Prisoners wait in line for breakfast at California Men's Colony prison on Dec. 19, 2013. (Photo by Andrew Burton/Getty Images)

Keith Humphreys is a Professor of Psychiatry and Mental Health Policy Director at Stanford University.  

After decades of growth, the U.S. imprisonment rate has been declining for the past six years. Hidden within this welcome overall trend is a sizable and surprising racial disparity: African-Americans are benefitting from the national de-incarceration trend but whites are serving time at increasingly higher rates.

The pattern of results, evident in a series of reports from the Bureau of Justice Statistics, is most stark among women. Since 2000, the imprisonment rate among African-American women has dropped 47 percent, while the rate among  white women has risen by 56 percent. These trends have combined to shrink the racial disparity in women’s imprisonment by two-thirds.

A similar pattern emerges for men, who compose a much larger share of the prison population. The rate of imprisonment among African-American men remains very high, but nonetheless it has tumbled 22 percent since 2000. The rate for white men in contrast is 4 percent higher than it was in 2000. As a result, the racial disparity has shrunk by nearly one quarter.

In responding to the data, Fordham University Professor John Pfaff echoed several criminologists when he said that“This is one of the most surprising pattern of results I have seen in corrections in a long time.”  Pfaff said that “law enforcement attitudes getting tougher in rural areas and softer in urban areas may be contributing to this change."

Adam Gelb, who directs the public safety performance project of the Pew Charitable Trusts, suggested that “changes in drug use and enforcement over the past 15 years could be at play.” Gelb said  the methamphetamine, prescription opioid and heroin epidemics have affected whites more than did the crack cocaine epidemic, which increased incarceration among blacks in the 1980s and 1990s but has since waned.

Stanford Law School Professor Joan Petersilia noted another possible cause: “sex offenders, who are disproportionately white and tend to receive long sentences, are a new target for the war on crime.” Consistent with this explanation, a larger proportion of white inmates have been convicted of sex crimes (16.4 percent) than have black inmates (8 percent).

More generally, multiple indicators of health and well-being have shown evidence of deterioration in white Americans over the past 15 years, including rising rates of suicide, drug overdose, poor mental health and inability to work.

Whatever cultural and macroeconomic forces are producing these changes could conceivably also be driving increased involvement in the criminal justice system by whites, including rising imprisonment in an era of de-incarceration.