We report the use of a combination of self- and peer-assessment in an undergraduate social psychology laboratory course. Students worked in small groups on a self-directed empirical project that they each wrote up independently as a laboratory report. Marks for the written assignment were moderated by a contribution index measure based on the self- and peer-assessment measures. Our analyses indicated that: (i) students took the peer-assessment process seriously, clearly differentiating between group members on the contributions questionnaires; (ii) students show a self-bias, rating their own contribution to the group task higher than that of other group members; (iii) for a large majority of students the contribution index resulted in very little moderation of the final assignment marks; (iv) there was a strong correlation between the contribution index and the overall assignment score. Implications for the assessment of group work are considered.