Ever since the Warren Commission concluded that a lone gunman assassinated President John F. Kennedy, people who doubt that finding have been widely dismissed as conspiracy theorists, despite credible evidence that right-wing elements in the CIA, FBI, and Secret Service—and possibly even senior government officials—were also involved. Why has suspicion of criminal wrongdoing at the highest levels of government been rejected out-of-hand as paranoid thinking akin to superstition?

Conspiracy Theory in America investigates how the Founders’ hard-nosed...