Conspiracy theories might seem to be the preserve of internet geeks and Oliver Stone fans, but they are no longer a cultural phenomenon that can be dismissed easily. International polls have found in the past few months that an extraordinary number of people believe that secret, conspiratorial forces lie behind world events.

To list just a few of the more widespread theories: a near-majority of the Arab world believes that Jews were warned away from the World Trade Center on 11 September; an actual majority believes that Diana, Princess of Wales, was murdered because of her involvement with a Muslim man; more than half of all black Americans believe that the CIA makes drugs easily available in their communities to keep them quiescent, and one-fifth believe that it deliberately introduced the Aids epidemic; 80 per cent of all Americans believe that the US government is conspiring to withhold information about Gulf war syndrome; and a bestseller in France claims that Osama Bin Laden was a US agent who was used by President Bush to destroy secret CIA offices in the twin towers.

The events of 11 September were a gift to budding Mulders and Scullys everywhere. David Corn, a writer for the left-leaning independent website AlterNet.org which seeks to expose corruption and secrecy, has been bombarded, to his growing horror, with e-mails purporting to disclose "what really happened". He explains the most popular theories circulating at the moment. "There are e-mails about a fellow imprisoned in Canada who claims to be a former US intelligence officer and who supposedly passed advance warning of the attack to jail guards in mid-August. And there are others citing an Italian newspaper report that, last July, Bin Laden was treated for kidney disease in Dubai and met a CIA official."

The most common theories of all, however, derive from Thierry Meyssan's bestselling book L'effroyable imposture (The Frightening Fraud). Meyssan is the director of the Voltaire Network, a prominent left-wing think-tank, and it is hard to overestimate the impact of his book in France. It were as though Matthew Taylor of the IPPR or Mark Leonard of the Foreign Policy Centre had suddenly announced that the US government was involved in the 11 September attacks. Central to Meyssan's thesis is the argument that the Pentagon was not struck by a plane at all, but rather that a carefully planned truck bombing or missile strike was set up by the US government to look like a plane crash. He believes that the government plotted the whole affair, in part to control rogue agents within the security services.

To justify these claims, Meyssan points to discrepancies in eyewitness statements, the lack of footage of any plane wreckage, and images from CCTV cameras across Washington which seem to show that there was no plane approaching the Pentagon. These theories have been popularised in the English-speaking world by cult websites ("Hunt the Boeing!" at www.asile.org/citoyens/ numero13/pentagone/erreurs_en.htm).

But Meyssan's theories are often circumlocutory and warped. They contain huge gaps: to give one example, if no plane smashed into the Pentagon, there's a stray Boeing out there carrying Barbara Olsen of CNN and a few hundred others. Where is it?

The French press have almost unanimously denounced the book and retaliated by publishing various pieces of evidence, including photographs, which show that the Pentagon was indeed hit by a plane. Eyewitnesses to the attack, such as the US journalist James S Robbins, have written eloquently about their anger at what they regard as tacit accusations that they are liars. George Bush felt the need to denounce the theory when he spoke to the UN General Assembly last October about the "malicious lies that attempt to shift the blame away from the terrorists themselves".

The Guardian columnist George Monbiot caused outrage when he seemed to imply that the US government might have had a role in the "convenient" anthrax attacks which followed 11 September. And sometimes it has suited right-wingers in forums such as the US National Review to include among "conspiracy theories" claims that the real motivation for the war in Afghanistan is the wish to build an oil pipeline through that country.

But in truth the most implausible conspiracy theories surrounding 11 September have emanated from the hard right, which has been detecting clandestine enemies for more than a century. The US militia movement has been quick to argue that the attacks were a ruse by the US government, mimicking Hitler's burning of the Reichstag, to justify the seizure of significant state powers to detain and control "dissidents" (that is, the militiamen themselves). Bush is acting as a proxy for "the communist United Nations" (the object of the militiamen's fanatical hatred), which is seeking to establish a "new world government" and "hates American traditions of liberty and freedom".

