He cited the shameful case of Kalief Browder, who was arrested in New York City at the age of 16 in 2010 and jailed for three years without trial for allegedly stealing a backpack. Mr. Browder spent two of those years in solitary confinement, endured “unspeakable violence at the hands of inmates and guards” and tried to kill himself several times. He was released in 2013, but never fully recovered, and he hanged himself last year.

Despite horror stories like this, as many as 100,000 people — including juveniles and people with mental illnesses — are held in solitary confinement and other forms of restrictive housing in American prisons, according to a new report by the Justice Department. Inmates often spend months or even years in small, cramped cells with virtually no human contact.

According to the report, the Bureau of Prisons in recent years has cut the number of inmates in solitary confinement and other restrictive housing. Federal officials believe that the new policies recommended in the report will lead to additionall reductions in restrictive housing populations. One recomendation is to divert inmates with serious mental illness to mental health units; another is to ban the use of “punitive segregation” for low-level infractions.

The new federal policy sets an excellent example for states and local governments that have yet to undertake reform.