The Penrose Hypothesis suggests that the decrease in psychiatric hospitals in the US is linked to the increase of mentally ill people in prison and unhoused. Statistics from the 1880s to 2005 demonstrate that the percentage of mentally ill people in prison increased from less than 1% to 21%, and there was a significant rise in the US unhoused population from 100,000 in 1980 to 400,000 in 1988.