The United States, which has less than 5 percent of the world’s population, has about one-quarter of its prisoners. But the relentless rise in the nation’s prison population has suddenly slowed as many states discover that it is simply too expensive to overincarcerate.

Between 1987 and 2007 the prison population nearly tripled, from 585,000 to almost 1.6 million. Much of that increase occurred in states  many with falling crime rates  that had adopted overly harsh punishment policies, such as the “three strikes and you’re out” rule and drug laws requiring that nonviolent drug offenders be locked away.

These policies have been hugely costly. According to the Pew Center on the States, state spending from general funds on corrections increased from $10.6 billion in 1987 to more than $44 billion in 2007, a 127 percent increase in inflation-adjusted dollars. In the same period, adjusted spending on higher education increased only 21 percent.

In 2008, the explosion of the prison population ground to a near halt, according to data released last month by the Bureau of Justice Statistics. About 739,000 inmates were admitted to federal and state facilities, only about 3,500 more than were released.