Wikipedia, the “free encyclopedia that anyone can edit,” has more than 3.5 million articles in English covering nearly every subject under the sun. Every day, hundreds of thousands of people go there to add information or create new pages (the site offers straightforward instructions). Yet despite the site’s openness, surveys suggest that less than 15 percent of Wikipedia’s contributors are women. The Wikipedia Foundation has set a goal to raise the share of female contributors to 25 percent by 2015. What accounts for this imbalance? Is there something about Wikipedia’s format and purpose that attracts more male contributors (other sites like Flickr and Yelp do not appear to have this gender gap)? Are there ways to alter this gap?