For decades now, social scientists and criminologists have been railing against America's sky-high incarceration rate. There are 1.6 million adults in state and federal prisons around the country, and many experts believe the costs now vastly outweigh the benefits.

So at first glance, this December report from the U.S. Justice Department looks like encouraging news. After years of relentless increases, the number of adults in state and federal prisons has finally started dropping, declining slightly in 2010 and then falling 0.9 percent in 2011 (or 15,023 fewer prisoners):

Why the decline? As the report details, about 70 percent of the state-level drop was due to California. Back in 2011, the state legislature passed new laws to shrink the prison population in response to a court order. As a result, California slowed down the rate of admissions and had 15,000 fewer prisoners by the end of the year. (Here's an analysis from the ACLU on the ups and downs of California's policy—many would-be prisoners are now being placed instead in county jails or shifted to post-release programs.)

But it wasn't just California. Twenty-five other states also saw their prison populations drop slightly, with New Jersey, New York, Florida, and Texas each shedding at least 1,000 state prisoners. In general, states appear to be locking up fewer drug offenders and focusing more heavily on violent offenders, the report said.

The picture is very different at the federal level, however. Federal prisons actually added 6,409 new inmates in 2011, an increase of 3.4 percent. That was driven by yet another steep rise in drug sentencing — drug offenders now make up nearly half of the 198,000 federal inmates. So far, Congress hasn't felt the same budget pressures that states have to thin out its prisons.

Will the incarceration drop last? That's the big question. At the moment, a huge portion of the decline is being driven by California, where the state legislature had to be prodded by the courts to do something about its overstuffed prisons. Other states are moving much more slowly. And as a recent report from the Urban Institute explained, federal prisons will keep ballooning unless Congress changes its sentencing guidelines.

For a more optimistic take, see Kevin Drum, who argues that a decline in the U.S. prison population is inevitable because lead levels in the environment are falling—he just wrote a big piece on the link between lead and crime. "Young people," he points out, "who were raised in the post-lead era, have sharply lower arrest rates than in the past and sharply lower prison rates."

Either way, it's a notable trend—and a largely under-reported one. As Keith Humphreys points out, this is probably because state and local officials don't want to trumpet the fact too loudly:

Most of the state, local and federal officials who have helped reduce incarceration are scared to publicly take credit for it. In general, reducing incarceration is a good thing, but probability dictates that in particular cases it will be a horrible thing. At least a handful of the roughly 100,000 fewer people under correctional supervision in 2011 versus 2010 for example will do something extremely violent and high-profile, and no politician wants to risk being in a story headlined “Convict released by thug-loving governor murders nun.”

Another reason to wonder if the drop could still reverse itself at some point.

Further reading:

–Hat tip to Alex Tabarrok and his commenters for the links, although they're discussing a slightly different report looking at all adults in prison, jails, and under probation and parole. About 83 percent of the drop in his graph is due to a decline in people under probation.

–Jim Webb held some key Senate hearings on the need for prison reform back in 2007, but the issue never became a top priority for Congress.

–Mark Kleiman explains how smarter parole and probation policies can cut the U.S. incarceration rate.