These beliefs are being openly propagated to an audience of millions by shock-jock radio hosts across the Deep South - many of whom can be listened to over the web, if you can bear it. The world was alerted to this paranoid US subculture when one of its most loyal "soldiers" - the Oklahoma bomber Timothy McVeigh - staged the largest pre-11 September terrorist attack on the US mainland. He displayed the movement's hatred of the government by attacking a federal building that he believed was a focus for furtive UN activity to suspend the US constitution.

Anti-Semitism informs the theories of McVeigh's friends and successors. Many of the survivalists' and militias' sites still distribute the Protocols of the Elders of Zion, a ludicrous forgery dating back to early 20th-century Russia, as an "expose". Even some of the more mainstream groups clumsily secularise their hatred of Jews into a hatred of international financiers, who are always identified by their Jewish surnames. The director of the US Anti-Defamation League, Abraham H Foxman, has been forced to issue a plea that "blaming Jews for 9/11 must stop".

Foxman has shown that some Arab governments and elements of Arab public opinion are also proponents of anti-Semitic conspiracy theories at present. Tim Sullivan, professor of political science at the American University in Cairo, explains that "conspiracy theories are a stock-in-trade here. When you feel you don't have control over your life and events, then conspiracy theories explain what is happening." Syria's ambassador to Tehran, Turki Muhammad Saqr, reflected a significant section of Arab opinion when he claimed that he had "documented evidence" that "Zionists conducted these attacks" to distract attention from the UN World Conference Against Racism.

Professor Robert Alan Goldberg of the University of Utah explains that, in the decades following the Second World War, conspiracy theories have become more commonly believed and more deeply embedded in American culture. He identifies a number of reasons for this, including the huge growth in ways to transmit these theories (most obviously the internet). He also points out, however, that conspiracy theories have long been part of US and European culture. "For example," he writes, "in the 18th and 19th centuries there was a fear of Catholics in the US as well as in Europe. The fear was . . . that Irish Catholics were coming to take over and make America a papal province."

Conspiracy theories have grown in popularity in part because a number of genuine conspiracies have come to light since the 1970s. Woodward and Bernstein may have been mocked initially as crazy conspiracy theorists, but Richard Nixon really was at the heart of a dangerous anti-democratic conspiracy. Similarly, the Warren Commission actually was a whitewash - the truth about JFK's death is being concealed by somebody out there; and Hillary Clinton identified a "vast right-wing conspiracy" seeking to bring down a twice-elected and popular president only a few years ago.

Indeed, the 11 September attacks were themselves the result of an al-Qaeda conspiracy that few people would have credited on 10 September. Similarly, the US far right's fears of persecution by the government are lent a sliver of credence by the CIA's bungling (and conspiratorial lies about the events) at Ruby Ridge and Waco, which ended in the deaths of law-abiding citizens, including children.

Why is there a need for the more fantastical theories in this (supposedly) rational age? Partly, they are an expression of the decline in trust across all sectors of society, and towards governments in particular. As governments' power over our lives seems to decrease, people begin to believe that the power has gone somewhere else: to people in the shadows.

But there are other, more personal reasons, too. Cary Cooper, professor of psychology at University of Manchester Institute of Science and Technology, believes this is a way of coping with the unbearable reality that we can be victims of random death, or at the mercy of a handful of knife-wielding hijackers. Many people, he says, "can't come to terms with the fact that it could just be a few people. If this can happen, what sense of security can you have?" There is a perverse comfort in the view that there is a controlling force behind it all.

The mentality of active conspiracy theorists is brilliantly dissected in Mark Lawson's 1995 novel, Idlewild (or everything is subject to change). Lawson shows that the most ardent theorists - the ones who attend conferences and write books - almost always have some significant trauma within their own lives which they need to rationalise. One character's response to each increasingly mad theory is to say: "Listen, they killed my daughter. They even managed to persuade one of the best coroners in the county that it was suicide. So these people are capable of anything."

Lawson connects the need for such theories with the decline of religious faith. As we lose faith in an omnipotent God pulling our strings from heaven, we convince ourselves there is another, secular, "ordering force behind the mess of existence". For some, it is more reassuring to believe in an elaborate conspiracy than to confront the cold post-religious reality that, often, horrific things just happen, with no cause, rhyme or reason.