Reformers and policymakers who are concerned about the vast U.S. prison system have called for reducing the number of people behind bars. By that standard, they've made progress over the past several years, as the incarcerated population has declined from its peak in 2009.

Yet even as fewer people are behind bars, the number going to prison nationally changed little during that time — outside of California, where the Supreme Court ordered major reforms to the state's overcrowded system in 2011.

John Pfaff, a legal scholar at Fordham University, pointed out the paradox in a series of tweets on Tuesday. While more people are being sent to prison than in 2010, the total population declined because prisoners are serving shorter terms, partly as a result of lawmakers' efforts to reduce minimum sentences. The reduced sentencing are welcome for convicts and their families, but incarceration is not affecting fewer lives.

When California is included, the number of admissions to state and federal prisons declined from 754,000 in 2006, the most in any year, to 631,000 in 2014, the most recent year for which data are available from Department of Justice. Yet that decline occurred almost entirely in California. Between 2006 and 2014, the number of new admissions in California dropped from 139,000 to 39,000.

Prison admissions in the rest of the country have also dipped from their apex of 616,000 in 2006, but that trend seemed to falter around 2010, when 590,000 new inmates were admitted. Excluding California, the number of admissions increased slightly to 593,000 in 2014.

"We’ve seen a decline in the total number in, and an increase in the total number passing through," Pfaff told Wonkblog. He summarized his review of the federal data in a series of tweets.

Pfaff said he was alarmed by what he found, since the data suggest that the costs of incarceration to society — losing your job, being separated from your family, having a felony on your record — could be increasing even as the imprisoned population declines. "We've been looking at it the wrong way," Pfaff said. "We need to really think more about what we should be measuring."

Pfaff argues that instead of focusing on time in prison, reformers should focus their efforts on decisions to prosecute. Prosecutors choose whether to seek incarceration for defendants, so they make some of the most important decisions affecting the number of admissions to prison.

As sentences become less stringent, prosecutors might become more aggressive in seeking prison time, Pfaff argues. Defendants might be more willing to plead guilty in a deal with prosecutors when the law allows prosecutors to offer shorter prison terms. Voters might be more willing to accept a district attorney's choice to incarcerate if the sentence is more lenient — or they might demand more sentences if the community is tough on crime.

Whatever the reason, the number of people going to prison has increased dramatically in several states despite declines in the population behind bars. In Alaska, for example, there was a 45 percent jump in admissions between 2010 and 2014, despite a 3 percent decrease in the incarcerated population. The number behind bars declined by 8 percent in North Carolina over that period, while the number of people sent to prison increased 34 percent.

"Prosecutors continue to avoid any sort of reform," Pfaff said. "That's a huge oversight."

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