Sexual jealousy arises in response to a perceived threat to a valued relationship with an individual, through perceptions or suspicions of a partner’s sexual infidelity. An evolutionary psychological perspective predicts that men are more sensitive to the sexual aspects of their partner’s infidelity than women, whereas women are more sensitive to the emotional aspects of their partner’s infidelity than men. This evolutionary perspective can be referred to as the sex-specific evolved jealousy mechanism (EJM). Many studies have verified the EJM. In this chapter, findings of studies on the EJM are discussed from two perspectives –the psychometric properties and participants’ and rivals’ characteristics – focusing on the findings from a meta-analysis in our laboratory (87 articles, k = 164, N = 125,698). Findings on the EJM are discussed from a psychometric perspective, focusing on (a) the rating methods of responses to a partner’s infidelity (i.e., forced-choice paradigm vs. continuous measures), (b) the validity of hypothetical infidelity scenarios used to have participants imagine a partner’s infidelity, (c) participants’ physiological responses to their partners’ infidelity, (d) participants’ response times to their partners’ infidelity, (e) participants’ recall performance for stimuli regarding infidelity, (f) participants’ responses to their partners’ infidelity under cognitive load, (g) heritability in behavior genetics, and (h) participants’ behaviors following their partners’ infidelity (e.g., morbid jealousy, violence, sexual coercion, and forgiveness). Findings on the EJM are discussed with respect to characteristics, focusing on (a) sexual orientation of participants, (b) participants who had nonmonogamous relationships, (c) children’s or siblings’ partners’ infidelity, and (d) participants’ digit ratio instead of sex. The EJM has been examined by many studies using various methods; however, conflicting findings have been reported. These differences seem to be caused by conflicting interpretations about the EJM and the rating methods of responses to infidelity.

Despite a tendency to form socially monogamous pair-bonds that carry expectations of sexual exclusivity, infidelity has been a recurrent feature of human mating across societies. The attitudes, social cognition, affect, and behavior associated with infidelity vary in patterned ways between women and men. In the current chapter, we use an evolutionary perspective to make sense of the historical and cross-cultural ubiquity of extradyadic behavior, as well the adaptative costs and benefits of men’s infidelity. Specifically, we review theory and research pertaining to men’s extra-pair mating and consider salient individual differences, romantic relationship dynamics, and social–ecological factors that influence mating strategies and extradyadic involvement. Following other scholars, we argue that men have evolved adaptations for short-term mating that facilitate opportunistic extra-pair behavior in a “quantity-over-quality” reproductive strategy. Consequently, on average, men are predicted to express a stronger desire to engage in sexual infidelity and to have more permissive attitudes toward extradyadic involvement than women. However, only particular men appear to execute a mixed mating strategy involving a long-term mate and an extra-pair partner, such as those with greater mate value. Satisfaction with and commitment to the relationship appear to be crucial in preventing men’s infidelity, and socio-ecological factors, including cultural dynamics (e.g., norms surrounding infidelity) and sex ratios that create conditions of mate scarcity, are inextricably tied to men’s extra-pair mating.