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Introduction xiii and care. As such, they tend to feel more jealous over emotional rather than sexual infidelity, because emotional attachments could lead their mate to commit himself to a sexual rival. Given that parental investment is longer for women, and the risk of engaging in casual or uncommitted sex is greater, women are theorized to be more selective than men when choos- ing among prospective mates. Empirical research supports this proposition. In two separate studies, the first across 33 countries and the second across 52, Buss (1989) and Schmitt and Dovidio (2003) found that men prefer more sex partners than women do, prefer to have sex after a shorter time knowing a partner, and are more likely than women to seek short-term partnerships, irrespective of their current relationship status. Although evolutionary theories describe greater reproductive benefits for promiscuous men than promiscuous women—because promiscuous men will produce abundant offspring— researchers also identify at least one benefit for promiscuous women: By having sex with multiple partners during her fertile period, a woman can elicit sperm competition and enhance her chances of birthing optimal off- spring (Pham and Shackelford 2015). Sexual selection theory also describes gender differences regarding intra- and intersexual selection. According to the theory, men and women faced different challenges in their ancestral history, which led them to develop distinct strategies for reproductive success (Buss et al. 2017). Intrasexual selection involves competing with members of one’s own sex for a potential mate, whereas intersexual selection refers to the manner in which each sex demonstrates its desirability to potential mates. Given that women value resources, status, and physical strength, particularly during their fertile period, men compete with each other on these traits by boasting about their financial accomplishments and demonstrating their physical prowess. Men are also expected to derogate their rivals on precisely these traits. Women, on the other hand, are expected to com- pete with each other on traits that men prioritize, such as those signaling youth and fertility. They are predicted to wear clothing that accentuates their figure and to invest in makeup, hair products, and even cosmetic procedures that will enhance their outward appearance. They derogate their rivals on the qualities valued by men, heavily criticizing for example, the outward appearance of other women (Buss 2012). Although the ideas stemming from this theory focus on gender stereotypes and do not often apply to same-sex relationships, research supports its propositions.