Is the rate of incarceration in the united states one of the lowest in the world?

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 and author of “Prisoners of Politics: Breaking the Cycle of Mass Incarceration.”

Like others who study the United States prison population, Ms. Barkow saw the significance of Thursday’s report not in the decline itself, but in how minor it was.

“The kinds of reforms we’re seeing now are really modest,” she said. “I’m glad were getting them. But this is not transformative yet.”

Slightly under 1.5 million people were in prison at the end of 2017, a slight decrease from 2016 but still a population that, if gathered in one place, would be one of the largest cities in the country.

County and city jails held around 750,000 inmates in mid-2017.

Combined, the United States would remain the world’s leader in incarceration, according to data collected by the Institute for Criminal Policy Research at Birkbeck, University of London. It is unclear, though, exactly how many people are held in detention in China, a country with a similarly high prison population.

The incarceration rates for both jails and prisons in the United States have declined by more than 10 percent over the past 10 years, the federal report found. (Prison is for people serving sentences longer than one year, while jails typically hold those awaiting trial or sentencing, or those serving shorter sentences.)

The decline in the prison population is not connected to the crime rate, which has fallen steadily over the past decades. Instead it is it the result of policy changes and court orders, and has been markedly uneven.

A drop in the federal prison population, due in large part to a 2014 decision by the U.S. Sentencing Commission to reduce sentences for drug crimes, accounts for a third of the year-over-year decline. And while some states have significantly reduced their prison populations in recent years, others continue to set records for the number of people they are keeping locked up.

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.

The rate of black adults serving prison sentences declined by nearly a third over the past decade. But the racial disparity among men remains stark, with black men serving prison sentences at almost six times the rate of white men.

The racial disparity among women in prison, however, has considerably narrowed.

Black women are still incarcerated at around double the rate of white women. But the number of white women in prison has increased by more than 40 percent since the turn of the century, while the number of incarcerated black women has dropped by nearly half.

Wendy Sawyer, a senior policy analyst at the Prison Policy Initiative, suggested that several trends may explain this. Arrests of women for drug violations have spiked over the last decade, even as drug-related arrests have declined for men, she said. Separately, statistics show a shift generally “from urban areas to rural areas in light of the opioid crisis.” It is possible, she said, that this was driving the increased incarceration of white women.