A just released report by the Bureau of Justice Statistics shows that the number of Americans under criminal justice supervision has dropped for the sixth year in a row. At the end of 2013, the proportion of Americans under supervision had returned to its 1995 level. To put it another way, the 12-year rise in the rate of criminal supervision from 1995-2007 was wiped out in 6 years of more rapid decline.

How do we reconcile an 18-year low in the rate at which Americans are under criminal justice supervision with despairing commentary published only a few months ago saying that mass incarceration was on the rise? Those stories focused on the fact the number of people in state prison rose slightly in 2013. But that data point didn’t give a good picture of where the correctional system is going overall, for two reasons.

First, although it is small by comparison to the state prison system, we have a federal prison system and its population declined for the first time in decades in 2013, cancelling out some of the impact of the rise in state prisoners. Also, the size of the U.S. population grew in 2013, meaning that the rate of imprisonment actually dropped that year despite a small increase in the raw number of prisoners.

More importantly, the correctional system has three components of which prison is only one. The others are jails (local facilities generally used for shorter sentences and less severe offenders) and probation/parole (offenders live in the community but are monitored). Many people do not understand the dynamic flows between these three parts of the system. Every day thousands of people move from one to the other — from prison to parole, from parole back to prison, from jail to probation and so on.

As a result, even in a period of overall decline in the correctional population such as we are experiencing now, one can often find one component or subcomponent of the system that is growing. A good example comes from 2012, in which after shrinking for a few years the jail population surged. But the correctional system still shrank that year because the jail surge was due to California realignment: The new people in jail had been moved out of prisons. By the next year many of those individuals were released from jail and the jail numbers returned to their declining trend.

[Cross-posted at The Reality-Based Community]

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Keith Humphreys is a Professor of Psychiatry at Stanford University and served as Senior Policy Advisor in the White House Office of National Drug Control Policy in the Obama Administration. @KeithNHumphreys