Penrose's Law in Ireland: an ecological analysis of psychiatric inpatients and prisoners.

Penrose's Law states that as the number of psychiatric inpatients declines, the number of prisoners increases. We studied data from the annual census of psychiatric inpatients and prison statistics in Ireland. Between 1963 and 2003, the number of psychiatric inpatients decreased by 81.5% (a five-fold decrease) and the average number of prisoners increased by 494.8% (a five-fold increase) (Spearman's rho=-0.992, P<0.001). The absolute decline in psychiatric inpatients greatly exceeded the increase in prisoners. This net de-institutionalisation appears particularly marked in Ireland compared to England; this may relate to ecological study designs or differences in prison, health or re-institutionalisation practices.