Prison as a Criminal School: Peer Effects and Criminal Learning behind Bars

We investigate peer effects on crime-specific recidivism using register data for the entire Danish prison population. We find that inmates do not acquire new criminal capital in prison (no introductory peer effects). For certain types of crimes, we find that inmates strengthen their criminal capital in prison because of exposure to offenders in their field of specialization (reinforcing peer effects). Our results accord with a theory of crime-specific knowledge transmission and network building: we find reinforcing peer effects for crimes that require specific capital, planning, and networks (for example, drug crimes, theft, burglary, and fencing) and/or are more effective when committed in groups (for example, threats and vandalism). Reinforcing peer effects are especially strong because of exposure to more experienced criminals. By contrast, we find that exposure to offenders in the same specialization deters inmates from recidivism for violent offenses, including sexual offenses and robbery.