"Studies in New York and Connecticut from the 1920s through the 1940s showed a much lower arrest rate for the mentally ill. In an era when involuntary commitment was relatively easy, those who were considered a danger to themselves or others would be hospitalized at the first signs of serious mental illness. The connection between insanity and crime was apparent, and the society took a precautionary approach. Mentally ill persons who were not hospitalized were those not considered a danger to others. This changed as deinstitutionalization took effect."
From "Madness, Deinstitutionalization & Murder," by Clayton Cramer.