The Washington PostDemocracy Dies in Darkness

Wonder why prisons stay full when crime goes down? Here’s the real reason.

(Benjamin C. Tankersley/The Washington Post)

Crime and punishment — and specifically, the United States’ high rates of incarceration — have been in the news lately. In the presidential campaign, protesters have demanded an accounting for the 1994 Crime Bill, which President Bill Clinton signed, former secretary of state Hillary Clinton supported, and Sen. Bernie Sanders voted for. Presumptive GOP presidential nominee Donald Trump has drawn a distinction between punishing violent crimes and nonviolent crimes. And we have seen bipartisan efforts in Congress to scale back mass incarceration.

But most people — scholars, policymakers, and involved citizens alike — misunderstand the relationship between crime and punishment. The conventional wisdom has it that when crime goes up, we should see more people in prison, and when crime goes down, we should see fewer people in prison.  But for two decades, as crime rates have plummeted the prison population has soared.

Scholars have puzzled over this pattern. The economist Glenn Loury writes, “For two generations, crime rates have fluctuated with no apparent relationship to a steady climb in the extent for imprisonment.” David Garland summarizes the scholarly view by explaining, “One axiom of the sociological literature is that punishment and penal measures are, to a considerable degree, independent of crime.”

This view is wrong. Crime and imprisonment are linked. But not in the way most people expect. Let me explain.

Here’s how to measure the way crime and imprisonment are linked

Most prison sentences in the United States are for more than one year. Thus, even if crime goes down, and the number of new incarcerations goes down, the total prison population can still increase — because most of those incarcerated in previous years are still behind bars. Suppose 100 crimes are committed in a given year and all 100 perpetrators are arrested, convicted, and sentenced to three years in prison (with no parole). Now, suppose in the next year the crime rate drops tenfold and only 10 crimes are committed, with all 10 perpetrators arrested, convicted, and sentenced to three years. Because the 100 individuals sentenced last year are still behind bars for another two years, the 10 new convictions will bring the total up to 110. Even though crime went down dramatically, the prison population grew because more people entered prison than were released.

In other words, because of the United States’ extremely long sentences, numbers in prison can go up while crime is going down — even though the crime rate is directly linked to the incarceration rate.

So how can we measure exactly how crime and punishment are linked? Instead of examining how the incarceration rate tracks the crime rate each year, we should examine how the change in the incarceration rate tracks the crime rate.

The reason is simple. If the crime rate influences the incarceration rate, when more crimes are committed, the prison population should increase even more; we will observe a larger change in the incarceration rate. If the crime rate goes down, the incarceration rate can still increase, because many of those in prison are still serving long sentences — but the increase should be much less because there are fewer new incarcerations. We will observe a smaller change in the incarceration rate.

We can see how this works in the two figures below. The first figure shows the standard crime rate/incarceration rate figure that we see in most criminal justice research. From the mid-1970s to 2010, we see the familiar “X” pattern, which makes it look as if the crime rate and incarceration rate are entirely unrelated.

But in the second figure, we see the crime rate compared with the change in the incarceration rate. (Technical note: All series have been standardized to a common range to aid over-time comparisons.) In that second comparison, we see something striking: When there are more crimes, more people are punished, leading the incarceration rate to grow even faster, and when there are fewer crimes, there are fewer people punished, leading to smaller changes in the incarceration rate. Changes in the incarceration rate closely track the crime rate.

As Marie Gottschalk explains in “The Prison and the Gallows,” the standard view holds that “mass imprisonment is only weakly related to the underlying crime rates.” But this is not the case. The number of crimes and the change in the incarceration rate are closely linked.

But that doesn’t explain why so many people in the United States are behind bars for so long

But the amount of crime is not the whole story. The crime rate cannot fully explain why so many people stay behind bars for so long — which gives us the level of mass incarceration to which protesters and reformers object so strongly.

What does explain why so many people in the U.S. are locked up for so long? In “Incarceration Nation: How the United States became the Most Punitive Democracy in the World,” I show how during the last 60 years, media coverage of the crime rate influenced the public’s attitude to punishment, which in turn was the driving force behind the many punitive political decisions that led to mass incarceration.

Many of these political decisions changed how the justice system dealt with crimes. Politicians passed mandatory sentences for many minor crimes that previously only resulted in fines or probation. These same politicians also lengthened sentences for major crimes.

Thus it would be wrong to conclude that the incarceration rate simply reflects the crime rate. Changes in how we punish lawbreakers are the primary reason the overall incarceration rate increased while the crime rate decreased.

Yet just as it is wrong to conclude that crime rates tell the whole story, it is equally problematic to underestimate the relationship between the crime rate and the incarceration rate. The United S now incarcerates more people per capita than any other nation. Protesters and politicians alike have been pointing to the tremendous human and financial costs of mass incarceration.

For years, however, scholars have failed to fully examine how crime influences the incarceration rate because the data suggested that crime had a weak influence, at best.

When we analyze changes in the incarceration rate, we see that conventional wisdom misses an important part of the story. When we study these changes, we find that that crime, media coverage of crime, and the public’s punitiveness are critical to understanding the development — and future — of the U.S. incarceration state.

Peter K. Enns, author of “Incarceration Nation: How the United States became the Most Punitive Democracy in the World,” is an associate professor in the Department of Government at Cornell University and executive director of the Roper Center for Public Opinion Research. Follow him on Twitter @pete_enns.