Cooperate without looking: Why we care what people think and not just what they do Moshe Hoffman', Erez Yoeli', and Martin A. Nowak' I' •Peogram for Evolutionary Dynamics, Oeparunent of Mathematics and 'Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138 Edited by Michael S. Gananiga, University of California, Santa Barbara, CA, and approved December 24. 2014 (received for review September 30, 2014) Evolutionary game theory typically focuses on actions but ignores motives. Here, we introduce a model that takes into account the motive behind the action. A crucial question is why do we trust people more who cooperate without calculating the cost? We propose a game theory model to explain this phenomenon. One player has the option to 'look" at the costs of cooperation, and the other player chooses whether to continue the interaction. If it is occasionally very costly for player 1 to cooperate, but defection is harmful for player 2, then cooperation without looking is a subgame perfect equilibrium. This behavior also emerges in population-based processes of learning or evolution. Our theory illuminates a number of key phenomena of human interactions: authentic altruism, why people cooperate intuitively, one-shot cooperation, why friends do not keep trade of favors, why we admire principled people, Kant's second formulation of the Categorical Imperative, taboos, and love. game theory I evolution I emotion I motive I cooperation C ooperation OMITS when we lake on costs to help others. A key mechanism by which cooperation is sustained is reciprocity: Individuals cooperate with those who have cooperated in the past (1-14). However, we care about not only whether others cooperate but also. their decision-making process: wc place more trust in cooperators who do not strategically weigh the arts and make an effort to collect than before deciding whether to cooperate. For example. we are impressed by colleagues