A nominee for Sunday’s Oscar as Best Picture, Zero Dark Thirty, reignited debate about whether the waterboarding of terrorism suspects was torture. This practice, which ended in 2003, was used on only three suspects. Meanwhile, tens of thousands of American prison inmates are kept in protracted solitary confinement that arguably constitutes torture and probably violates the Eighth Amendment prohibition of “cruel and unusual punishments.”

Noting that half of all prison suicides are committed by prisoners held in isolation, Sen. Richard Durbin, D-Ill., has prompted an independent assessment of solitary confinement in federal prisons. State prisons are equally vulnerable to Eighth Amendment challenges concerning whether inmates are subjected to “substantial risk of serious harm.”

America, with 5 percent of the world’s population, has 25 percent of its prisoners. Mass incarceration, which means a perpetual crisis of prisoners re-entering society, has generated understanding of solitary confinement’s consequences when used as a long-term condition for an estimated 25,000 inmates in federal and state supermax prisons — and perhaps 80,000 others in isolation sections within regular prisons. Clearly, solitary confinement involves much more than the isolation of incorrigibly violent individuals for the protection of other inmates or prison personnel.

Federal law on torture prohibits conduct “specifically intended to inflict severe physical or mental pain or suffering.” And “severe” physical pain is not limited to “excruciating or agonizing” pain, or pain “equivalent in intensity to the pain accompanying serious physical injury, such as organ failure, impairment of bodily functions, or even death.” The severe mental suffering from prolonged solitary confinement puts the confined at risk of brain impairment.

Supermax prisons isolate inmates from social contact. Often prisoners are in their cells, sometimes smaller than 8 feet by 12 feet, 23 hours a day, released only for a shower or exercise in a small fenced-in outdoor space. Isolation changes the way the brain works, often making individuals more impulsive, less able to control themselves. The mental pain of solitary confinement is crippling: Brain studies reveal durable impairments and abnormalities in individuals denied social interaction. Plainly put, prisoners often lose their minds.

The first supermax began functioning in Marion, Ill., in 1983. By the beginning of this century there were more than 60 around the nation, and solitary-confinement facilities were in most maximum-security prisons. In an article ( Hellhole) in the March 30, 2009, New Yorker, Atul Gawande, a surgeon who writes on public health issues, noted, “One of the paradoxes of solitary confinement is that, as starved as people become for companionship, the experience typically leaves them unfit for social interaction.” And those who are most incapacitated by solitary confinement are forced to remain in it because they have been rendered unfit for “the highly social world of mainline prison or free society.”

Last year, The New York Times reported that of the prisoners sent to solitary confinement in California’s Pelican Bay prison because of gang affiliation, “248 have been there for five to 10 years; 218 for 10 to 20 years; and 90 for 20 years or more.”

Two centuries ago, solitary confinement was considered a humane reform, promoting reflection, repentance — penitence; hence penitentiaries — and rehabilitation. Quakerism influenced the design of Philadelphia’s Eastern State Penitentiary, which opened in 1829 with a regime of strict solitude. In 1842, Charles Dickens visited it:

Mass incarceration is expensive (California spends almost twice as much on prisons as on universities) and solitary confinement costs, on average, three times as much per inmate as in normal prisons. And remember: Most persons now in solitary confinement will someday be back on America’s streets, some of them rendered psychotic by what are called correctional institutions.