Many of us are being misled. Claiming to know the “pals” of presidential aspirants, dark secrets about public officials, and hidden causes of the current economic crisis, those who spread rumors know precisely what they are doing. They are sometimes able to derail political candidates, injure companies and reputations, even damage democratic governance. And in the era of the Internet, they know more about manipulating the mechanics of false rumors—social cascades, group polarization, and biased assimilation—than you do. They also know that the presumed correctives—publishing balanced information, issuing corrections, and trusting to the marketplace of ideas—do not always work.