Cass R. Sunstein, the Robert Walmsley University professor at Harvard Law School, is a former administrator of the White House Office of Information and Regulatory Affairs and is the author of "Conspiracy Theories and Other Dangerous Ideas" and, most recently, "Wiser," co-authored with Reid Hastie.

A lot of conspiracy theorists are neither ignorant nor ill-educated. On the contrary, they can be spectacularly well-informed, at least on the topics that interest them. (Try arguing with one; they probably know a lot more than you do.) Why, then, do they accept theories that are patently inconsistent with reality?

One reason involves their suspicious and in some cases paranoid natures. Want to know whether your neighbors will accept a particular conspiracy theory? Just ask them what they think about other conspiracy theories. Those who insist that the Apollo moon landings were faked are more likely to believe that the United States caused the 9/11 attacks.

Horrible events can make people who are scared and angry look for someone to blame.

In fact, people who embrace one conspiracy theory are also inclined to embrace another conspiracy theory that cannot simultaneously be true. In one study, people who said they believed that Osama bin Laden is alive and well were more likely to believe that he was dead before U.S. forces invaded his compound. The belief in a more central idea — that authorities are engaged in deceptive cover-ups — supports any number of skeptical theories, even leading suspicious individuals to disregard contradictions between them.

But conspiracy theories are not only a product of people’s natures. Social conditions matter. Horrible events — economic collapse, an assassination, a grievous loss of some kind — can make people who are scared and angry look for someone to blame, not least to assert a kind of mastery of the circumstances. The human mind is drawn to think that whenever something bad has happened, it is because someone bad wanted it to happen.

Conspiracy theories are especially likely to spread within isolated communities and social networks, including those online. Most people do not know the causes of horrible events, and so we tend to rely on those we trust. If a few people within a network — call them “conspiracy entrepreneurs” — point to an alleged conspiracy, others within that network might well follow them. After a while, people can become committed to that belief and treat official denials as yet additional evidence of conspiracy, leading to the conspiracists’ triumphant question: Why would they deny it if it weren’t true?



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