In a six-year period, California simultaneously cut its prison population by 23 percent and its violent crime rate by 21 percent — both over the national average, according to a new report by the Sentencing Project.

The group, which advocates for sentencing reform, found that during 2006-2012, the national violent crime rate dropped by about 19 percent, while the nationwide state imprisonment rate (meaning not including federal prisons and local jails) fell 1 percent. The property crime rate in California also fell, but less than the national average (13 versus 15 percent).

California's success — along with New York and New Jersey, which experienced even greater crime drops — "demonstrate that it is possible to substantially reduce prison populations without harming public safety," the report says.

Different policy changes accounted for the prison population drops in each state, the report says.

In New York, the New York City Police Department's shift in enforcement priorities away from drug crimes helped lower the prison population. In New Jersey, changes in parole policies and drug crime sentences helped empty the prisons. California's drop largely came through realignment, a policy implemented in 2011 which shifted state prisoners to county control--a result of the U.S. Supreme Court ordering the state to relieve prison overcrowding.

Though realignment has not been statistically associated with a rise in crime, it has presented California counties with budgetary and policy challenges.

Recently, Stanford Law Professor Joan Petersilia, one of the state's most well known experts on the policy, wrote in the Harvard Law & Policy Review that realignment gets "mixed reviews" from county leaders in California.

"It is one thing to urge prison downsizing, but such pronouncements will be hugely counterproductive if policymakers act without giving serious thought to how communities will deal with all the offenders who are released," she wrote.

Petersilia also put forth a number of widely endorsed tweaks to realignment, including capping the time an inmate can spend in county jail, collect more county-level data to see what's working and what's not, and consider an offender's history rather than most recent crime during sentencing.

Find the full Sentencing Project report here.