Mass incarceration of women was one of the important issues highlighted by the Women’s March on Washington. The U.S. imprisonment rate has been declining for almost a decade, yet the number of women in prison at the end of 2015 (about 105,000) is virtually identical to what it was when the de-incarceration trend began. The size of the female inmate population is being maintained by surging imprisonment among women of a particular race: whites.

Bureau of Justice Statistics data allow a 30-year perspective on the imprisonment rates of black and white women. In 1985, the rate for white women was extremely low (10 per 100,000), whereas for black women it was much higher (68 per 100,000). In the ensuing 15 years both groups experienced sharply increased imprisonment rates, but the burden was particularly acute for black women, who at a rate of just over 205 per 100,000 were imprisoned in 2000 at an unprecedented rate.

But the black female imprisonment rate then began falling and has kept doing so to this day, whereas the white female rate continued the rise it began 30 years ago. Activists who think the prison population can be cut by 50 percent are sometimes dismissed as unrealistic, yet this is precisely what black women experienced over the past 15 years. In contrast, the current rate of white female imprisonment (52 per 100,000) is almost certainly a high.

Black women remain imprisoned at a higher rate than white women, but the gap has shrunk from about 7 to 1 in 1985 to about 2 to 1 in 2015. Because whites are a much larger population, the increase in white female imprisonment has easily refilled the prison cells that black women have been vacating. At the end of 2015, white women (52,700) outnumbered black women (21,700) in prison by about 2.5 to 1.

This stunning change in the racial makeup of the female inmate population mirrors and may well be at least partially caused by changes on other indicators of economic and physical well-being. Over recent decades, life expectancy among women without a college education has increased for blacks but decreased for whites. Problems with alcohol — the drug most closely linked to arrests, violence and incarceration — are up among white women but down among black women. White woman have also been disproportionately affected by the methamphetamine and prescription opioid epidemics, both of which raise the risk of contact with the criminal justice system.

The decline in black women’s incarceration is surely a positive development even though the high imprisonment rate relative to white women remains disturbing. Given present trends, that racial gap may soon disappear, but if that distance is closed due to continued growth of white female imprisonment, the total number of women behind bars will dramatically expand rather than shrink.

Keith Humphreys is a professor of psychiatry and mental-health-policy director at Stanford University.