By Peter Knight
Senior lecturer in American Studies, University of Manchester
There have undoubtedly been conspiracies throughout history, and people have probably always speculated about a hidden, sinister hand behind events.
But it was only in the late 18th Century that the idea of a vast plot shaping the course of history began to grip European and American imaginations with fears that the French Revolution was the result of a plot by secret societies such as the Freemasons or the Illuminati.
The first recorded use of the phrase "conspiracy theory" dates back to a history article from 1909.
But it is only since the 1960s that it entered popular usage.
Indeed, conspiracy theory, only entered the supplement to the Oxford English Dictionary for the first time in 1997.
Conspiracy theories have served different political and cultural functions during the last two centuries.
Often minority groups or outsiders are blamed for social woes, in a familiar process of scapegoating that helps to provide a comforting sense of coherence and community to those who believe their way of life is imperilled by conspiring forces.
The principal target of conspiracy theories has shifted from the "red scare" to the " fed scare".
Moreover, these popular fears are exploited by those in positions of power in order to further their own vested interests.
But in the 20th Century, and particularly since the 1960s, conspiracy theories have increasingly pointed the finger of blame at the powerful, usually the government and the intelligence agencies.
In effect, the principal target of conspiracy theories has shifted from the "red scare" of McCarthy's distrust of communism to the "fed scare" - a deep distrust of anything official.
This is partly a result of a loss of faith in authorities in recent decades: opinion polls show that in the US in the early 1960s three quarters of Americans trusted the government, but by the 1990s only a quarter claimed to do so.
There was a resurgence of trust following 9/11, but that has faded with the war in Iraq.
Why have conspiracy theories become so prominent since the 1960s?
The rise in conspiracy thinking has usually been seen by academics as a sign of popular paranoia in those on the margins of society.
Richard Nixon resigned over Watergate in 1974
Condemned as paranoid thinking, conspiracy theories are dismissed as mental aberrations which could do only harm.
But a different explanation for its increasingly mainstream popularity is the trickle of revelations about government and corporate wrongdoing that emerged in the 1970s and after, with Watergate the prime example.
Another reason is that conspiracy theories have now become a form of entertainment, not least in Hollywood, making them both more widespread but also less dangerous.
You now need, as the popular wisdom has it, to be a little paranoid to remain sane
Conspiracy culture has become a surprisingly self-conscious phenomenon, with TV series such as the X-Files, and films such as the 1997 Conspiracy Theory starring Mel Gibson and Julia Roberts, and Oliver Stone's JFK.
Often people believe in a conspiracy when no other explanation seems to fit the bill, particularly when trying to work out the exact linkages of blame and causation in a globally connected world.
Following the assassination of President Kennedy in 1963 in particular, conspiracy theories have become a regular feature of everyday political and cultural life.
Conspiracy theories have become part and parcel of people's normal way of thinking about who they are and how the world works.
Certainty has given way to doubt, and conspiracy has become the default assumption in an age which has learned to distrust everything and everyone
They have become a semi-respectable way of analysing how the world of complex political power works, with even Hillary Clinton seeing fit to claim - not without some justification - that there was a "vast right-wing conspiracy" out to get her husband during his impeachment.
You now need, as the popular wisdom has it, to be a little paranoid to remain sane.
A certain kind of world-weary paranoia has become the norm, in both entertainment culture and popular politics.
The theories sometimes express a sense of connectedness that is both scary and transcendent: the American novelists Thomas Pynchon and Don DeLillo have explored this possibility in depth.
It is also noticeable that since the paradoxically secure paranoia of the anti-Communist decade of the 1950s, conspiracy theories now often provide a sense of insecure paranoia in which it is no longer easy to distinguish between Them and Us in the current pervasive environment of risk.
Certainty has given way to doubt, and conspiracy has become the default assumption in an age which has learned to distrust everything and everyone.
It is also arguable that the current climate of fear post-9/11 marks a return to the conspiratorial certainties of McCarthyism.
Why do Americans seem particularly prone to believing in conspiracy theories?
Many people question the official analysis of 9/11
With their Puritan legacy of always seeking the Devil behind inexplicable events, a Republican distrust of secrecy and special privileges, a recurring - but usually unfounded - sense that the exceptional mission of America is threatened by un-Christian enemies, and an abiding faith in the power of individualism closely followed by a fear of anything that threatens the individual, Americans have often been quick to turn to conspiratorial explanations that blame all manner of external forces for why things have not worked as hoped.
Although Americans have had a special affinity with a conspiracy talk, they no longer have a monopoly on the paranoid style.
With the growth of new media technologies conspiracy theories now circulate around the globe at lightening pace, and in recent years the UK has become increasingly attracted to rumours of conspiracy.
Peter Knight is a Senior Lecturer in American Studies at the University of Manchester. He is the author of Conspiracy Culture (2000), The Kennedy Assassination (To be published in 2007), and the editor of Conspiracy Nation (2002) and Conspiracy Theories in American History: An Encyclopaedia (2003).
The first episode of The Conspiracy Files: How Diana Died was broadcast on Sunday, 10 December 2006 at 2100 GMT on BBC Two.