Jordan, Jillian J.Hoffman, MosheNowak, MartinRand, David Gertler2017-04-072016Jordan, Jillian J., Moshe Hoffman, Martin A. Nowak, and David G. Rand. 2016. “Uncalculating Cooperation Is Used to Signal Trustworthiness.” Proceedings of the National Academy of Sciences 113 (31) (July 20): 8658–8663. doi:10.1073/pnas.1601280113.0027-8424http://nrs.harvard.edu/urn-3:HUL.InstRepos:32094205Humans frequently cooperate without carefully weighing the costs and benefits. As a result, people may wind up cooperating when it is not worthwhile to do so. Why risk making costly mistakes? Here, we present experimental evidence that reputation concerns provide an answer: people cooperate in an uncalculating way to signal their trustworthiness to observers. We present two economic game experiments in which uncalculating versus calculating decision-making is operationalized by either a subject’s choice of whether to reveal the precise costs of cooperating (Exp. 1) or the time a subject spends considering these costs (Exp. 2). In both experiments, we find that participants are more likely to engage in uncalculating cooperation when their decision-making process is observable to others. Furthermore, we confirm that people who engage in uncalculating cooperation are perceived as, and actually are, more trustworthy than people who cooperate in a calculating way. Taken together, these data provide the first empirical evidence, to our knowledge, that uncalculating cooperation is used to signal trustworthiness, and is not merely an efficient decision-making strategy that reduces cognitive costs. Our results thus help to explain a range of puzzling behaviors, such as extreme altruism, the use of ethical principles, and romantic love.en-USreputationsocial evaluationdecision-makingexperimental economicsmoral psychologyJournal Article2017-03-2620162017-04-0710.1073/pnas.1601280113