Collaborative learning is often used in higher education to help students develop their teamwork skills and acquire curricular knowledge. In this paper we test a mediation model in which the quality of group discussions mediates the impact of gender diversity and group motivation on collaborative learning effectiveness. Our results show that the proportion of women in groups, and the group level need for cognition and core self-evaluations (within group average) positively predict discussion quality that in turn predicts group (academic) performance. Our results show that discussion quality fully mediates the effects of need for cognition and core self-evaluations on group performance. The effect for gender diversity on group performance is only partly mediated by discussion quality.