One salient aspect of most world religions is an emphasis on reproductive morality—rules about which types of sexual behaviors and familiar structures are acceptable. In Chapter 1, I introduce the theoretical background of the dissertation, including the Reproductive-Religiosity Model. Then, in one theoretical paper (Chapter 2) and two empirical papers (Chapters 3 and 4), I consider the cultural and social implications of religious proscriptions on sexual behavior. In Chapter 2, I review the Reproductive-Religiosity Model, which posits that religions are especially attractive to people who desire monogamous, long-term mating strategies. I also discuss the implications of this model for cultural evolution. In Chapter 3, I look at the social implications of these religious proscriptions. That is, if restricted attitudes toward sexuality are strongly linked to religious belief, it follows that people’s stereotypes of religious people may track this relationship. Three studies showed that people tended to trust religious targets more than nonreligious targets, but that this effect seems to be due to inferences about religious targets’ reproductive strategies—that is, people trusted religious targets because they perceived them more likely to be interested in starting a family. In Chapter 4, I examine patterns of religiosity across the world through a rational choice lens, positing that people are more likely to be religious when religion can help them fulfill their goals. Analysis of two global datasets shows that men, more so than women, tend to be less religious in countries with greater gender equality. Finally, Chapter 5 summarizes results and discusses future directions for this line of research.

This study explores aggression among siblings. Specifically, this study tests whether different types of aggression among siblings occur more or less frequently than toward friends and acquaintances. This study also examines whether sex differences in aggression between siblings resemble sex differences in aggression among friends and acquaintances. Results suggest that direct aggression occurs much more frequently among siblings than among friends and acquaintances, but that indirect aggression occurs far less frequently among siblings than among friends and acquaintances. In addition, the findings suggest that certain sex differences in aggression, such as males directly aggressing more often than females, are diminished when the aggression occurs between siblings. Both males and females report high levels of direct aggression toward their siblings.