Some 80,000 people are held in solitary confinement in U.S. prisons, according to the latest available census. The practice has grown with seemingly little thought to how isolation affects a person's psyche. But new research suggests that solitary confinement creates more violence both inside and outside prison walls.

Prisoners in solitary confinement—also known as administrative segregation—spend 22 to 24 hours a day in small, featureless cells. Contact with other humans is practically nonexistent. Because solitary confinement widely occurs at the discretion of prison administration, many inmates spend years, even decades, cut off from any real social interaction. More than 500 of the prisoners at Pelican Bay State Prison in California, for example, have been in isolation units for over a decade, according to the California Department of Corrections and Rehabilitation.

You might assume all inmates sent to solitary are the “worst of the worst”—rapists and murderers who continue their violent ways even behind bars. But in fact, many are placed in solitary for nonviolent offenses, and some are not even criminals, having been arrested on immigration charges. Others are thrown into isolation cells “for their own protection” because they are homosexual or transgendered or have been raped by other inmates.

Whatever the reasons, such extreme isolation and sensory deprivation can take a severe, sometimes permanent, toll on emotional and mental health. Researchers have found that prisoners in solitary quickly become withdrawn, hypersensitive to sights and sounds, paranoid, and more prone to violence and hallucinations. Craig Haney, a professor of psychology at the University of California, Santa Cruz, has documented several cases of individuals with no prior history of mental illness who nonetheless developed paranoid psychosis requiring medical treatment after prolonged solitary confinement. As damaging as the consequences are for otherwise healthy adults, they are even worse for adolescents, whose brains are still in their final stages of development, and the mentally ill, who already struggle to maintain a solid grasp on reality. About half of all prison suicides occur in isolation cells.

The U.S. justice system once understood that long stretches in solitary served no good purpose. In 1890 the U.S. Supreme Court addressed the pernicious nature of solitary confinement in the case of a man who had murdered his wife. In their decision, the justices noted that “a considerable number of the prisoners fell, after even a short confinement, into a semi-fatuous condition, from which it was next to impossible to arouse them, and others became violently insane; others still committed suicide, while those who stood the ordeal better were not generally reformed, and in most cases did not recover sufficient mental activity to be of any subsequent service to the community.”

Nearly a century later this wisdom was all but lost. The use of solitary grew in the 1980s after white supremacists murdered two prison guards in the federal penitentiary at Marion, Ill. Officials responded by placing the entire facility on permanent lockdown. What started out as a stop-gap measure to address prison violence soon became institutionalized; so-called supermax prisons were built that encased all inmates in solitary cells whose only window was often just the slot for food found in the steel door.

Yet strangely, no one knows if segregating prisoners reduces violence. Indeed, evidence suggests that the opposite is true. After the state of Mississippi reduced the number of prisoners in solitary confinement at its Parchman facility and developed new units for prisoners with mental illness, the number of violent attacks plummeted from a high of 45 in March 2006 to five in January 2008. (Mississippi also saved more than $5 million.) A 2007 study of Washington State's prison population found that 69 percent of those who were released directly to the community from solitary—a dishearteningly regular practice—committed new crimes that landed them back in jail within three years, compared with 46 percent of those who had been allowed to readjust to the general prison population before release.

Solitary confinement is not only cruel, it is counterproductive. The U.S. should reclaim the wisdom it once held and dramatically limit the practice.