The report compiled and analyzed data from several sources, including the federal Bureau of Justice Statistics and Bureau of Prisons and each state's department of corrections. It did not include individuals detained for noncriminal immigration violations.
Although studies generally find that imprisoning more offenders reduces crime, the effect may be less influential than changes in the unemployment rate, wages, the ratio of police officers to residents and the proportion of young people in the population, report co-author Adam Gelb said.
In addition, when it comes to preventing repeat offenses by nonviolent criminals -- who make up about half of the incarcerated population -- less-expensive punishments such as community supervision, electronic monitoring and mandatory drug counseling might prove as much or more effective than jail.
For instance, Florida, which has almost doubled its prison population over the past 15 years, has experienced a smaller drop in crime than New York, which, after a brief increase, has reduced its number of inmates to below the 1993 level.
"There is no question that putting violent and chronic offenders behind bars lowers the crime rate and provides punishment that is well deserved," said Gelb, who as director of the Center's Public Safety Performance Project advises states on developing alternatives to incarceration. "On the other hand, there are large numbers of people behind bars who could be supervised in the community safely and effectively at a much lower cost -- while also paying taxes, paying restitution to their victims and paying child support."
Sociologist James Q. Wilson, who in the 1980s helped develop the "broken windows" theory that smaller crimes must be punished to deter more serious ones, agreed that sentences for some drug crimes were too long. However, Wilson disagreed that the rise in the U.S. prison population should be considered a cause for alarm: "The fact that we have a large prison population by itself is not a central problem because it has contributed to the extraordinary increase in public safety we have had in this country."
About 91 percent of incarcerated adults are under state or local jurisdiction. And the report also documents the tradeoffs state governments have faced as they devote larger shares of their budgets to house them. For instance, over the past two decades, state spending on corrections (adjusted for inflation) increased 127 percent; spending on higher education rose 21 percent.
Five states -- Vermont, Michigan, Oregon, Connecticut and Delaware -- now spend as much as or more on corrections as on higher education. Locally, Maryland is near the top, spending 74 cents on corrections for every dollar it spends on higher education. Virginia spends 60 cents on the dollar.
Despite reaching its latest milestone, the nation's incarcerated population has been growing more slowly since 2000 than it did during the 1990s, when harsher sentencing laws began to take effect. These included a 1986 federal law (since revised) mandating prison terms for crack cocaine offenses that were up to eight times as long as for those involving powder cocaine. In the 1990s, many states adopted "three-strikes-you're-out" laws and curtailed the powers of parole boards.
Many state systems also send offenders back to prison for technical violations of their parole or probation, such as failing a drug test or missing an appointment with a supervisory officer. A 2005 study of California's system, for example, found that more than two-thirds of parolees were being returned to prison within three years of release, 40 percent for technical infractions.
"We're just stuck in this carousel that people get off of, then get right back on again," said Los Angeles Police Chief William J. Bratton, who as New York City police commissioner in the 1990s oversaw a significant reduction in crime.
Because of these policy shifts, the nationwide prison population swelled by about 80 percent from 1990 to 2000, increasing by as much as 86,000 a year. By contrast, from 2007 to 2008, that population increased by 25,000, a 2 percent rise.
The U.S. Supreme Court has recently issued decisions giving judges more leeway under mandatory sentencing laws, and a number of states -- including Texas, which has the country's second-highest incarceration rate -- are seeking to reduce their prison population by adopting alternative punishments.
Last year, Maryland officials began developing a new risk-assessment system to ensure that low-level offenders are not kept in jail longer than necessary, said Shannon Avery, executive director of a policy planning division of the state's Department of Public Safety.
"That's what you have to do when you don't have enormous amounts of tax dollars available for building prisons," she said.
Among the early innovators that states can look to is Virginia, which overhauled its system for sentencing nonviolent offenders in the mid-1990s. Although the state's incarceration rate remains relatively high, Virginia has managed to slow the growth of its prison population substantially and reduce the share of its budget spent on corrections while still reducing its crime rate.
State judges use a point system to weigh factors believed to predict a lawbreaker's likelihood of becoming a repeat offender or otherwise pose a threat to public safety. Those deemed low risk are given alternative sentences. As a result, the share of Virginia prison beds occupied by nonviolent convicts has dropped, from 40 percent in 1994 to 23 percent in 2007.
"The idea is to make a distinction between the people we're afraid of and the ones we're just ticked off at," said Rick Kern, director of the Virginia Criminal Sentencing Commission. "Not that you shouldn't punish them. But if it's going to cost $27,500 a year to keep them locked up, then maybe we should be smarter about how we do it."