In order to motivate individuals to share their knowledge in online communities, the use of extrinsic rewards and goals is a typical approach. However, extrinsic motivation may have unintended consequences. Although past studies have examined the direct effect of extrinsic motivation on intrinsic motivation, no research to date has investigated how extrinsic motivation moderates the impact of intrinsic motivation on knowledge sharing, or how the effect of extrinsic motivation on intrinsic motivation is contingent upon whether a member is active or not. Drawing on attribution theory and theory of planned behavior, the study was conducted with data collected from a large social Q&A site consisting of multiple online communities with millions of registered users; the data were analyzed with moderated regression and structural equation modeling. Results show that the effect of enjoyment in helping others on attitude toward knowledge sharing is undermined by virtual organizational rewards, while the effect of knowledge self-efficacy on attitude toward knowledge sharing is undermined by reciprocity. The results also show that the effect of virtual organizational rewards on enjoyment in helping others is contingent upon whether members are active or not. Specifically, for active members, virtual organizational rewards undermine enjoyment in helping others; for inactive members, however, virtual organizational rewards increase enjoyment in helping others. These findings enrich the research on unintended consequences of extrinsic motivation specifically, and the theory of motivation in general. Additionally, these findings provide practical insights on how and when to use extrinsic rewards/goals to motivate individuals to share knowledge in social Q&A sites.