Is the debate about why the United States imprisons so many people having an impact?

Yes, but slowly, although some states have significantly reduced their prison populations over the last decade, according to a report released Wednesday by The Sentencing Project, the Washington, D.C.-based advocate for sentencing and prison reform.

Nationally, the total U.S. prison population of 2.2 million people has dropped 2.4 percent since 2009 – still no threat to the current U.S. title for imprisoning more of its citizens than any other country. A total of about $80 billion a year in taxpayer money is spent on maintaining inmates and the prisons and personnel who hold them.

Among the states, however, the declines have been more dramatic. The Sentencing Project report finds that 34 states have had a "modest decline" in their prison populations since 1999 and nine – led by New Jersey, with 28.7 percent – showed double-digit declines in their inmate rolls.

The report is based on data from 1999 to 2013 gathered by the U.S. Bureau of Justice Statistics.

Pennsylvania experienced a 2.5-percent decrease in its prison population, a number that may be explained by the state's prison construction boom over the last decade. Pennsylvania, for example, hit its peak inmate population in 2011; New Jersey's peak prison population was in 1999. Delaware's inmate population has dropped 2.1 percent since its peak in 2007, according to the Sentencing Project report.

Sixteen states, most in the South and West, saw their prison populations increase since 2008, the report continues, with four -- Utah, Wyoming, Kansas and Nebraska -- experiencing increases of more than 10 percent and Arkansas leading the nation with a 17-percent increase.

According to the U.S. Bureau of Justice Statistics, the total U.S. prison population remained constant from 1925 until the mid-1970s when politicians began promoting "law and order" platform that favored incarceration and discounted inmate and post-prison rehabilitation programs.

"Just as mass incarceration has developed primarily as a result of changes in policy, not crime rates, so too have declines reflected changes in both policy and practice," the Sentencing Project report concludes.

The report cited the new interest in drug policy sentencing reforms, reductions in returning people to prison for "technical parole violations" and diversion programs for people convicted of lesser drug and property crimes.