(Rhonda Oaks/The Daily News via AP)

Throughout the presidential campaign season, both Hillary Clinton and Bill Clinton have been excoriated for supporting the 1994 Violent Crime Control and Law Enforcement Act, which critics charge is fueling mass incarceration of African Americans. Defenders of the law, including Bill Clinton himself, maintain that it was a necessary response to rising violent crime and that it passed with substantial support from Black voters and elected officials. This heated debate about whether the 1994 law is responsible for African Americans increasingly being behind bars can never be resolved, for a reason that may surprise many observers: The African American imprisonment rate has been declining for many years. Indeed, the likelihood of African American men and women being in prison today is lower than it was a generation ago when the law was passed, as these two charts show.

The quarter-century of data in the charts come from the Bureau of Justice Statistics series of annual reports on the state and federal prison population. The rate of imprisonment of black men is in absolute terms consistently much higher than that of black women, but the shape of the two curves is remarkably similar. In the 1990s, the explosive growth in imprisonment that began in the mid-1970s was slowing but still underway, affecting people of all races but African Americans worst of all. But around the turn of the millennium, the African American imprisonment rate began declining year after year. Other government reports document that a parallel decline has occurred in the county and city jail population as well.

At the end of 2014, the African American male imprisonment rate had dropped to a level not seen since early 1993. The change for African American women is even more marked, with the 2014 imprisonment rate being the lowest point in the quarter-century of data available. It can’t be overemphasized that these are trends unique to blacks rather than being part of a broader pattern of de-incarceration: The white imprisonment rate has been rising rather than falling.

A 23 percent decline in the black male imprisonment rate and a 49 percent decline in the black female imprisonment rate would seem to warrant some serious attention. But if you point out to the average person or even a seasoned criminologist that the United States is at a more than 20-year low in the black incarceration rate, you are likely to be met with stunned silence. Why?

Psychological research shows that human beings are not particularly good at updating their assumptions about the world in light of new data (e.g., crime is getting worse or better, the economy is strong or weak), often leading them to ignore or discount any information that doesn’t fit their standing views. If for example you feel strongly that Michelle Alexander’s much-discussed book "The New Jim Crow" was the definitive statement on the state of black incarceration, you would probably have trouble taking in the fact that the black imprisonment rate had been falling for a decade prior to that book’s 2010 appearance and has continued to drop every year since.

It can be emotionally and intellectually challenging to break free from long-held assumptions, but there can be benefits as well. The high rate of black incarceration remains a horribly destructive social problem. Yet somehow the juggernaut reversed direction 15 years ago. Analyzing how and why that happened could point the way to further reform, but of course this can happen only if the progress is acknowledged in the first place. Also, the fuel of much social change is optimism that the future can be better than the past. The widespread belief that black incarceration gets worse every year is not only profoundly wrong, it may also be crushing to the spirit of the many reformers who are striving to create a safer, more equitable and freer society.

Keith Humphreys is a professor of psychiatry at Stanford University.