Several readers have asked whether prison deters crime, some saying that correlation is not the same as causation. Social scientists have known that dictum for the better part of a century and have worked hard at finding out whether prison deters crime among would-be-offenders (it obviously prevents it among people already locked up). Steven Levitt, principal author of Freakonomics, discusses the issue in that book and summarizes the evidence, now very strong, in the chapter he wrote from a book I and Joan Petersilia edited entitled Crime. The econometric technique is simple: construct an equation that asks what can explain the crime rate using, as explanations, everything that we think causes crime (for example, urbanization, unemployment, and the like) and adds the chances of going to prison. When you do this, you find that the higher the chances of going to prison in a state, the lower the crime rate, other things being equal. (You also discover that the unemployment rate has very little effect on crime.)