The number of American adults is about 230 million, meaning that one in every 99.1 adults is behind bars.

Incarceration rates are even higher for some groups. One in 36 adult Hispanic men is behind bars, based on Justice Department figures for 2006. One in 15 adult black men is, too, as is one in nine black men ages 20 to 34.

The report, from the Pew Center on the States, also found that one in 355 white women ages 35 to 39 is behind bars, compared with one in 100 black women.

The report’s methodology differed from that used by the Justice Department, which calculates the incarceration rate by using the total population rather than the adult population as the denominator. Using the department’s methodology, about one in 130 Americans is behind bars.

The increase in the number of prisoners over the last 18 months, the Pew report says, pushed the national adult incarceration rate to just over one in 100.

“We aren’t really getting the return in public safety from this level of incarceration,” said Susan Urahn, the center’s managing director.

But Paul Cassell, a law professor at the University of Utah and a former federal judge, said the Pew report considered only half of the cost-benefit equation and overlooked the “very tangible benefits: lower crime rates.”

In the past 20 years, according the Federal Bureau of Investigation, rates of violent crimes fell by 25 percent, to 464 per 100,000 people in 2007 from 612.5 in 1987.

“While we certainly want to be smart about who we put into prisons,” Professor Cassell said, “it would be a mistake to think that we can release any significant number of prisoners without increasing crime rates. One out of every 100 adults is behind bars because one out of every 100 adults has committed a serious criminal offense.”

The United States imprisons more people than any other nation in the world. China is second, with 1.5 million people behind bars. The gap is even wider in percentage terms.

Germany imprisons 93 out of every 100,000 people, according to the International Center for Prison Studies at King’s College in London. The comparable number for the United States is roughly eight times that, or 750 out of 100,000.

Ms. Urahn said the nation could not afford the incarceration rate documented in the report.

“We tend to be a country in which incarceration is an easy response to crime,” she said. “Being tough on crime is an easy position to take, particularly if you have the money. And we did have the money in the ’80s and ’90s.”

Now, with fewer resources available, the report said, “prison costs are blowing a hole in state budgets.”

On average, states spend almost 7 percent of their budgets on corrections, trailing only health care, education and transportation.

In 2007, according to the National Association of State Budget Officers, states spent $44 billion in tax dollars on corrections. That is up from $10.6 billion in 1987, a 127 percent increase when adjusted for inflation. With money from bonds and the federal government included, total state spending on corrections last year was $49 billion. By 2011, the Pew report said, states are on track to spend an additional $25 billion.

It cost an average of $23,876 dollars to imprison someone in 2005, the most recent year for which data were available. But state spending varies widely, from $45,000 a year in Rhode Island to $13,000 in Louisiana.

“Getting tough on crime has gotten tough on taxpayers,” said Adam Gelb, the director of the public safety performance project at the Pew center. “They don’t want to spend $23,000 on a prison cell for a minor violation any more than they want a bridge to nowhere.”

The cost of medical care is growing by 10 percent annually, the report said, and will accelerate as the prison population ages.

About one in nine state government employees works in corrections, and some states are finding it hard to fill those jobs. California spent more than $500 million on overtime alone in 2006.

The number of prisoners in California dropped by 4,000 last year, making Texas’ prison system the nation’s largest, at about 172,000. But the Texas Legislature last year approved broad changes to the state’s corrections system, including expansions of drug treatment programs and drug courts and revisions to parole practices.

“Our violent offenders, we lock them up for a very long time — rapists, murderers, child molesters,” said State Senator John Whitmire, Democrat of Houston and the chairman of the Senate’s Criminal Justice Committee. “The problem was that we weren’t smart about nonviolent offenders. The Legislature finally caught up with the public.”

Mr. Whitmire gave an example.

“We have 5,500 D.W.I offenders in prison,” he said, including people caught driving under the influence who had not been in an accident. “They’re in the general population. As serious as drinking and driving is, we should segregate them and give them treatment.”

The Pew report recommended diverting nonviolent offenders away from prison and using punishments short of reincarceration for minor or technical violations of probation or parole. It also urged states to consider earlier release of some prisoners.

Before the recent changes in Texas, Mr. Whitmire said, “we were recycling nonviolent offenders.”