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Over the past 30 years, two powerful trends in criminal justice have been playing out in America, touching every part of the country, red states and blue, big cities and small ones. The first is the astonishing rise of incarceration, a trend that has made the U.S. the most punitive country in the world. The second is the sweeping decline in crime, a phenomenon that has encompassed petty thefts and violent offenses alike.

We often make a terrible mistake in thinking about the relationship between the two, in assuming that the first trend created the second one — that we have less crime precisely because we've locked up all these people. Recent evidence, particularly from states that have reduced their prison rates, refutes this logic. But if anyone is still unconvinced, this jaw-dropping finding from the Brennan Center for Justice at NYU should undermine once and for all the notion that mass incarceration makes us safer: We've locked up so many people in the U.S., that increasing incarceration long ago ceased to have any real impact on crime in this country.

As economists would put it, there are diminishing returns to incarceration. Lock up one criminal in town, and crime will decline. Lock away two, and it will probably decline further. But each criminal in prison yields a smaller and smaller impact outside of it — until finally, there's no new impact at all. Now we're effectively imprisoning more and more people with no benefit to public safety.

According to the Brennan Center, in a new report out today, we reached that point in the U.S. around the year 2000. And we've been experiencing these diminishing returns — incarceration has been losing its power as a crime-fighting tool — since at least the early 1980s:

As a result, the Brennan Center concludes after analyzing state crime data, incarceration can explain only a fraction of the nationwide decline in crime in the 1990s — and it can explain hardly any of it since 2000. Here is a very simple, very blunt illustration of that fact:

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The new report also dives in some fascinating detail into what some of those other factors might be, and how much credit they deserve for making cities in particular safer since the 1990s. The authors, Oliver Roeder, Lauren-Brooke Eisen and Julia Bowling, calculate that declining alcohol consumption can explain about 5-10 percent of the decline in the 1990s and 2000s. They attribute a similar effect to income growth in the 2000s (the more financially stable people are, the less need they have to turn to crime).

And they find that more data-driven policing — using a system known as CompStat — has had a significant impact in cities that adopted it. Not surprisingly, smarter policing (by which the Brennan Center does not mean stop-and-frisk) has a greater impact on reducing crime than ubiquitous and harsh prison sentences.

Much of the decline still remains unexplained. And some politically popular explanations — for instance, that concealed-weapons laws drove down crime — appeared to have no impact at all.

The incarceration finding is particularly important because it gives further support to the idea that we can reduce prison populations and roll back aggressive sentencing laws without endangering public safety. The theory behind incarceration is two-fold: Lock criminals up, and they're not available to commit crime. They — and other people — may also be less likely to commit crimes in the future because they know they'll be punished.

That we would see diminishing returns to these effects makes a lot of sense. Over the years, the prison population has ballooned in large part because we've been locking up more nonviolent and drug offenders — people who weren't committing violent crimes anyway when they were on the street. Evidence also suggests that the threat of prison isn't as effective at deterring future crime as people often think, in part because would-be criminals are bad at gauging the risk of being caught and what will happen when they are.

It's also entirely likely that prison has the opposite effect on some people, reinforcing criminal behavior rather than deterring it. A criminal record effectively makes alternatives to crime less feasible. A man who's spent several years in prison for a drug offense will have a harder time finding a stable job when he leaves. And perversely, as we've gotten awfully good at imprisoning people in the U.S., we've gotten worse at rehabilitating them while they're there. The more people we crowd into prisons, the less likely each one is to get the mental health care, counseling, or job training that would make it possible to re-enter society on the outside.