So how much of the reduction in violent crime was produced by increased incarceration? This kind of analysis doesn’t tell us. But neither am I sure that the armory of social science quantitative techniques adequately models what has gone on. Here is my simple-minded thought: Suppose we had maintained imprisonment for violent crime at the rate that applied in 1974. In that case, we would have had 276,769 state and federal prisoners in 2010 instead of the 1,518,104 we actually had. Suppose tomorrow we freed 1.2 million inmates from state and federal prisons. Do we really think violent crime would continue to drop at a somewhat slower pace?
In one sense, it is a silly question, as all counter-factuals must be. And I’m not saying that our current incarceration rates are appropriate. We may very well have been in a state of diminishing returns to incarceration for the last decade, as the experts DiIulio cites have argued. But I continue to harbor the belief that without the massive increases in incarceration after the mid 1970s, crime rates wouldn’t have turned around at all. Higher imprisonment was the necessary condition for 100 percent of the reduction in violent crime.