That would be prison, according to sociology professor Donald T. Hutcherson II, who mined data from the US government’s National Longitudinal Survey of Youth:
Included in the survey are questions about how much money individuals make legally and illegally. Because the survey also ascertains whether people have spent time in prison, Hutcherson pored through data from tens of thousands of queries to a large number of young people to establish whether illegal earnings went up or down after individuals served time.
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.”