more institutionalization, less homicide?

bernard harcourt is guest-blogging at volokh this week, offering engaging posts on deinstitutionalization, incarceration, and homicide. he nicely explicates a new state-level analysis of the national evidence discussed in his january times op-ed and texas law review piece.

the pattern appears to hold up under a more stringent state-level panel specification: aggregate institutionalization (prisons plus mental institutions) bears a strong inverse relationship to homicide rates over a long historical period. moreover, the correlation between homicide and aggregate institutionalization is far stronger than the correlation between homicide and imprisonment. my sense is that the individual-level literature shows rather modest associations between violence and mental illness. what might account for the strong aggregate relationship?