Fifty years ago this month, Democrats made a historic mistake.

Daniel Patrick Moynihan, at the time a federal official, wrote a famous report in March 1965 on family breakdown among African-Americans. He argued presciently and powerfully that the rise of single-parent households would make poverty more intractable.

“The fundamental problem,” Moynihan wrote, is family breakdown. In a follow-up, he explained: “From the wild Irish slums of the 19th-century Eastern seaboard, to the riot-torn suburbs of Los Angeles, there is one unmistakable lesson in American history: a community that allows large numbers of young men to grow up in broken families ... never acquiring any stable relationship to male authority, never acquiring any set of rational expectations about the future . that community asks for and gets chaos.”

Liberals brutally denounced Moynihan as a racist. He himself had grown up in a single-mother household and worked as a shoeshine boy at the corner of Broadway and 43rd Street in Manhattan, yet he was accused of being aloof and patronizing, and of “blaming the victim.”

“My major criticism of the report is that it assumes that middle-class American values are the correct values for everyone in America,” protested Floyd McKissick, then a prominent African-American civil rights leader.

The liberal denunciations of Moynihan were terribly unfair. In fact, Moynihan emphasized that slavery, discrimination and “three centuries of injustice” had devastated the black family. He favored job and education programs to help buttress the family.

But the scathing commentary led President Lyndon Johnson to distance himself from the Moynihan report. Scholars, fearful of being accused of racism, mostly avoided studying family structure and poverty.

In 1992, Vice President Dan Quayle stepped into the breach by emphasizing the role of the family in addressing poverty, including a brief reference to Murphy Brown, a television character who was a single mom. Liberals rushed to ridicule Quayle for sexism and outdated moralism, causing politicians to tread this ground ever more carefully.

The taboo on careful research on family structure and poverty was broken by William Julius Wilson, an eminent black sociologist. He has praised Moynihan’s report as “a prophetic document,” for evidence is now overwhelming that family structure matters a great deal for low-income children of any color.

In 2013, 71 percent of black children in America were born to an unwed mother, as were 53 percent of Hispanic children and 36 percent of white children.

Indeed, a single parent is the new norm. At some point before they turn 18, a majority of all American children will likely live with a single mom and no dad.

My point isn’t to cast judgment on nontraditional families, for single parents can be as loving as any. In fact, when one parent is abusive, the child may be better off raised by the other parent alone. And well-off kids often get plenty of support whether from one parent or two.

One kind of nontraditional household does particularly well. One study found that children raised by same-sex couples excelled by some measures, apparently because the parents doted on their children . most gay couples don’t have unwanted children whom they neglect.

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.

What can be done?

In line with Moynihan’s thinking, we can support programs to boost the economic prospects for poorer families. We can help girls and young women avoid pregnancy (30 percent of American girls become pregnant by age 19). If they delay childbearing, they’ll be more likely to marry and form stable families, notes Isabel Sawhill of the Brookings Institution.

So let’s learn from 50 years of mistakes. A starting point is to acknowledge the role of families in fighting poverty. That’s not about being a moralistic scold, but about helping American kids.