This paper examines a half century of trends in family attitudes and beliefs in the United States, including attitudes towards gender, marriage, childbearing, cohabitation, sex outside marriage, divorce, and same-sex relations. We trace attitudes from the 1960s through the 2010s using four data sources: Intergenerational Panel Study of Parents and Children, Monitoring the Future, General Social Survey, and International Social Science Project. We find profound and largely consistent changes in Americans’ attitudes. We argue these changes demonstrate the expansion of developmental idealism in the United States. Americans increasingly endorsed longstanding “modern” family attributes, as well as newly “modern” attributes viewed as extensions of freedom and equality and linked to seemingly natural progress of society. At the same time, sizable majorities remained committed to marriage and children. While Americans increasingly supported all individuals’ freedom to choose among a diversity of family behaviors, most continued to choose marriage and children for themselves.