Several experiments on the effects of pair versus solo programming have been reported in the literature. We present a metaanalysis of these studies. The analysis shows a small significant positive overall effect of pair programming on quality, a medium significant positive overall effect on duration, and a medium significant negative overall effect on effort. However, between-study variance is significant, and there are signs of publication bias among published studies on pair programming. A more detailed examination of the evidence suggests that pair programming is faster than solo programming when programming task complexity is low and yields code solutions of higher quality when task complexity is high. The higher quality for complex tasks comes at a price of considerably greater effort, while the reduced completion time for the simpler tasks comes at a price of noticeably lower quality. We conclude that greater attention should be given to moderating factors on the effects of pair programming.