The Supreme Court has known for over a half century that the survival of our constitutional polity ultimately depends on the proper cultivation of children's "hearts and minds." This idea was expressed most directly in Brown v. Board of Education, where a unanimous Supreme Court concluded that segregated schooling affects the "hearts and minds" of African-American schoolchildren in a way that undermines "the very foundation of good citizenship." On many other occasions as well, the Justices have formulated constitutional doctrine to foster democratic skills of mind in future citizens. Yet for all the normative force of this idea, courts and commentators have failed to analyze with care the relationship between child development and democratic citizenship in constitutional law. This Article provides a comprehensive conceptual and empirical account of the process of becoming a citizen in the full psychological sense of the term, in other words, the process by which individuals acquire the integrated cognitive and emotional capacities of mature reasoned thinking. Research in developmental psychology provides strong empirical support for the proposition that the mature capacity of reasoned thinking has its roots in the emotional embrace of the early caregiving relationship. An understanding of the fundamental connection between the early caregiving relationship and the adult skills of reasoned thinking lays the foundation for a new, comprehensive and integrated constitutional family law, one that identifies the essential role of early caregiving in the political socialization of children. In illuminating the vital connection between families and citizenship, this approach offers a substantial reworking of constitutional doctrines in the area of family privacy, parental rights and federalism. More broadly, the developmental approach taken is a starting point for investigating the political socialization of children in our constitutional scheme of government.