Three experimental studies were conducted to examine two alternative explanations for the widely established positive eect of social identi®cation in promoting cooperation in social dilemmas. We hypothesised that social identi®cation eects could be either ascribed to (1) an increase in the value assigned to the collective good (i.e. goaltransformation hypothesis) or (2) an enhancement of trust in the cooperation of other group members (i.e. goal-ampli®cation hypothesis). To disentangle these two explanations, we examined the eects of social identi®cation on the contributions to a public good of people with a dierent social value orientation (i.e. pre-existing dierences in preferred outcome distribution between self and others). Following the goal transformation hypothesis, we predicted that an increased group identi®cation would raise contributions, in particular for people essentially concerned with their personal welfare (i.e. pro-self value orientation). Alternatively, following the goal ampli®cation hypothesis it was expected that increased group identi®cation would primarily aect decisions of people concerned with the collective welfare (i.e. prosocial value orientation). The results of all three studies provided support for the goal-transformation rather than goal-ampli®cation hypothesis, suggesting that `sel®sh' individuals can be encouraged to cooperate by increasing the salience of their group membership.