The effect of gender composition on group performance was investigated over three years using the MARKSTRAT 2 simulation. Senior undergraduates formed groups and competed within five-firm (group) industries over 10 weeks, as well as submitting group reports on their performances. Performance and report marks were independent of group size and individual performance in other course assessment. Women per group (numbers, percentage, gender ratio) was positively related to simulation performance, with few women-dominated groups ranking lower than first or second within each industry. Very few all-men groups achieved first or second rankings. There were no significant correlations between group gender compositions and marks for the group written report.Women's more interactive, people-oriented and co-operative work styles apparently facilitate the MARKSTRAT 2 group process and, when combined with men's more analytical decision-making tendencies and competitive orientation, appear to explain the superior performances of mixed groups. The emergence and utilization of these tendencies within groups are discussed in terms of Social Contact Theory and Competition Theory. Although generalizations from student-based studies to the workplace is problematic, the results indicate that groups may be more effective when women outnumber or equal men, especially in complex management activities requiring extensive information management and processing, planning and decision-making over protracted periods.