Why do today's religions look and function the way they do? Presenting research primarily on religion’s effects on prosocial behavior and prejudice toward outgroups, I will argue that the form and function of modern religions can be understood as the legacy of a millennia-long process of cultural evolution. Our recent research has begun to empirically test perennially debated questions about whether religions make people act more ethically, what functions religions have served, and why some religious traditions have fared better than others. The results reveal that while the social consequences of religion are not always desirable, they can be explained as the product of cultural adaptations that served vital social functions. In particular, I’ll discuss how recurrent elements throughout religions have served to stabilize cooperation among large groups of unrelated strangers, and maximize survival in intergroup competition. Finally, I’ll speak about how this cultural evolutionary perspective informs predictions about the future of religion. Altogether, this research demonstrates how social psychological research can add important empirical data to heated debates about the values and vices of religion in the modern world.