If prison reformed criminals, illegal earnings once people were released ought to have gone down. But if prison was a "finishing school" for criminals, illegal earnings after serving time should have increased.
"Spending time in prison leads to increased criminal earnings," Hutcherson says. "On average, a person can make roughly $11,000 more [illegally] from spending time in prison versus a person who does not spend time in prison."
This is a good argument for shorter sentences, as well as an argument for punishment other than incarceration.
If we want criminals to leave prison and lead better lives, we need to consider what they are learning while in prison.