These days, discussions about family structure tend to focus on two key facts. First, about 40 percent of children in the U.S. are born to unmarried parents — a figure that was around 5 percent in the 1950s. Second, 40–50 percent of first marriages end in divorce.

In and of themselves, these are incredibly important parts of the picture. Both are ways that children end up living apart from one of their biological parents, usually their father. (Unmarried parents normally welcome their children into the world together, but two-thirds of them are neither married nor cohabiting five years later.) And in …