A new book by economist Melissa Kearney makes a readily apparent but still-provocative argument: Children raised by two married parents do better than children raised by single parents. The difference, Kearney argues, isn’t just financial (although the money matters a lot); the “two-parent privilege,” as her book calls it, is also about parental time investment and familial stability. The problem with the point she makes — and with the firestorm of discussion it’s touched off on the left and the right — is that there are many things that are statistically better for us, but if those things are not...