Two studies investigated what motivates knowledge sharing in online knowledge forums. Based on the uses and gratifications model, we hypothesized that individuals would respond to information requests broadcast by unknown others to fulfill their needs for social interaction (affiliative tendency), to maintain a positive self-image (self-esteem), or to proclaim one's uniqueness (public individuation). Consistent with the hypotheses, a web-based survey with current users of a public knowledge sharing site found that those with stronger affiliative tendency, higher self-esteem, or stronger public individuation were more likely to contribute to the open information repository (Study 1). However, a 2 (social presence: low vs. high) x 2 (recognition rewards: absent vs. present) between-subjects design experiment also showed that these psychological traits significantly enhanced individuals' intention to share knowledge on a public web site, only when other users' presence was rendered salient and individual contributions were visibly acknowledged (Study 2).