Yolanda Martinez Graphics by Justice Lab is a column that examines the science, social science and technology of criminal justice

One of the most damning features of the U.S. criminal justice system is its vast racial inequity. Black people in this country are imprisoned at more than 5 times the rate of whites; one in 10 black children has a parent behind bars, compared with about one in 60 white kids, according to the Stanford Center on Poverty & Inequality.

The crisis has persisted for so long that it has nearly become an accepted norm.

So it may come as a surprise to learn that for the last 15 years, racial disparities in the American prison system have actually been on the decline, according to a Marshall Project analysis of yearly reports by the federal Bureau of Justice Statistics and the FBI’s Uniform Crime Reporting system.

This story was published in collaboration with Washington Post's Wonkblog

Between 2000 and 2015, the imprisonment rate of black men dropped by more than 24 percent. At the same time, the white male rate increased slightly, the BJS numbers indicate.

Among women, the trend is even more dramatic. From 2000 to 2015, the black female imprisonment rate dropped by nearly 50 percent; during the same period, the white female rate shot upward by 53 percent. As the nonprofit Sentencing Project has pointed out, the racial disparity between black and white women’s incarceration was once 6 to 1. Now it’s 2 to 1.

205 103 52 34 0 2000 2005 2010 2015 205 103 52 34 0 2000 2005 2010 2015 205 103 52 34 0 2005 2000 2010 2015 Source: Bureau of Justice Statistics

Similar patterns appear to hold for local jails, although the data are less reliable given the “churn” of inmates into and out of those facilities. Since 2000, the total number of black people in local detention has decreased from 256,300 to 243,400, according to BJS; meanwhile, the number of whites rose from 260,500 to 335,100. The charts below from the Vera Institute of Justice also reveal significant drops in the jailing of blacks from New York to Los Angeles, coinciding with little change for whites. (In both the prison and jail data, the total number of incarcerated Latinos has increased, but their actual incarceration rate has remained steady or also fallen, attributable to their increasing numbers in the U.S. population generally.)

Taken together, these statistics change the narrative of mass incarceration, and that may be one reason why the data has been widely overlooked in policy debates. The narrowing of the gap between white and black incarceration rates is “definitely optimistic news," said John Pfaff, a law professor at Fordham University and an expert on trends in prison statistics. "But the racial disparity remains so vast that it’s pretty hard to celebrate. How exactly do you talk about ‘less horrific?'”

According to Pfaff, “Our inability to explain it suggests how poorly we understand the mechanics behind incarceration in general.” In other words, how much of any shift in the imprisonment rate can be attributed to changes in demographics, crime rates, policing, prosecutors, sentencing laws and jail admissions versus lengths of stay? And is it even possible to know, empirically, whether specific reforms, such as implicit bias training, are having an effect on the trend line?

3,457 2,613 449 457 0 2000 2005 2010 2015 3,457 2,613 449 457 0 2000 2005 2010 2015 3,457 2,613 457 449 0 2000 2005 2010 2015 Source: Bureau of Justice Statistics

“If we want to continue or accelerate [the trend], we need to acknowledge it and figure out why it’s happening,” said Adam Gelb, director of the public safety performance program at The Pew Charitable Trusts and an expert on prison data. “Maybe we can bottle it up and replicate it.”

In that spirit, here are four (not mutually exclusive or exhaustive) theories, compiled from our research and interviews with prison system experts, to explain the nearly two-decades-long narrowing of the racial gap in incarceration.

1) Crime, arrests and incarceration are declining overall.

Those decreases benefit the most incarcerated group: African Americans. Crime rates have been on the decline since just after 1990, as have arrests. Given that both measures disproportionately affect the black community, one theory goes, the overall drop should shrink the racial gap in incarceration, too.

According to the Marshall Project analysis of federal data, arrests for nearly every type of crime (one exception was prostitution, which peaked earlier) were on the rise until the early to mid-1990s, followed by a steep decline that affected African Americans more significantly than whites. From 2000 to 2009, for example, there was a 22 percent drop in arrests of black people for violent crime; for whites, it was 11 percent. Since these offenses are most likely to result in substantial prison sentences, the shift most likely transformed the racial makeup of incarceration, as well.

