FIFTY years ago this week President John Kennedy was assassinated by Lee Harvey Oswald, a Castro-supporting communist who had learned to shoot straight while in the US Marines. Books such as Gerald Posner’s “Case Closed” have painstakingly debunked the various alternative theories: that he fell victim to multiple gunmen, elaborate plots involving the CIA, the Mafia and who knows what other shadowy groups. The “magic bullet”, modern ballistics show, behaved normally. There was no second gunman. Nothing interesting happened on the grassy knoll. And the idea that Dallas, the “city of hate”, was somehow collectively to blame, is absurd.

Yet still the conspiracy theories live on, inspired by popular books such as Mark Lane’s “Rush to Judgment”, by Oliver Stone’s preposterous movie “JFK”, and by endless speculation online. Was Lyndon Johnson involved in a dastardly coup d’état, as Mr Stone hints? Of course not. But half a century later, 61% of Americans believe in a conspiracy. Amazingly, this is the lowest level since the late 1960s.