This chapter examines several demographic trends relevant to parenthood in individual lives and to the social ambiance surrounding childbearing and rearing in contemporary society. A good starting place for understanding change in gender and parenting roles is several demographic trends: Longevity and the sex ration, marriage and fertility, and household composition. The chapter reviews gender differences in parenting as reflected in recent research on traditional and nontraditional family arrangements and the effect of significant male investment in parenting for child outcome. It then assesses the adequacy of current explanations of gender differences in parenting and demonstrates the relevance of an expanded explanatory model that draws upon bioevolutionary theory and the neurosciences. Three types of research permit a close-up view of what it is that men do when they carry primary child-care responsibility and how they differ from the more traditional circumstance of women carrying this responsibility. They are solo fathers, men in nontraditional family circumstances, and men in intact marriages.