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...