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Solitary confinement is a form of torture used in prisons in which a prisoner is isolated from others, generally in a dark room. Solitary confinement has been in use since the 19th century, originally as a way for prisoners to "seek forgiveness from God".
In 1829, Eastern State Penitentiary in Philadelphia, USA, began doing the first experiments about solitary confinement, based on the Christian belief that prisoners isolated in small cells with only a Bible would seek forgiveness from god and thus change their ways. The practice was slowly abandoned for a time after this initial experimentation in Philadelphia. In 1890, a case was brought before the United States Supreme Court, in which a prisoner petitioned for Habeus Corpus. Besides the solitary confinement prior to the prisoner's death sentence being found unconstitutional on an ex post facto basis, the court 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.' This being something that was apparent in 1890, you would think that the American People would have figured it out by now.
Psychological effects on prisoners
The conditions of solitary confinement units have extreme psychological effects on prisoners. A recent study showed that "inmates ever assigned to solitary confinement were 3.2 times as likely to commit an act of self-harm per 1000 days at some time during their incarceration as those never assigned to solitary". A research paper by the American Academy of Psychiatry and the Law stated that solitary confinement can be "as clinically distressing as physical torture". Prisoners inside solitary confinement units with little or no human contact can develop serious mental disabilities, such as schizophrenia, if in solitary confinement for long periods of time. Many prisoners with pre-existing mental conditions can "deteriorate mentally in isolation, requiring crisis care or psychiatric hospitalization". 
In a nutshell
- In re Medley, 134 U.S. 160 (1890)