Some of the states with notable drops, including New Jersey, New York and Connecticut, pushed through significant policy changes, such as reclassifying felonies as misdemeanors and giving more discretion to sentencing judges.

But one of the large reductions over the past decade was in California, which was ordered by the U.S. Supreme Court in 2011 to reduce its overcrowded prisons by 30,000 inmates. That this was done under court order, rather than voluntarily, shows the political difficulty of taking aggressive steps to curb mass incarceration.

At the end of 2016, the federal report said, more than half of state prisoners had been convicted of violent offenses, even though the country’s violent crime rate has dropped drastically since the early 1990s. So many experts say that a dramatic reduction in the prison population is unlikely if policy changes address only nonviolent offenses — what Ms. Barkow refers to as “the low hanging fruit.”

The size of the United States prison population has resulted from not only locking more people up, but also keeping them locked up longer.

A record number of people are serving life sentences, according to the Sentencing Project, a group that advocates improvements to the criminal justice system. In fact, while the United States accounts for about 4 percent of the world’s population, it has more than a third of the estimated number of people serving life sentences, according to “Life Imprisonment: A Global Human Rights Analysis,” an international survey by a professor and researcher at the University of Nottingham.

The statistics released on Thursday showed that the nature of the prison population has changed in a number of ways.

As measures like parole and compassionate release have been curtailed, or even eliminated in some places, prisoners have become older and more costly. According to the report, more than one in 10 prison inmates in 2017 were 55 years or older.