After rising inexorably for more than three decades, the U.S. prison population has declined three years in a row. Few people see this as anything other than an extremely positive development. But will it last? Some prison policy watchers are more optimistic than others.

Mike Konczal is one of many smart people who has raised the worry that as The Great Recession disappears into the rear-view mirror, states will return to fiscal health and consequently lose interest in slashing prison budgets.

I don’t share Konczal’s anxiety, for two reasons. First, because the general pattern in U.S. history is for prison populations to grow rather than shrink during economic downturns, I am not convinced that The Great Recession was very important to the reversal of the 30+ year mass incarceration trend. Second, states like South Dakota whose public finances are already in rude health are nonetheless taking major steps to reduce incarceration.

A different case for pessimism, among some liberals at least, is that now that some prisons are privatized, the powerhouse lobbyists of that industry will prevent further de-incarceration. Some people on the political right are too reflexively fearful of government and too trusting of the private sector. Prison policy is a case where the opposite set of biases afflicts some analysts on the left. Over 90% of U.S. inmates are in public prisons. The political power of public sector unions on incarceration-related issues thus dwarfs that of the small private sector. If the private prison operators and public sector prison employees unions allied in the cause of preventing de-incarceration, it could be a significant political problem, but that’s not very likely because they hate each others’ guts.

Turning to the case for optimism, Charles Lane persuasively cites the impact of our still-falling crime rate:

Less crime leads to declining incarceration in two ways. First, and most obvious, there are fewer law-breakers to lock up. Second, safer streets reduce the public’s demand for tough “law and order” policies – like the stiff mandatory minimum sentences that helped drive the U.S. rate of incarceration up in the 80s and 90s.

Kevin Drum is even more upbeat based on his analysis of lead exposure research. He argues that the generation that grew up in the leaded gasoline era was uniquely violent. As they age out and are replaced by non-exposed generations, Kevin expects crime and incarceration rates to continue their fall.

You don’t have to accept the lead explanation to make an equally positive projection about the future. Prisoners tend to have long criminal histories that began when they were teenagers. As a result, the current size of the prison population reflects the crime rate of a decade or two ago better than it does that of the present moment. Ten years from now, the prison population will better reflect the low crime rate we have been enjoying in recent years, which translates into many fewer people serving hard time.

[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