-male, the infanticide risk is high and infanticide, not food, may limit group size. This proposal requires further testing and may also apply to uni-male frugivores. This overview of the socio-ecological patterns in Asian colobines highlights that ecological and social selection pressures in interaction determine primate social organization.