Atul Gawande has a piece in the New Yorker arguing that lengthy periods of solitary confinement are so debilitating that it basically amounts to torture. You can read the whole thing and decide for yourself, but I actually found this short passage to be the most convincing argument:
The wide-scale use of isolation is, almost exclusively, a phenomenon of the past twenty years. In 1890, the United States Supreme Court came close to declaring the punishment to be unconstitutional. Writing for the majority in the case of a Colorado murderer who had been held in isolation for a month, Justice Samuel Miller noted that experience had revealed “serious objections” to solitary confinement:
“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 suffcient mental activity to be of any subsequent service to the community.”
If you go down the whole list of accepted norms in treating people — child labor, civil rights, treatment of the mentally ill, minimum housing standards, workplace safety, etc. — virtually everything that was even a close call in 1890 is universally reviled today. Nobody’s in favor of kids working in mills, Jim Crow laws, packed lunatic asylums, rat-infested slums, or miners dying of black lung. Our penal system is apparently the exception. But if we knew, even in 1890, that long-term solitary confinement is essentially barbaric, can there really be any question about it in 2009?