Statistics

According to Jeffrey Sedgwick, Director of the Bureau of Justice Statistics,

In 2006, the most recent year for which complete data are available, police received the fewest reports of violent crime and property crime since 1977. What was the cause? Research has shown that, with some exceptions, crime rates decline as the incarceration rate rises. In other words, while the number of people under correctional supervision has gone up, crime has gone down.

So there you go. Correlation = causation.