His bottom line:
Prison incarceration alone does not predict homicide, but when mental health institutionalization is combined with the prison rate to form an aggregate instituaionlization measure, that measure is significantly and robustly related to homcide rates over a 68 year period across the fifty separate states, holding constant a number of leading control variables.
The match isn't really quite the match that it is as there is a lot of controlling for independent factors going on.
His main table holds "constant three leading structural covariates of homicide (youth demographics, unemployment, and poverty)."
In order to test the national-level findings, I collected state-level panel data and ran clustered regressions. The results were truly remarkable. Using state-level panel data spanning the entire period from 1934 to 2001, including all 50 states, and controlling for economic, demographic, and criminal justice variables, I again found a large, robust, and statistically significant relationship between aggregated institutionalization and homicide rates. The findings are not sensitive to weighting by population and hold under a number of permutations, including when I aggregate jail populations as well.
To help visualize the relationship, I plotted the predicted values of homicide in the final model (Model 6) against the aggregated institutionalization rate. These, then, are the predicted values of homicide from the model including all the independent variables (aggregated institutionalization, real per capita income, demographics, execution rate, proportion urban, proportion black, and state and year fixed effects).
An important observation of the study is that total mental health and prison institutionalization rates in the United States in the 1940s and 1950s were even higher than they are now.
The study also highlights the important role that deinstitutionalization of the seriously mentally ill has played in our country's social ills.
The question is, how do you square this with international comparisons? The strong corollations with race, high poverty levels and urban settings controlled out of this analysis, sweep the economic causes of crime under the rug. It also suggests a more probing look at institutionalized treatment of mental health in other nations touted for low incarceration rates. Is the main difference between the U.S. and Western Europe that the Europeans are more likely to provide inpatient mental health treatment, while the U.S. lets those same people rot in prison without treatment?
At any rate, it is a study worth a closer look.