It would seem common sense not to place people with mental illness or intellectual disabilities in solitary confinement, but the Pennsylvania Department of Corrections has only recently come around to realizing that fact. And not on their own, either. It took a lawsuit and a federal civil rights probe to bring the message home.

The probe, which began in 2013, found that state prisons were using solitary confinement instead of treatment services to handle mentally-ill inmates, shutting them away for 23 hours a day alone, in a small cell.

On Thursday the Department of Justice announced it was formally closing its investigation into Pennsylvania’s use of solitary confinement, saying the state had taken “significant steps” to reform its use of solitary confinement.

The state has more than 4,000 mentally-ill inmates. Those people are now protected under a settlement agreement with the Disability Rights Network which bars the DOC from putting them into solitary confinement.

This is a step forward, but more work remains to be done. There are still about 2,000 inmates in what the department calls “restricted housing units.” The state needs to do more to expand and streamline treatment services for mentally-ill inmates, and a broader reform of the use of solitary confinement in general is necessary as well.

In February, President Barack Obama barred federal prisons from holding juvenile inmates in solitary confinement and ordered officials to make sweeping changes to how the system uses solitary confinement to punish inmate infractions, protect them from abuse, and publishes information on how widespread the use of solitary actually is.

The truth is, it’s far too widespread. While most countries have discontinued the practice of shutting inmates away, the United States has become an outlier and chosen to expand its use of the practice.

According to a Justice Department report, as many as 100,000 people are held in solitary confinement or similar situations, and often spend months or even years in small cells with virtually no human contact.

In 2014 a panel of psychologists and neuroscientists confirmed what should be obvious: shutting human beings — who are social creatures — away alone causes severe psychological stress that begins when they’re put into isolation and doesn’t subside over time.

Prisoners subjected to these conditions often become prone to violence and hallucinations; some become prone to suicide. A 2007 study of Washington State’s prison population found that prisoners released directly from solitary had a recidivism rate more than 20 percent higher than the state average.

Once upon a time, our nation’s justice system understood that solitary confinement was a useless, cruel and counterproductive practice. An 1890 ruling by the U.S. Supreme Court notes how damaging it is to prisoners subjected to its use.

Somehow between then and now, that knowledge has been either lost or disregarded.