1. Are people with blue eyes more successful?  Many studies of the effects of physical attractiveness have endogeneity problems.  But given the randomness of eye color (conditional on parents with recessive genes), siblings make a great control.  Someone must have looked at this.
  2. Procrastination is a best-response to perfectionism.  A perfectionist spends too much time on a task, so she should optimally procrastinate so that the deadline disciplines her to work quickly and settle for imperfection.
  3. When secretly hiding Halloween candy from your kids you face a non-convexity:  either hide just a little bit at a time, or go all the way and hide it all.  Leaving too little is the worst of both worlds:  the stash itself reminds them that they have Halloween candy, then the noticeably small remainder cues them that you are hiding the rest.
  4. In sports like golf, bowling, and chess players are rated in order to match players of similar skill or handicap the weaker players.  Then there are kludgy rules to prevent “sandbagging:”  playing poorly in one tournament in order to get an advantage in the next tournament.  These systems could surely be improved by formally analyzing the strategy-proof mechanism design problem.