Public policymakers and political pundits tend to focus on problems — understandably, because if things are going right they aren’t thought to need attention. Yet positive developments can teach us things as well, when, for reasons not necessarily clear, great masses of people start to behave more constructively.

One such trend is the better behavior of the young Americans of today compared to those 25 years ago. Almost no one anticipated it, the exception being William Strauss and Neil Howe in their 1991 book Generations, who named Americans born after 1981 the Millennial generation and predicted that “the tiny boys and girls now playing with Lego blocks” — and those then still unborn — would become “the nation’s next great Civic generation.”

The most obvious evidence of the Millennials’ virtuous behavior is the vast decline in violent crime in the last 25 years. The most crime-prone age and gender cohort — 15-to-25-year-old males — are committing far fewer crimes than that cohort did in 1990.

Statistics tell the dramatic story. In two decades the murder rate fell 49 percent, the forcible rape rate 33 percent, the robbery rate 48 percent, the aggravated assault rate 39 percent. Government agencies report that sexual assaults against 12-to-17-year-olds declined by more than half and violent victimization of teenagers at school declined 60 percent.

Binge-drinking by high school seniors is lower than at any time since 1976, sexual intercourse among ninth graders and the percentage of high school seniors with more than three partners has declined.

There has been much ado about rape on college campuses today, with President Obama among others stating that one in five women students will be raped or sexually assaulted. But that statistic is based on a bogus survey, covering just two colleges, with self-selected rather than randomly selected respondents and a laughably broad definition of “sexual assault.” A recent Justice Department report showed that the rate rape on campus was not 20 percent but 0.6 percent.

And today’s young are better behaved despite what blind statistical trends might seem to hint at. Compared to the young Americans of 1990, their ranks include a higher percentage of Hispanics and blacks, who statistically tend to have above-average crime rates. Today's young are also more likely to come from single-parent households—another high-risk factor. Demographics suggested there would be more bad behavior. Instead, there is much less.

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.

It’s true that unmarried parenthood has risen. But teen births, like violent crime, have been in sharp decline. Now the latest statistics tell us that birth rates are, unusually, up among married women and down among unmarried women.

There remain stark differences between the experiences and behavior of high-education and -income and low-education and -income Americans, as Charles Murray showed in his 2012 book Coming Apart. But perhaps they are starting to converge.

Liberals and conservatives often assume that moves away from traditional moral rules must inevitably continue. How can you keep them down on the farm once they’ve seen “Paree?”

But today’s America, like Victorian England, shows that virtuous cycles are possible as well. People can learn from experience, and those who have seen the downside of bad behavior may choose to behave better.