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Cooperating Without Looking

Moshe Hoffman,1,2*† Erez Yoeli, 2,3* Martin Nowak2

1Department of Computer Science and Engineering,
University of California at San Diego, La Jolla, CA 92093
2Program for Evolutionary Dynamics,
Harvard University, Cambridge, MA 02138
2Federal Trade Commission,
600 Pennsylvania Ave. NW, Washington, DC 20004
*These authors contributed equally to this work.
†To whom correspondence should be addressed; E-mail: hoffman.moshe@gmail.com.

Cooperation occurs when we take on costs to help others. A key
mechanism by which cooperation is sustained is reciprocity: indi-
viduals cooperate with those who have cooperated in the past. In
reality, we not only condition on others’ past cooperative actions,
but also on the decision making process that leads to cooperation:
we trust more those who cooperate without calculating the costs
because they will cooperate even when those costs are high. We
propose a game theory model to explain this phenomenon. In our
model, player 1 chooses whether or not to cooperate with player 2.
Player 1 faces a stochastic temptation to defect and, before choos-
ing whether to cooperate, also decides whether to “look” at the
realized temptation. Player 2 observes not only whether player 1
ultimately cooperated but also whether she looked, then decides

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