For all the talk of curbing America’s appetite for mass incarceration and bipartisan support for reducing prison sentences, the number of people incarcerated in the United States declined only slightly in 2017, according to data released on Thursday by the federal Bureau of Justice Statistics.

The United States still has the largest known incarcerated population in the world.

“If we keep working on the kinds of criminal justice reforms that we’re doing right now, it’s going to take us 75 years to reduce the population by half,” said Rachel Barkow, a sentencing expert at New York University School of Law...

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.