"As marriage has declined, though, childbearing has not, which means that more children are living in families without two parents and the resources they bring."

Er, no, it doesn't mean that at all. I mean, I 100% support anyone doing the ridiculously hard job of raising a kid all by themselves, but just because Justin Wolfers and Betsey Stevenson aren't married it doesn't mean they aren't committed to each other and their children.

Anyway, maybe the truth is: people don't have to get married, and it's expensive to do so. I don't think there's any great mystery here.