As Attorney General Eric Holder observed last summer, "too many Americans go to too many prisons for far too long, and for no truly good law enforcement reason." A new report from the National Research Council (NRC) analyzes the causes and consequences of the "historically unprecedented and internationally unique" expansion in the U.S. prison population since the early 1970s. That punitive binge left us with 2.2 million Americans behind bars and an incarceration rate "5 to 10 times higher than the rates in Western Europe and other democracies." In 2012, the report notes, "close to 25 percent of the world's prisoners were held in America's prisons, although the United States accounts for about 5 percent of the world's population."

How did that happen? In brief, American voters and politicians freaked out about crime, demanding ever tougher policies that increased the likelihood and length of incarceration. The report highlights three policies in particular: mandatory minimum sentences, "truth in sentencing" laws that limited or abolished parole, and "three strikes" laws imposing long terms (including life sentences) on repeat offenders. Stepped-up enforcement of drug prohibition magnified the impact of all those policies.

Contrary to what you might think, there is no clear relationship between the intensity of this response and the crime rates that ostensibly provoked it. "Over the four decades when incarceration rates steadily rose," the NRC report says, "U.S. crime rates showed no obvious trend: the rate of violent crime rose, then fell, rose again, then declined sharply." While most studies of the question find that all this imprisonment had some impact on crime through deterrence and incapacitation, the report says, the magnitude of the effect is "highly uncertain." In any event, the authors conclude, long prison terms are not a cost-effective way of preventing crime, given research showing that recidivism falls sharply as convicts age and that the likelihood of apprehension is more important in deterrence than the severity of the punishment. Furthermore, locking up the wrong people—in particular, nonviolent drug offenders—accomplishes nothing in terms of preventing predatory crime.

The crime-reducing effects of incarceration, of course, have to be weighed against the costs, not just in tax dollars but in lost liberty, missed earnings, abandoned families, weakened communities, alienation, and permanently impaired employment prospects. The authors emphasize that such costs are borne disproportionately by poor people with little education, blacks and Hispanics in particular. "We believe that the policies leading to high incarceration rates are not serving the country well," they conclude. "We are concerned that the United States has gone past the point where the numbers of people in prison can be justified by social benefits. Indeed, we believe that the high rates of incarceration themselves constitute a source of injustice and, possibly, social harm."

For Reason readers, who might use somewhat stronger language to describe this situation, the thought that our government too readily locks people in cages and keeps them there too long is not exactly novel. But the 464-page NRC report, which is available for free online (or for $75 as a paperback!), gathers together a wealth of detail about the costs and benefits of incarceration that can be used to win over anyone who still thinks building more prisons is a sound investment in public safety.

The report is well timed (I hope) to influence the current congressional debate over sentencing reform. So far, with the exception of the 2010 law that reduced federal crack sentences, state legislators have taken the lead in this area. "In recent years," the NRC report notes, "the federal prison system has continued to expand, while the state incarceration rate has declined. Between 2006 and 2011, more than half the states reduced their prison populations, and in 10 states the number of people incarcerated fell by 10 percent or more."