Deterrent effects persist for a period after enforcement is withdrawn, then decay as offenders learn it has gone.
Sherman's analysis of crackdowns found that the effect commonly outlasts the operation, because offenders update slowly on the absence of something they cannot observe directly. The practical implication is counter-intuitive: short, unpredictable and rotated crackdowns can extract more deterrence per hour than continuous presence, which offenders learn to read accurately.
In practice
- A ticket inspection blitz keeps fare evasion low for weeks after the inspectors have moved on.
- An expenses audit announced and completed changes behaviour for two quarters and then stops.