In practically all [of the studies that measure the social effects of incarceration in America], we have used the imprisonment rate to measure society’s level of incapacitation. But the prison rate alone may not capture what we were trying to measure. The most straightforward interpretation of my findings is that neither the rate of imprisonment alone, nor the rate of mental hospitalization alone are good predictors of serious violent crime over the period 1934-2001. In contrast, the aggregated institutionalization rate (aggregating the mental hospitalization and prison rates) is a strong predictor of homicides. This suggests that there is something going on in the relationship between mental hospitalization and prison — perhaps a form of substitution — that should make us rethink entirely how we measure social control and incapacitation. [italics mine - bc]
But since practically none of our studies on prisons, guns, abortion, education, unemployment, capital punishment, etc., controls for institutionalization writ large, most of what we claim to know about these effects may be on shaky ground.
Hat tip: Reason's Hit and Run blog