The practice of isolating prisoners began in America in the late 18th century, when Quakers advocated it as a means of sparing inmates from the whippings and public humiliation common back then. In 1829, Pennsylvania's Eastern State Penitentiary began experimenting with lockdowns, but found inmates committed suicide or became socially dysfunctional, and so eventually abandoned the practice. But a century later, in 1934, solitary was tried again at Alcatraz, where the most problematic prisoners were isolated in D Block.
Still, the practice remained rare until 1983, when two corrections officers were killed by inmates on the same day, in separate incidents, at Illinois' Marion Federal Prison. After the killings, all of the facility's prisoners were put into lockdown — and kept there, even after things had quieted down. Marion thus became America's first "supermax" prison, where all inmates were kept in their cells 23 hours per day [sources: Infographic World, The Guardian].
Soon supermax facilities, or control-unit prisons, were being built across the nation. In 1994, then-President Bill Clinton signed a crime bill that offered federal monies to states that increased prison sentences. The effect? Even more states constructed supermax prisons or added solitary confinement units, and the number of prisoners so confined skyrocketed.
While the exact number of inmates held in solitary confinement today is difficult to ascertain, experts say around 80,000 to 100,000 American prisoners are locked up in some form of restrictive house. This estimate doesn't include those in juvenile facilities, local jails or immigrant detention centers [sources: Infographic World, The Guardian, Solitary Watch].