Johann Hari: Diana, Osama and the rise of conspiracy theories

A huge number of genuine conspiracies have been exposed over the past 30 years

We are living in a Golden Age of conspiracy theories. The lingering stain of green ink has been scrubbed away; all over the world, the maddest ideas have gone mainstream. Some 27 per cent of Brits believe Diana Spencer was murdered - presumably by the Windsor mafia - and 86 per cent in the Arab world agree.

The announcement this week of a new head for the Diana inquiry, due to report in two years' time, is hardly likely to dampen their delusions. Some 36 per cent of American citizens - more than the number who voted for either Kerry or Bush - believe it is "likely" or "very likely" the US government staged the attack on the World Trade Centre themselves as a Reichstag fire to justify eternal war. Fat majorities all over the developing world agree. David Icke's 12-foot lizards are no doubt watching this bush-fire of fantasies from their lair beneath the White House, licking their reptilian lips with glee.

Why this rise in shadow-politics? This could be a case for Mulder and Scully, but the first reason is slightly encouraging: a huge number of genuine conspiracies have been exposed over the past 30 years. Richard Nixon bugging and burgling his political opponents, while ordering secret coups to kill the elected Prime Minister of Chile and replace him with a fascist junta? It happened. A far-right plot by colonels and hacks at the heart of the British establishment to oust Prime Minister Harold Wilson? It happened. George Bush ordering an invasion of Iraq on the basis of fictitious Weapons of Mass Destruction? It happened.

Once real plots like this have been exposed, many people inevitably become radically sceptical - and some become so open-minded their brains fall out. They begin to believe that Donald Rumsfeld plotted to fly a plane into the building he was sitting in, or that Elizabeth Windsor has the power to order assassinations, and that nobody would ever leak it. David Shayler, the former MI5 spy who is the darling of the "9/11 Truth Movement" in Britain, says there were no planes flown into the World Trade Centre. No, they "were missiles surrounded by holograms made to look like planes" - and cites Bush's lies on Iraq as evidence that anything is possible. Necessary scepticism towards power becomes credulousness towards any oppositional theory, however mad.

There are other, sadder explanations too. Conspiracy theories are the ideologies of the impotent. They are the political theory people turn to when they feel they have no control over events and no hope for change, but still pine for an over-arching and intellectually satisfying explanation for the state of the world. I can't do anything - and it's because These People (Jews/ Freemasons/ Chinese/ lizards) control everything. That's why conspiracy theories dominate the hushed political discussions that happen in tyrannies. And it's why they have ballooned in popularity in the democratic world as trust in our political institutions has haemorrhaged over the past 50 years, and we increasingly feel subject to rulers we cannot influence.

The susceptibility of so many people to the plainly false belief that the Bush administration ordered 9/11, for example, is partly the result of a natural psychological process. When trauma whacks us in the face, our brains naturally try to associate it with what we knew before. An evil event has occurred; who do we know who is evil? The Bush administration. They must be responsible. It cannot be seemingly random people Out There, particularly not people we see as inherently good - the wretched of the earth. Rather than adjust our old thinking to the new realities, conspiracists try to adjust the new realities to our old thinking.

But there is a deeper, almost-religious impulse behind conspiracy theories. They are a natural attempt to rebel against the cruel randomness of life. Patrick Leman, a psychology lecturer at Royal Holloway University, explains: "We tend to associate major events - a president or princess dying - with major causes. If we think big events like a president being assassinated can happen at the hands of a minor individual, that points to the unpredictability and randomness of life and unsettles us."

So Diana's death is a big event; it can't have been caused by a drunk driver taking a wrong turn in a tunnel. So the massacres in Manhattan and Washington five years ago today were a huge event; they can't have been the result of 17 Arabs working for a guy in a cave in Tora Bora. If we admit to ourselves that the very big can be wiped out by the unbearably small, then we have to admit the ground beneath our feet is very fragile.

Conspiracists continue to believe, in defiance of all the evidence, for the same reason Creationists continue to believe in defiance of all the evidence. As Mark Lawson puts it in his brilliant novel Idlewild: "Just as the pious made sense of bereavement through God and heaven, so did the suspicious find purpose in chaos through the invocation of the CIA, the FBI and the military-industrial complex. There was the same comfort in believing there was an ordering force behind the mess of existence."

He points out that some hardcore conspiracists believe in the "Four Men from Texas" theory of history, arguing that all of America's modern horrors - from Watergate to Abu Ghraib - are the work of four men meeting in country clubs in Dallas. He asks: "What was this invisible committee if not a proxy God?"

To many of us, it is more comforting to believe there is a malevolent force controlling events than to acknowledge that we are simply arbitrarily evolved bits of carbon floating in a void. It is easier to confect an endless contorted conspiracy theory - the real planes disappeared, the driver was paid by MI5, the Queen is a lizard - than to admit the truth. Sometimes a car crash is just a car crash. Sometimes a massacre is just a massacre. Sometimes nobody is in control.