Lord Stevens' report into the death of Princess Diana yesterday appeared to put an end to the numerous conspiracy theories surrounding her fate. Or did it? Lindsay Jennings reports on the theories which still, today, surround the world's most famous deaths and events.

ACCORDING to one television programme on conspiracy theories, half of us believe that Princess Diana was murdered by a French onion-seller on a moped. A third of us are convinced the Kennedys killed Hollywood actress Marilyn Monroe, and more than three million Americans believe they have been abducted by little green men.

They are the same people who, no doubt, think Elvis will be buying his Christmas turkey from M&S this year (those Twiggy adverts have really boosted the store's popularity) and Hitler is plotting a comeback and an appearance on Celebrity Big Brother - direct from his bunker where he's been hiding under the A1 all these years (he's been terrified of it being upgraded in case they find him).

Conspiracy theorists may be dismissed by some as 'nutcases' or 'paranoid', but it doesn't stop them speculating and pontificating on what is often simply an awful accident or a tragic piece of bad luck.

The ink had barely dried on Lord Stevens' inquiry into the death of Princess Diana yesterday than the speculation began again, led by Mohammed Al Fayed, Dodi Al Fayed's father. No longer is a report by a distinguished establishment figure enough to signal an end to the speculation. Instead, Mr Al Fayed branded the report a "cover up" and insisted that his son and prospective daughter-in-law were killed on the orders of the Royal Family with help from MI6.

Other theories around on the internet - and this is where most theories get a jolly good airing - are that Diana faked her own death after being fed up with the constant intrusion into her private life by the media. There are even sick joke theories: that all the London florists were in cahoots and, in a fiendish act of flower power, killed the couple in order for their profits to shoot up like a rampant clematis.

Whatever their roots, conspiracy theories are often an attempt to rationalise what causes a major event in history. They are usually associated with influential figures (when was the last time you blamed your Great Aunt Pauline's fall down the stairs on MI6?) from the world of politics or celebrity. According to Wikipedia, the online encyclopedia, the term was first coined in the 1920s but it was only in 1960 that it entered popular usage. The advent of the internet and the media's willingness to give space to them (albeit in an often patronising way) have only served to increase the speculation and the speed with which they are transmitted.

Initially in America, 9/11 conspiracy theories received little attention in the media, but as the weeks passed and more space in the media was given to theories, the Bush administration had to issue a series of statements refuting various spurious allegations.

Theories abounded that individuals associated with the US government knew of the impending attacks on the World Trade Center and refused to act with the intention of stirring up patriotism. Others claim the collapse of the World Trade Center was the result of a controlled demolition exercise and that United Airlines Flight 93 was shot down.

Occasionally, a good old conspiracy theory doesn't have to come from a live event. When Dan Brown wrote The Da Vinci Code it was based on a theory - that Jesus Christ had had a child and his descendants walk the Earth today. By the time Brown had woven the theory into a convincing story, quoting rumoured instances in history as fact, his novel had an air of legitimacy which gave his theory credence. Countless number of articles and television programmes have been given over to probing the 'truth' in his work.

So, why are we so obsessed with conspiracy theories? According to Dr Joan Harvey, a chartered psychologist at Newcastle University, it's because we are incapable of believing major incidents and accidents can happen simply because of chance.

"We need to seek some other justification for something rather than accept the fact that it was simply bad luck," she says. "It's a way of making sense of things and find a reason for them."

Dr Harvey says that we often tend to underestimate the role of chance in what happens in our lives.

"There are some people who just like the mystery and the notion of a conspiracy theory," she says. "But generally we want our world to be stable and we like it to be predictable and it isn't when it has a chance element in it.

"Because of that, we're uncomfortable just accepting that these things are anything other than unfortunate events. It's like if there's an accident, someone has got to be blamed. We're really very uncomfortable that our world can be torn apart for absolutely no reason."

There may also be some who focus on conspiracy theories because they cannot accept the death of those who have been loved by the public. It could even be seen as a way to keep their memory alive.

As Mr Al Fayad spoke of MI6 and plots to assassinate his son yesterday, there is no doubt thousands of people will be agreeing with him. They are probably online now, adding their own theories.