DALLAS — The United States may soon see its prison population drop for the first time in almost four decades, a milestone in a nation that locks up more people than any other.

The inmate population has risen steadily since the early 1970s as states adopted get-tough policies that sent more people to prison and kept them there longer. But tight budgets now have states rethinking these policies and the costs that come with them.

“It’s a reversal of a trend that’s been going on for more than a generation,” said David Greenberg, a sociology professor at New York University. “In some ways, it’s overdue.”

The U.S. prison population dropped steadily during most of the 1960s, and there were a few small dips in 1970 and 1972. But it has risen every year since, according to the Bureau of Justice Statistics.

About 739,000 prisoners were admitted to state and federal facilities last year, about 3,500 more than were released, according to new figures from the bureau. The 0.8 percent growth in the prison population is the smallest annual increase this decade and significantly less than the 6.5 percent average annual growth of the 1990s.

Overall, there were 1.6 million prisoners in state and federal prisons at the end of 2008.

The economic crisis is forcing states to reconsider who they put behind bars and how long they kept them there, said Kim English, research director for the Colorado Division of Criminal Justice.

In Texas, parole rates were once among the lowest in the nation, with as few as 15 percent of inmates being granted release as recently as five years ago. Now, the parole rate is more than 30 percent after Texas began identifying low-risk candidates for parole.

In Mississippi, a truth-in-sentencing law required drug offenders to serve 85 percent of their sentences. That’s been reduced to less than 25 percent.

California’s budget problems are expected to result in the release of 37,000 inmates in the next two years. The state also is under a federal court order to shed 40,000 inmates because its prisons are so overcrowded that they are no longer constitutional, Austin said.

States also are looking at ways to keep people from ever entering prison. A nationwide system of drug courts takes first-time felony offenders caught with less than a gram of illegal drugs and sets up a monitoring team to help with case management and therapy.

Studies have touted significant savings with drug courts, saying they cost 10 percent to 30 percent less than it costs to send someone to prison.