This paper reports a systematic literature review examining empirical studies on the effects of peer assessment for learning. Peer assessment is fundamentally a social process whose core activity is feedback given to and received from others, aimed at enhancing the performance of each individual group member and/or the group as a whole. This makes peer assessment an interpersonal and interactional process. Using this social perspective in order to study learning effects, we focus on the impact of the structural arrangement of peer assessment on learning, and the influence of interpersonal variables.the literature search, focusing on empirical studies measuring learning outcomes in a peer assessment setting, resulted in 15 studies conducted since 1990 dealing with effects (performance or perceived learning gains) of peer assessment. Our analysis reveals that, although peer assessment is a social process, interpersonal variables have hardly been studied; more specifically, they were measured in only 4 out of 15 studies. Moreover, they are not used to explain learning gains resulting from peer assessment. Finally, comparing the studies with respect to structural features reveals that, although the differences between the studies are significant, there seems to be no relation with the occurrence of learning benefits. The results of this review seem to indicate that research on peer assessment from a social perspective is still in its infancy and deserves more attention.