Of course, there are important demographic differences between the two populations. In 1937, women represented 48 percent of residents in state mental hospitals. In contrast, new prison admissions have consistently been 95 percent male. Also, the mental health patients from the 1930s to the 1960s were older and whiter than prison inmates of the 1990s.

But the graph poses a number of troubling questions: Why did we diagnose deviance in such radically different ways over the course of the 20th century? Do we need to be imprisoning at such high rates, or were we right, 50 years ago, to hospitalize instead? Why were so many women hospitalized? Why have they been replaced by young black men? Have both prisons and mental hospitals included large numbers of unnecessarily incarcerated individuals?

Whatever the answers, the pendulum has swung too far — possibly off its hinges.

It would be naïve, today, to address any of these questions without also considering the impact of imprisonment on crime. One of the most reliable studies estimates that the increased prison population over the 1990s accounted for about a third of the overall drop in crime that decade.

However, prisons are not the only institutions that seem to have this effect. In a recent study, I demonstrated that the rate of institutionalization — including mental hospitals — was a far better predictor of serious violent crime from 1926 to 2000 than just prison populations. The data reveal a robust negative relationship between overall institutionalization (prisons and asylums) and homicide. Preliminary findings based on state-level panel data confirm these results.

The effect on crime may not depend on whether the institution is a mental hospital or a prison. Even from a crime-fighting perspective, then, it is time to rethink our prison and mental health policies. A lot more work must be done before proposing answers to those troubling questions. But the first step is to realize that we have been wildly erratic in our approach to deviance, mental health and the prison.