Mark Kleiman: UCLA Department of Public Policy

The threat of punishment can facilitate cooperation by discouraging defection and aggression. Because punishment is scarce, costly, and painful, optimal enforcement strategies will minimize the amount of actual punishment required to effectuate deterrence. If potential offenders are deterrable, increasing the conditional probability of punishment (given violation) can reduce the amount of punishment actually inflicted, by “tipping” a situation from its high-violation equilibrium to its low-violation equilibrium. Compared to random or “equal opportunity” enforcement, dynamically concentrated sanctions can reduce the punishment level necessary to tip the system.
http://www.bec.ucla.edu/kleimanpaper.doc