Giving Compass' Take:

• Researcher David Roodman found, even using the most favorable cost-benefit valuation for the "deterrence" hypothesis, that even if increasing incarceration rates leads to a decrease in crime, it would not decrease crime enough to justify the costs. The central estimate is still that the marginal effect is zero.

• How can funders use this information to guide prison reform funding efforts? 

• Read about prison reform strategies to reduce incarceration rates

With 698 inmates per 100,000 citizens, the U.S. is the world’s leader in incarcerating people. But what effect does this actually have on crime?

According to David Roodman, Senior Advisor to the Open Philanthropy Project, the marginal effect is zero.

In his comprehensive review of the evidence, David says the effects of crime can be split into three categories; before, during, and after.

Does having tougher sentences deter people from committing crime? After reviewing studies on gun laws and ‘three strikes’ in California, David concluded that the effect of deterrence is zero.

Does imprisoning more people reduce crime by incapacitating potential offenders? Here he says yes, noting that crimes like motor vehicle theft have gone up in a way that seems pretty clearly connected with recent Californian criminal justice reforms (though the effect on violent crime is far lower).

Finally, do the after-effects of prison make you more or less likely to commit future crimes?

This one is more complicated.

His literature review suggested that more time in prison made people substantially more likely to commit future crimes when released. But concerned that he was biased towards a comfortable position against incarceration, David did a cost-benefit analysis using both his favored reading of the evidence and the devil’s advocate view; that there is deterrence and that the after-effects are beneficial.

For the devil’s advocate position David used the highest assessment of the harm caused by crime, which suggests a year of prison prevents about $92,000 in crime. But weighed against a lost year of liberty, valued at $50,000, and the cost of operating prisons, the numbers came out exactly the same.

So even using the least-favorable cost-benefit valuation of the least favorable reading of the evidence — it just breaks even.

Read the full article about incarceration rates by Robert Wiblin and Keiran Harris at 80000 Hours.