One of Dahl and Moretti's most striking bits of evidence comes from shotgun marriages. Take a typical unmarried couple who are expecting a child and have an ultrasound, which more often than not reveals the child's sex. It turns out that such couples are more likely to get married if the child is a boy. Apparently, for unmarried fathers, the prospect of living with a wife and a son is more alluring than the prospect of living with a wife and a daughter.
So, what's the bottom line? Dahl and Moretti are quick to acknowledge that they've found no smoking guns; if you're sufficiently clever you can probably concoct alternative explanations for everything they've observed. But the most natural way to interpret their data is that parents, on average, prefer boys to girls. The preference is stronger elsewhere in the world, but it's plenty strong in the United States too.
That seems to answer one question: Boys preserve marriages by making marriages better, not by making divorces worse. But it also raises a new question: What's so great about a boy? Why do parents prefer boys to girls?
Maybe boys grow up to be better economic providers for their parents' old age. (This would explain why the preference for boys is stronger in countries where men hold more economic power.) Maybe boys are just more fun to have around. Maybe parents want a child who can carry on the family name. Or maybe there's something deep in our psyches that tells us a family just isn't a family without a son. Which is it?
Dahl and Moretti wisely decline to speculate, and I will follow their example. I don't know any evidence that could settle this question. All we know is that for some reason, parents prefer boys—by enough that boys hold a lot of shaky marriages together.
Years ago on the schoolyard, we used to chant that girls are good but boys are better. It looks like our parents agreed with us.