Crazy as these theories are, those propagating them are not — they’re quite normal, in fact. But recent scientific research tells us this much: if you think one of the theories above is plausible, you probably feel the same way about the others, even though they contradict one another. And it’s very likely that this isn’t the only news story that makes you feel as if shadowy forces are behind major world events.

“The best predictor of belief in a conspiracy theory is belief in other conspiracy theories,” says Viren Swami, a psychology professor who studies conspiracy belief at the University of Westminster in England. Psychologists say that’s because a conspiracy theory isn’t so much a response to a single event as it is an expression of an overarching worldview.

As Richard Hofstadter wrote in his seminal 1965 book, “The Paranoid Style in American Politics,” conspiracy theories, especially those involving meddlesome foreigners, are a favorite pastime in this nation. Americans have always had the sneaking suspicion that somebody was out to get us — be it Freemasons, Catholics or communists. But in recent years, it seems as if every tragedy comes with a round of yarn-spinning, as the Web fills with stories about “false flag” attacks and “crisis actors” — not mere theorizing but arguments for the existence of a completely alternate version of reality.