Yet Moynihan was absolutely right to emphasize the consequences for low-income children of changing family structure. Partly because there is often only one income coming into a single-parent household, children of unmarried moms are roughly five times as likely to live in poverty as children of married couples.

Causation is difficult to tease from correlation. But efforts to do that suggest that growing up with just one biological parent reduces the chance that a child will graduate from high school by 40 percent, according to an essay by Sara McLanahan of Princeton and Christopher Jencks of Harvard. They point to the likely mechanism: “A father’s absence increases antisocial behavior, such as aggression, rule-breaking, delinquency and illegal drug use.” These effects are greater on boys than on girls.

Conservatives shouldn’t chortle at the evidence that liberals blew it, for they did as well. Conservatives say all the right things about honoring families, but they led the disastrous American experiment in mass incarceration; incarceration rates have quintupled since the 1970s. That devastated families, leading countless boys to grow up without dads.