The Internet has been called the universal marketplace of ideas. But as Cass Sunstein asserts in On Rumors, the World Wide Web has also worsened the human tendency toward destructive, misleading gossip. The Harvard Law professor identifies the cascading effect of the Internet, allowing rumors—defined as unsubstantiated claims of any kind—to spread across the glob with tsunami force and speed. What results is “group polarization.” The like-minded end up with a more extreme version of what they already believed through the echo chamber of online “communities,” which can turn into ghettos of the mind. Despite the “self-correcting” facet of the Internet, the old rule still applies: It’s hard to wash off mud once it sticks.