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Despite what you may have heard from Donald Trump, crime rates in the U.S. have declined steadily since the mid-1990s, and are currently at their lowest levels in more than four decades.

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And yet, the country's prison population has continued to hold steady. As of 2015, nearly one in every 200 people in America is incarcerated.

In a new paper, sociologist Ryan King of Ohio State University offers an explanation for what he calls the "persistence of mass incarceration in a low crime era."

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His hypothesis is that even though more crime isn't being committed, the type of person who's now in prison has changed over time. Namely, they are more likely to be repeat offenders, who are subject to rigid sentencing guidelines that put undue weight on an individual's prior crimes.

To test his theory, King gathered data case-level information on each of the approximately 355,000 felony convictions in Minnesota between 1981 and 2013. He found that in 1981, fewer than 40% of sentenced offenders had prior criminal records, but almost 60% did in 2001.

"Criminal activity can decrease, but the criminal record only goes up," he said in a release. "These records do not vanish; they are sticky, persistent, and accrue with subsequent arrests and convictions over time," he writes.

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Here's the chart showing the number of prior offenses for a given defendant over time.

King looked at other factors that might be correlated with increasing incarceration rates, including race, type of crime, and the gender of defendants, and found most of these correlations were not nearly as strong as with the defendants' prior offense history.

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Looked at through the lens of King's data, the narrative about America's incarceration problem is much different. Basically: the increased crime rates of the 1970s through the early 1990s, combined with the War on Drugs, left a legacy of individuals with criminal records. And because of the way sentencing guidelines are structured, those people are likely to spend more time in prison on future offenses than first-time offenders.

"Judges and prosecutors are now dealing with a different sort of offender today than in years past," King writes.

In an email, Marc Mauer, executive director of the Sentencing Project, a nonprofit focused on criminal justice reform, said King's study provided valuable new information on America's mass incarceration phenomenon.

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"We need to be cautious in interpreting it, of course, since the data are only from one state, but they are consistent with other trends that we've seen," he wrote. "In particular, the average age of individuals in prison has been increasing, which suggests that there are a greater number of people admitted to prison for repeat offenses."

King said he hopes his research will help direct future policies meant to reduce the incarceration rate.

"Any discussion that focuses only on leniency for first-time offenders is bound to fail, at least if significant declines in incarceration are the goal," he writes. "An implication of this research is that a serious attempt to bring down incarceration must take into consideration how the prior record affects imprisonment rates."

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Rob covers business, economics and the environment for Fusion. He previously worked at Business Insider. He grew up in Chicago.