Grim Trigger

We have seen that actors cannot credibly commit to cooperation in a repeated prisoner’s dilemma if the game has a definite ending. But can they cooperate when their interaction will go on indefinitely?

Takeaway Points

  1. Grim trigger is a policy of tough love; you start by cooperating and continue to cooperate as long as everyone has cooperated in the past. If someone has defected, then you defect forever.
  2. The threat of forever conflict keeps everyone cooperating today as long as everyone cares enough about the future.
  3. This is a critical result from The Evolution of Cooperation, one of the most influential political science books ever written.