What accounts for this virtuous cycle? I am inclined to give some credit to better police tactics and welfare reform, the great positive conservative policy successes of the 1990s. Others might credit the Clinton administration’s increase in the Earned Income Tax Credit or bipartisan-supported education reforms. But partisan explanations, though plausible, seem inadequate.

I think what we are seeing is a mass changing of minds, something like the movement in Victorian England toward what historian Gertrude Himmelfarb described as “the morality that dignifies and civilizes human beings.”

My theory is that young people do what is expected of them, in two senses of the word “expected.” One is statistical expectation. Americans in 1990 expected young people, especially from disadvantaged backgrounds, to commit lots of crimes. They had been doing so, after all, for 25 years. But Rudy Giuliani and others adapting his methods reduced crime dramatically, and statistical expectations rapidly changed.

The other sense of the word “expected” is moral expectation. A parent tells a boy he is expected not to shoplift, bully, rob, rape or kill. She tells a girl she is expected not to sleep around or get pregnant. The parents of the last 25 years grew up in years of high crime, high divorce and high unmarried births. Evidently they wanted — expected — something better from their own children.