The chapter discusses how and why the United Stes went from a relatively undifferentiated family system in the middle of the last century to the present system of diverse family forms. Even conceding that the family system was always less simple than it now appears in hindsight, it began to depart from the dominant model of the nuclear family household in the late 1960s. The chapter first examines how this change is a result of adaptation by individuals and family members to change economic, demographic, technological and cultural conditions. The breakdown of the gender based division of labour was the prime mover. Next, the chapter examines family complexity in the United States as largely a product of growing stratification. It shows how family formation processes associated with low human capital produce complexity over time in family systems. The chapter then examines complexity in a changing global context.