Book excerpt: Conspiracies theories are some of the most striking features in the American political landscape: the Kennedy assassination, aliens at Roswell, subversion by Masons, Jews, Catholics, or communists, and modern movements like Birtherism and Trutherism. But what do we really know about conspiracy theories? Do they share general causes? Are they becoming more common? More dangerous? Who is targeted and why? Who are the conspiracy theorists? How has technology affected conspiracy theorising? This book offers the first century-long view of these issues.