Marriage is no longer what it once was. Since the 1970\u27s, and accelerating in recent decades, the link between formal marriage and family life has weakened dramatically. Nearly one-half of U.S. adults are now single at any given time, and two of five children are born to unmarried parents. At the same time, delayed marriage, divorce, remarriage, and changing gender roles have transformed the content of marriage itself. Despite these changes, the federal income tax and the Social Security system continue to define family based on formal marriage, and our textbooks treat the economic vulnerability of the married woman as the central problem of gender in tax policy design. In this Article, I argue that the growing gap between legal fictio...