United States Secretary of State John Kerry recently said he has ''serious doubts that Lee Harvey Oswald acted alone'' in the assassination of John F. Kennedy. While Kerry has no alternative explanation for who killed Kennedy, he has never been convinced by the official narrative of the ''lone gunman'' who was killed by another lone gunman. Kerry's beliefs are thoroughly mainstream.

According to a recent Gallup poll, 61 per cent of Americans believe there was more than one man involved in the assassination of Kennedy. Since Gallup first asked the question in 1963, the number of Americans who believe there was a conspiracy to kill Kennedy has never been less than 50 per cent. It has usually been much higher.

President Kennedy in 1963.

Like Kerry, not everyone has a fully developed conspiracy theory. In the most recent poll, 40 per cent of respondents who said there was a conspiracy had ''no opinion'' on who might have been involved. This is more than three times the numbers of people who said they believed the government, the Mafia or the Soviets killed Kennedy.

None of these alternative theories of Kennedy's death has ever gained much respectability. But the simple idea that Oswald could not have acted alone is the most widespread and persistent conspiracy theory in American life.