This is a lightly edited version of a keynote address Clay Shirky gave on Social Software at the O'Reilly Emerging Technology conference in Santa Clara on April 24, 2003, in which he talks about social software. “I want to talk about a pattern I've seen over and over again in social software that supports large and long-lived groups. And that pattern is the pattern described in the title of this talk: A Group Is Its Own Worst Enemy.”