Cooperating behavior may be fostered by personality traits re ecting either favorable inclination to others or willingness to comply with norms and rules. We test the relative importance of these two factors in an experiment where subjects provide real mental effort in two treatments with identical task, differing only by whether others' payment is affected. If the rst hypothesis is true, subjects reporting high Agreeableness score should put more effort; if the second is true, reporting higher Conscientiousness should predict more effort. We find experimental support for the second hypothesis but not for the first, as subjects reporting high Altruism do not behave consistently with this statement.
