We are two weeks away from the 50th anniversary of the death of John F. Kennedy, and the event is sure to dredge up the usual array of alternative histories as to what actually happened that day in Dallas. From the start, the assassination has provided extraordinarily fertile ground for conspiracy theorists.

Homo conspiracist can generate some wacky beliefs—in space aliens hidden away at Roswell, N.M., or in 9/11 as an inside job by the U.S. government. But there's a catch: Sometimes skepticism about official versions of things is justified and leads to the truth—to the conclusion that, say, the Earth circles the sun, or species evolve, or the weather sure has been getting warmer lately. The challenge is to differentiate such warranted doubt from the style of reflexive skepticism that leads to the belief that yetis trained by Mossad killed Elvis because he could prove that the moon landing was faked.

How do we distinguish between critical thinking and nutty conspiracism? Psychologists have repeatedly demonstrated a key feature of conspiracism, namely that people who believe in one type of conspiracy theory are more likely than chance to believe in other unrelated ones. In other words, it's a deep psychological trait, a world view that transcends any given case.

The philosopher Paul Thagard has written about our need for explanations that provide coherence to our view of the world. Conspiracism is an extreme expression of this need. It predisposes someone who intensely believes in one conspiracy theory to accept a similar theory about unrelated events: "There they go again, trying to pull off another fast one, so don't believe that official explanation either."

Recent research by Michael Wood and Karen Douglas of the University of Kent in the U.K. points to the distinguishing characteristics of this frame of mind. In a systematic analysis of online discussions of 9/11 conspiracy theories, they showed that critics of these theories (that is, supporters of the conventional account of 9/11) tend to cite evidence in support of their stance. Conspiracists, by contrast, devote more space to arguing against the conventional view and are far less interested in supporting the validity of their alternative explanations.