2) The war on drugs has shifted its focus from crack and marijuana to meth and opioids.

As Michelle Alexander (author of “The New Jim Crow”) and many others have extensively documented, the so-called war on drugs has been waged more aggressively against people of color. The arrest numbers bear this out: The black vs. white disparity for drug crimes remains extreme.

But the narrowing of that gap since the mid-1990s — right around the passage of the 1994 crime bill, which is often blamed for the spike in black incarceration — has been nearly as sharp. And in 2000, something else happened: White people started getting locked up for drugs more often. From 2000 to 2009, the black imprisonment rate for drug offenses fell by 16 percent. For white people, it climbed by nearly 27 percent, according to BJS.

What explains this? There is no reliable national data breaking down how many drug offenders are in prison for which particular substances. But experts hypothesize that it may have to do with the waning of one epidemic — crack, which devastated black communities in the 1980s and 1990s — and the emergence of another — meth and opioids, both used in greater numbers by whites more recently. Such a theory may also explain why the narrowing of the racial gap has been more dramatic among women, who are disproportionately incarcerated for drug crimes, according to the Sentencing Project.

3) White people have also faced declining socioeconomic prospects, leading to more criminal justice involvement.

Starting around 2000, whites started going to prison more often for property offenses: robbery, burglary, theft, motor vehicle theft, forgery, counterfeiting and selling or buying stolen property, often categorized as crimes of poverty. From 2000 to 2009, black incarceration for those crimes dropped nine percent, the BJS numbers show. It went up by 21 percent for whites.

Explanations for this shift are also speculative. But some analyses suggest that an overall decline in life prospects for white people over the past few decades has also led an increase in lawbreaking among that population, especially crimes of poverty.

One much discussed study by economists Anne Case and Angus Deaton found that between 1998 and 2013 — precisely when these racial shifts in incarceration were occurring — white Americans saw their rates of mortality, suicide and alcohol and drug abuse spike acutely.

Yet that cannot be the whole explanation for the rise in white incarceration rates. African Americans have long been worse off on most of those socioeconomic metrics: jobs lost to automation and globalization, homes lost to the 2008 mortgage crisis, declining unionization and wages, loans denied. Perhaps, says Marc Mauer, executive director of the Sentencing Project, whites are just newer to the experience of poverty, which could explain why their rates of drug use, property crime and incarceration have ticked up so suddenly.

4) Criminal justice reform has been happening in cities, where more black people live, but not in rural areas.

Starting in 2000, the rate of prison population growth in the United States finally began to slow, thanks not only to declining crime but also to efforts at reform. These have included treatment alternatives for drug offenders, sentence reductions for inmates who participate in educational programming, reentry services for former inmates and reduced sanctions for technical violations of parole.

Yet this is a tale of two Americas: urban and rural. From 2006 to 2014, according to a recent analysis by the New York Times, annual prison admissions plummeted in major cities such as Los Angeles and Brooklyn, due largely to criminal justice reform. But in counties with fewer than 100,000 people, the incarceration numbers have actually risen even as crime declined.

People in rural districts are now 50 percent more likely to be sent to prison than are city dwellers, as local prosecutors and judges there have largely avoided the current wave of reform. New York offers an illustrative example. It reduced its incarcerated population more than any other state during the 2000s — but almost entirely through reductions in the far more diverse New York City, not in the whiter and more sparsely populated areas of the state.

Even with all of these factors at work, the racial inequity of the American prison system remains vast and continues to wreak devastation on black and Latino communities nationwide. At the current rate, the disparities would not fully disappear for many decades.

Even more troubling, racial divides in the juvenile justice system are getting significantly worse. In 2003, black youth were incarcerated at 3.7 times the rate of white youth; by 2013, that number had grown to 4.3.

As Michelle Alexander wrote to The Marshall Project in 2015: “Until we learn the true value of the lives we have wasted, and until we truly reckon with our nation’s history… and until we muster, as a nation, a willingness to invest heavily in the communities that have suffered the most, we will find ourselves in an endless cycle of reform and retrenchment — periods of apparent progress followed by the creation of new systems of racial and social control."

An earlier version of this story included a chart of the imprisonment rates for women that instead showed the figures for men.