In this chapter, we propose to review a series of cooperative learning studies that allow to pinpoint that social skills development is crucial for group work to be efficient in terms of cognitive/academic outcomes and that teachers can further this social skills development with a reasonable investment. We start with some highlights of a research programme showing how easily students can switch to competition even with cooperative instructions. We document this phenomenon at both university and at primary school levels. We then use this set of results to underline the importance of preparing students to cooperate when they have to work together. Finally, we summarise and document the benefits of two short simple interventions, one at university and the other at middle school, developed to address some potential resistance of teachers to invest in the development of social skills. The implications for teachers' ability to accompany cooperative group work are discussed.