Incentives, Exogeneity, and Social Planning

Ian Lovett writes about neighbourhoods and sex offenders in Los Angeles (via):

So local residents and city officials developed a plan to force convicted sex offenders to leave their neighborhood: open a tiny park.

Parents here, where state law prohibits registered sex offenders from living within 2,000 feet of a school or a public park, are not the only ones seizing on this approach. From the metropolis of Miami to the small town of Sapulpa, Okla., communities are building pocket parks, sometimes so small that they have barely enough room for a swing set, to drive out sex offenders.

This nicely illustrates a three-step caveat it's useful to keep in mind when thinking about social processes: (1) People respond to incentives. (2) Hence, stuff you might have thought is exogenous actually turns out to be endogenous. (3) Hence steering society is harder than one might think.