Three decades ago, it would have been inconceivable for people to discuss seriously the idea of withdrawing the legal and financial support society gives to marriage. In recent years, however, thinkers and policymakers have given more serious thought to the possibility of eliminating marriage as a category entitled to the State’s support. An important consideration in this debate is whether keeping or eliminating the State’s support of marriage matters to the well-being of children. A wealth of studies contemplating modern family forms now exists, many of which invariably stack newer family structures up against the more traditional nuclear family. Until recently, such studies contrasted families that are so dissimilar that nothing meaningful could be drawn from them about the importance of marriage. This article evaluates the extent to which newer, more carefully constructed studies can assist us in isolating the impact on a child’s well-being of living in a marital home. Part I describes the limitations of earlier studies of family structure. Part II examines a pair of studies published in 2003 that compare children’s outcomes and parental investments in children in families that contain biological, married parents with those containing biological, unmarried parents. These studies conclude that “marriage per se confers advantage in terms of” how children thrive and to the extent to which parents are willing to invest in children. Part III evaluates the degree of reliance we should place on these newer studies. It first identifies various selection effects that may color the study results. It then reviews scholarship that suggests that the transformative power of marriage may lie first in the greater permanence of marital relationships and, secondarily, in the motivation of the parties to invest in these relationships. This article concludes that perceptions of enduringness may shape not only relationships between the adults, but may also frame the adults’ relationships to their children.