The new edition of the APA Monitor magazine has an article that discusses the psychological impact of solitary confinement in light of its growing use in American prisons.
One of the most interesting points is that evidence for the effect on solitary confinement on prisoners is actually quite limited due to difficulties studying incarcerated people.
Prisoners in administrative segregation are placed into isolation units for months or years. Corrections officials first turned to this strategy in response to growing gang violence inside prisons, Dvoskin says. Though critics contend that administrative segregation has never been proven to make prisons safer, use of this type of confinement has continued to rise. That’s worrisome to most psychologists who study the issue. Deprived of normal human interaction, many segregated prisoners reportedly suffer from mental health problems including anxiety, panic, insomnia, paranoia, aggression and depression, Haney says (Crime and Delinquency, 2003)…
However, much of the evidence of harm comes from cross-sectional studies or research done on people who are not in prison, such as the isolated elderly. Designing a long-term study to follow prisoners in solitary confinement is challenging. Each correctional system is unique, inmates move in and out of segregation, and many states prohibit or limit psychological studies of incarcerated individuals due to ethical concerns.
Most of our evidence, it turns out, comes from other people deprived of human contact, although the effects have been found to almost universally unpleasant until now.