Assessment of altruism depends on inferred ulterior motives.

There is no general agreement whether image scoring or good standing provide a better explanation for the perception of reputation and the way it influences cooperation. We studied the rules humans use to decide who is trustworthy or altruistic when the information they have is reports of others’ behavior. We found that our subjects used a decision rule which takes inferred motives into account, rather than only scoring observed behavior. This occurred both in scenarios in which our subjects judged failures to be altruistic and in scenarios in which they judged apparently altruistic behavior. Our findings provide support for the good standing strategy rather than image scoring.