This book examines the cultural, biological and other processes that explain how humans evolved into an exceptionally cooperative species. It advances two propositions, the first of which deals with proximate motivations for prosocial behavior and the second is concerned with the distant evolutionary origins and ongoing perpetuation of these cooperative dispositions. It argues that cooperation arose because it was highly beneficial to the members of groups that practiced it, and humans were able to construct social institutions that minimized the disadvantages of those with social preferences in competition with fellow group members, while heightening the group-level advantages associated with the high levels of cooperation that these social preferences allowed. The book explains why altruistic social preferences supporting human cooperation outcompeted unmitigated and amoral self-interest.