By Flickr user Sean Hobson. (Used under a Creative Commons license)

The U.S. imprisonment rate has fallen for a fifth straight year, a run not seen since Richard Nixon was in The White House.  According to data released Tuesday from the Bureau of Justice Statistics, while the U.S. incarceration rate is still remarkably high, 2013 marks a 10-year low.  For five reasons, the de-incarceration trend has an excellent chance of continuing.

First, crime is down by about 50% in the past two decades.  Practically, this reduces incarceration because there are fewer criminals to lock up.  Politically, lower crime rates eventually result in a less fearful and punitive public, which shifts elected officials’ incentives away from tougher sentences and toward de-incarceration.

Second, U.S. prison policy is primarily set by states and hence is less constrained than other potential reforms stalled by Washington's political gridlock.  The federal government has some influence over state correctional policy through the grants it gives and the political signals it sends, but states call most of the shots for the simple reason that they operate the facilities in which 85-90% of U.S. prison inmates are incarcerated.  And in states as different as South Carolina, Michigan and Texas, elected officials are in a de-incarcerating mood.

Third, even though conservatives and liberals are battling each other vigorously on many policy fronts, de-incarceration is not one of them.   Evangelical Christian Conservatives have long objected to the inhumanity of prison overcrowding while economic and libertarian conservatives have correctly diagnosed mass incarceration as a burden both on human freedom and the public purse.  In many states, they have joined forces with liberal politicians concerned about racial disparities in incarceration and whether prison costs could squeeze out funding for education and health care.

Fourth, a new generation of evidence-based community supervision programs such as drug courts, 24/7 Sobriety, and HOPE probation are increasingly attracting the attention and support of state and federal policymakers.  These programs monitor drug and alcohol-involved offenders more closely than traditional community supervision, providing a mixture of carrots and sticks that have protected public safety while simultaneously reducing substance use, crime and re-incarceration.

Last, just as a high crime rate can create the conditions for more crime (e.g., by overwhelming law enforcement) and a low crime rate can create the conditions for less crime (e.g., by encouraging more citizens to walk the streets at night), lower imprisonment rates also appear capable of creating virtuous self-reinforcing cycles.  Since the federal government began tracking incarceration almost 90 years ago, the years in which the incarceration rate dropped were concentrated in distinct eras rather than random periods of time. The first was during World War II, when the rate fell every year, dropping by 27% from 1940 to 1945.  The incarceration rate also declined every year from the early 1960s until the early 1970s, before a massive wave of rising imprisonment began a three-decade run.

De-incarceration appears to be on another of its multi-year runs, and it’s not hard to see why.  States are saving money by downsizing prisons and are not seeing crime rise.  That encourages those states to continue pursuing reform and makes onlooking states more likely to mirror their de-incarceration initiatives.

Keith Humphreys is the Director of Mental Health Policy and a Professor of Psychiatry at Stanford University.  He is on Twitter @KeithNHumphreys