A recent survey of contributors to Wikipedia found that less than 15% of contributors are women. This gender contribution gap has received significant attention from both researchers and the media. A panel of researchers and practitioners has offered several insights and opinions as to why a gender gap exists in contributions despite gender anonymity online. The gender research literature suggests that the difference in contribution rates could be due to three factors: (1) the high levels of conflict in discussions, (2) dislike of critical environments, and (3) lack of confidence in editing other contributors' work. This paper examines these hypotheses regarding the existence of the gender gap in contribution by using data from an international survey of 176,192 readers, contributors, and former contributors to Wikipedia, including measures of demographics, education, motivation, and participation. Implications for improving the design and culture of online communities to be more gender inclusive are discussed.