COLORADO SPRINGS — For years, the Colorado corrections system had a ready answer for inmates it wanted to punish. For almost any reason — smuggling drugs, talking back to a corrections officer, assaulting another prisoner — it would send an inmate to a cell the size of a parking spot. The inmate would stay there, alone, at least 22 hours a day, for two and a half years on average, but sometimes for decades. This is called administrative segregation, and shortly after I became Colorado’s head of corrections in 2013, I began to ask why we were doing it. Can you imagine spending years without having regular social interactions or without full access to basic human activities like showering and exercising? When did it become O.K. to lock up someone who is severely mentally ill and let the demons chase him around in the cell? What is wrong with us? I asked. Then, in 2015 I assisted the State Department with other United Nations countries in modernizing international standards for the treatment of prisoners, now known as the Nelson Mandela Rules. During the debates about the wording of the new standards, it was decided that keeping someone for more than 15 days in solitary was torture. After listening and being involved in those discussions, I agreed. There now is enough data to convince me that long-term isolation manufactures and aggravates mental illness. It has not solved any problems; at best it has maintained them. That’s why, in September, Colorado ended the practice. Inmates who commit serious violations like assault will now spend at most 15 days in solitary, and, if necessary, undergo therapy or anger management classes afterward. Today, there are only 18 inmates in such segregation, and we deal with minor offenses like mouthing off in less severe ways.