WASHINGTON — The most important social trend of the past 20 years is as positive as it is underappreciated: the United States’ plunging crime rate.

Between 1991 and 2010, the homicide rate in the United States fell 51 percent, from 9.8 per 100,000 residents to 4.8 per 100,000. Property crimes such as burglary also fell sharply during that period; auto theft, once the bane of urban life, dropped an astonishing 64 percent. And FBI data released Dec. 19 show that the trends continued in the first half of 2011. With luck, the United States could soon equal its lowest homicide rate of the modern era: 4.0 per 100,000, recorded in 1957.

To be sure, the United States is still more violent than Europe or Canada, and that’s nothing to brag about. But this country is far, far safer than it was as recently as the late 1980s, when the movie “Robocop,” set in a future dystopia of rampant urban mayhem, both expressed and exploited the public’s belief that criminals ruled the streets — and always would.

We are reaping a domestic peace dividend, and it can be measured in the precious coin of human life. Clearly the experts underestimated Americans’ capacity to take on a seemingly intractable problem and fix it.