The rate of violent crimes in the United States has been cut in half during the past 20 years.

That’s according to statistics released in November by the FBI.

What surprises some criminologists is that the decline has persisted against the backdrop of two other trends long believed to lead to an increase in crime: The US prison population is dropping and the number of young adults, who are considered more likely to commit a crime, has risen.

In 2013, the estimated number of violent crimes declined 4.4 percent, compared with the year before, according to FBI figures released in November. And the rate of violent crimes per 100,000 people in America has fallen to 368 per 100,000 US residents, down from about 714 in 1994.

The last time there were fewer than 400 violent crimes per 100,000 Americans was in 1971.

Those numbers suggest that, even though declining crime is a long-term trend, the reasons for the drop have shifted.

Criminologists say the latest gains against violent crime appear rooted especially in improved law enforcement, rather than a drop in the number of teenagers and 20-somethings.

The “community policing” movement has brought beat cops into closer ties with the neighborhoods they serve, for one thing, and a data-driven focus on crime “hotspots” has led to successful prevention efforts in high-risk areas.

“It’s certainly good news,” and it’s largely a tribute to hard work by police chiefs as change agents across the country, says Scott Decker, a criminologist at Arizona State University in Phoenix.

Among other factors contributing to the trend, Professor Decker says, are heightened efforts to prevent recidivism when inmates are released.

“We’re paying increased attention to getting them ready to not offend when they get back on the street,” he says.

In fact, most US states have reduced incarceration rates and simultaneously reduced crime during the period from 2008 to 2013, according to analysis of government data by the Pew Charitable Trusts.

“States are continuing to defy the common assumption that crime would keep shrinking only if prisons kept expanding,” Adam Gelb of Pew’s public safety performance project said in releasing the group’s analysis last month.

That notion – that crime can be controlled and even cut while reducing prison populations – “is really starting to sink in with voters and elected officials,” at a time when prison costs are a big part of strained state budgets, Mr. Gelb said.

The total number of US prisoners has fallen since 2009, although last year had a small uptick (0.3 percent).

For years, rising prisoner tallies and a decline in the number of younger, more crime-prone Americans were cited as important factors behind the drop in crime.

Over the past decade, though, the demographic part of the equation has been nuanced, as the children of baby boomers swelled the ranks of teens and and 20-somethings. In 2010, for example, the number of Americans age 15 to 29 was higher by 6 million than in 2000, and also a bit higher when measured as a share of the total population.

All this doesn’t mean the gains against crime will continue uninterrupted – or that prisons aren’t important for reducing crime. The FBI figures show some up years for violent-crime rates (as recently as 2012) as well as down ones in the past two decades.

Partly for state-budget reasons, the justice system is getting choosier about who gets put in prison.

“One of the most positive trends that you see in the United States ... is reserving those prison beds for either the violent” or serious repeat offenders, says Joan Petersilia, a Stanford University law professor who helped California reduce its prison population.

The strategy, she argues, can keep many low-level offenders from turning into repeat offenders.

Some states have combined lower prison populations with increased efforts at rehabilitation, to reduce the likelihood of repeat offenses once an inmate is released.

In Michigan, a “prisoner reentry initiative” helped lead to a roughly one-third percent reduction in parolees returning to prison in recent years. The program involves assessing inmate risks and needs, and helping to address them both during and after prison. Goals include helping former inmates to find jobs, housing, and counseling.

Other states including Ohio and New York have been working along similar lines to help former convicts stay out of prison.

Dr. Petersilia’s view, argued in a 2012 report, is that efforts to help former inmates with issues like jobs and drug abuse would pay for themselves by reducing future costs of crime, court proceedings, and prison terms.

Beyond the incarceration trends, one of the stunning features of the recent fall in crime is that it persisted generally even during the deep recession of 2007 to 2009 and the tough economic times that followed. That may be a testament, in part to the rise of security cameras on stores and city streets.

But it doesn’t mean there’s no tie between crime and economic conditions.

Falling crime rates can help a local economy revive. Just consider the resurgence of Times Square and other parts of New York City in recent years. And more jobs and capital investment can fuel further progress against crime.

That can go hand in hand with the policing efforts that will remain vital, experts say.

Events this past year have shown how, despite nationwide efforts to integrate police into their communities, police in many neighborhoods still have a long way to go to build and maintain trust. Two key examples are the controversial deaths of two unarmed black men, Michael Brown in Missouri and Eric Garner in New York. Both men’s deaths were caused by police officers and have sparked protests and loud calls for reforms of police use of force.

A final caveat: It’s important not to read too much into the headline crime numbers. Petersilia notes that prosecutors have been increasingly defining assault cases as misdemeanors, rather than felonies, as part of the effort to reduce prison populations. Those cases won’t show up under aggravated assault, the largest category in the FBI’s violent crime index.

And significant criminal activity is now happening in cyberspace, in categories that don’t show up in the FBI tracking, she adds. Despite the statistical question marks, though, she shares the view of other experts that violent crime has been heading downward.