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Understanding mass incarceration in the US is the first step to reducing a swollen prison population

in the United States fell in 2021 to its lowest levels since 1995—but the U.S. continues to imprison a higher percentage of its population than almost every .

The U.S. actually had the greatest percentage of its population imprisoned until 2019. This followed steady growth in prison and jail populations in the 1970s, after a wave of and policies swept the nation.

While there has been a of the need to reduce , experts agree on or the best path to reducing it.

As a former prosecutor and a researcher who studies the , I have found that understanding how the U.S. incarceration rate grew over the is the key to understanding its root causes—and what it will take to return to lower rates.

As I , "Mass Incarceration Nation, How the United States Became Addicted to Prisons and Jails and How It Can Recover," people tend to talk past one another when they discuss crime and punishment in the U.S. I think the public debate can improve if people develop a better understanding of how mass incarceration arose—and its tenuous connection to crime.

A growing prison population

The growth in mass incarceration began with a crime spike. Homicides, which averaged around , reaching over 24,000 in 1991.

The crime spike sparked a bipartisan wave of punitive laws, the hiring of and a that permeated every aspect of American criminal law. The system became more punitive, generating longer sentences, especially for repeat and violent offenses, as I show in my book.

Over time, this led to today's and many people being held long past the time they would have been released in other countries and at other times .

The number of people 55 or older in state and federal prisons increased 280% from 1999 to 2016,

Different kinds of crime

But longer sentences are only one factor in America's supersized incarceration rates.

There has also been a for which U.S. courts imprison people.

After the 1970s, more and more people went to prison for drug crimes and other offenses that rarely used to lead to prison time.

Serious violent crime, meanwhile, went . The crimes—like armed robbery and murder—that had sparked the march toward mass incarceration plummeted.

But prison populations didn't drop.

As a prosecutor in Washington, D.C. in the early 2000s, I saw this change firsthand. Our caseloads were increasingly dominated by drug sales, drug possession and gun possession cases—cases which, not coincidentally, are typically the easiest to detect and prove. These changes were happening on a national level.

The number of people incarcerated in state prisons for homicide increased by over 300% between 1980 and 2010, reflecting the temporary spike in homicides and longer sentences for those convicted of that offense.

But the scale of the increases for other offenses, like drug crimes, is even larger—rising 1,147% over this time frame.

Speaking the same language

While prison populations are finally starting to go down, progress is slow. At the current rate, it will take decades to reach the low incarceration rates the U.S. had for most of its history.

This dip is partially , which prompted some states to release prisoners to avoid overcrowding and health risks. It is not clear that these recent reductions in the incarcerated population will continue.

I think that substantially reducing prison and jail populations will require better understanding of the link between incarceration and crime. It is not simply the case that incarceration goes up because people commit crime; instead, the story is much more complicated. That is because we use incarceration for two purposes: to obtain justice on behalf of victims and to try to change people's behavior.

This distinction results in two kinds of cases flowing into this nation's criminal courts.

First, there are cases that involve the most serious harm to individuals, like crimes of sexual violence and murder. Second, there are cases like drug offenses and weapons possession, which are not typically about obtaining justice for victims but are supposed to further policy goals like preventing drug use.

Changes in how we treat both kinds of cases contributed to the nation's sky-high incarceration rate. American mass incarceration is a result of for people who commit serious violent crimes. But it is also a product of a stunning expansion of the system's reach in the form of more and more crimes leading to prison and jail.

Substantial progress at reducing the incarcerated population will require reversing both trends. First, returning sentence lengths for all offenses, including serious violent crime, to their historical norms. And second, resisting this country's growing habit of relying on incarceration as a tool for achieving policy goals.