“When they get out, they are broken,” said Dr. Terry Kupers, a psychiatrist in California who consults on prison conditions and mental health programs. “This is permanent damage.”

Cornell William Brooks, the president of the N.A.A.C.P., said prolonged solitary confinement amounted to torture. “Putting someone in solitary confinement does horrible things to a person’s personality, their psyche, their character,” he said. “It might be said that condemning a person to solitary confinement treats a person as an animal. And so that they emerge from such treatment exhibiting animalistic behavior can’t be surprising.”

Many corrections officials, even those who believe that solitary confinement is overused, caution that in some situations, it may be unavoidable.

“If someone has committed a violent assault, whether it be a staff member or another inmate, until you can somehow solve that problem, that person is going to need to be isolated,” said Rick Raemisch, executive director of Colorado’s corrections department. He pointed to an inmate who said he would kill someone if he were allowed out of solitary, a threat mental health professionals considered credible.

Mr. Raemisch has worked to substantially reduce the use of solitary confinement in Colorado but said groups that opposed it altogether should help develop other ways to handle inmates who pose a danger of violence. “There are those that say this is bad,” he said, “but when you look around for an alternative, people have left the room.”

Solitary confinement has been widespread since the 1980s, when many states built supermaximum-security prisons. They were intended to hold the most dangerous criminals, but corrections departments, dealing with gang violence and crowding worsened by stiffer sentences, began removing an increasing number of inmates from the general population, including some who did not pose a serious threat.