Family law is based on marriage, but family life increasingly is not. The American family is undergoing a seismic shift, with marriage rates steadily declining and more than four in ten children now born to unmarried parents. Children of unmarried parents fall far behind children of married parents on a variety of metrics, contributing to stark inequality among children. Poverty and related factors explain much of this differential, but new sociological evidence highlights family structure—particularly friction and dislocation between unmarried parents after their relationship ends—as a crucial part of the problem. As the trend toward nonmarital childbearing continues to spread across class lines, the effect will be most pronounced among children.

This shift is the single most important issue facing family law today, yet scholars have been slow to engage with the structure and substance of the law in response. In family law, the marital family serves as a misleading synecdoche for all families, not only marginalizing nonmarital families but also actively undermining their already tenuous bonds.

It is essential for family law to address the needs of both marital and nonmarital families. This entails a new theory of state regulation as well as new doctrines, institutions, and norms in practice. Some feminists argue that the state should privilege caregiving between parents and children instead of marital relationships, while other commentators stubbornly advocate marriage primacy—the elevation of marriage above other family forms—despite the evidence that marriage promotion fails. These responses fundamentally misunderstand nonmarital family life, in which dynamics between parents deeply affect children yet parents are unlikely to marry. We must instead understand that it is possible to separate marriage from parenthood but not relationships from parenthood. Accordingly, the state should help unmarried parents become effective co-parents, especially after their relationship ends, so they can provide children with the healthy relationships crucial to child development. This theoretical insight, and the family law that flows from it, will inaugurate a larger debate about how to prepare for a world in which marriage is not the defining institution of family life.