In arguing against allowing marriage between people of the same sex, Maggie Gallagher appeals to a definition of marriage rooted in traditional notions of the nuclear family and the importance of such family structures to the wellbeing of entire societies. Gallagher cites research showing that children raised by both of their biological parents tend to have better health, do better in school, are less likely to commit crimes, and thrive more by nearly any relevant assessment than children who are raised in any other environment. This much is not in dispute. However, it is worth asking, as Corvino does, whether such statistics indicate a causal link between family structures and children’s health and success, or whether the link is due, at least in part, to both measures being correlated to a different underlying cause (e.g. a family’s socioeconomic status). Suppose we assume the former, that differing family structures are in fact the main or only cause of the different outcomes for the children they produce.…