Of the many barriers to equal opportunity for African-Americans, differences of family background may well be the most consequential—and the least likely to yield to public policy. This is the gravamen of research made public in recent weeks, much of it collected in the fall 2015 issue of the academic journal the Future of Children.

Although there were signs of trouble to come in the 1960s, racial differences in marriage rates remained modest until 1970, when 95% of white women and 92% of black women had been married at least...