This paper investigates the effects that different patterns of similarity among group members have on a group's performance on a problem-solving task. I discuss and test hypotheses on the effects of similarity on group performance derived from two literatures: balance theory and research on homophily. In an experiment I found that the relative balance of the pattern of similarity was more important in predicting how quickly groups establish norms of interaction and complete a task than how similar group members were to each other. Neither balance nor the degree of similarity had a significant effect on the quality of the groups' work. I conclude that groups with balanced similarity structures produce task solutions that approximate the quality of those from other groups, but they do so in significantly less time. That is, balanced groups are more efficient than unbalanced groups. (Contains 8 footnotes, 2 tables, and 1 figure.)