Mark Steel: The madder the theory, the more people believe it

There's one question no one has raised at this inquest into Diana's crash. Is it really possible that an elaborate murder, involving flashing lights, redirected cameras and ghostly Fiat Unos, could be organised by a family made up of bumbling aristocratic idiots incapable of crossing the road without an equerry?

If they wanted to be rid of Diana, Prince Philip would have just yelled "Chase her through a tunnel? Stuff that, I'll do her with my twelve-bore." If only someone would make this obvious point, the whole thing would be settled and the country could save ten million quid.

But conspiracy theories are more popular than ever, and they're everywhere. By now there's almost certainly a website that starts "So Ming Campbell unexpectedly resigns just two days after England's unlikely victory over France! Coincidence? Do they think we're fools??!! Far more likely is that Ming's opposition to the proposed reform of certain Scottish bus routes was too much for MI6, who poisoned him on Saturday night while everyone was distracted by the rugby, which they'd paid the French to lose, and then replaced him with a clone who was programmed to resign. And that's a fact!!!!!"

Last week I spoke at a rally supporting a withdrawal of troops from Iraq, and did two separate interviews for websites that went the same way. About 30 seconds in, I was asked what I thought were the real reasons for the collapse of the twin towers. One of them kept saying with increasing volume "How many buildings collapsed that day Mr Steel, I ask you how many?" with such vigour I was on the point of confessing to blowing them up myself. In short, the conspiracy argument surrounding the twin towers seems to be: "First the planes full of fuel fly into the towers and explode. Then the towers collapse. And they expect us to believe these two events are connected? Do they think we're stupid?"

But it's a mistake to get into a discussion because then comes a battery of unanswerable questions, such as: "How could the steel have melted when it had a residual heat capacity of 145 oblm per 10,000 over nine with an impact resistance factor of Rockall, Finistaire, South-West, six rising to seven, moderate gales likely. Huh?"

There's such a relentless certainty about them. They might, as several websites do, suggest there were no planes involved at all. But if you said, "So where did the fire come from?", they'd look at you as if you were stupid and say: "The CIA bred dragons. And sent them to the 95th floor of both towers. Obviously."

There are otherwise rational people who claim the planes were holograms and the debris from the planes was planted. Next time someone tells me that, I'll reply that they've missed the point, because the towers themselves were holograms, and anyone who thinks they went up them was in fact hypnotised to think that by the lift operator who was, in fact, a CIA agent.

The more deranged theories might be limited to a few busy bloggers, but the basic conspiracy theories have become hugely popular. Millions believe Diana was murdered, and in America 36 per cent believe that federal officials took part in the attacks on New York.

Not all of these people can be mad, and part of the reason for these figures may be that the authorities have been blatant liars in recent years. The Vietnam War escalated after US ships were said to have been fired upon, when this was made up. The Falklands War depended on sinking the General Belgrano for heading towards the British fleet, though this was made up. And the Iraq war depended on Saddam's weapons of mass destruction... do you detect a pattern here?

But these are lies that not only illustrate the willingness of governments to deceive their population. They also show their limitations, because a government can't rule through a handful of loyal robotic agents, who convey a series of fairy stories to the easily duped masses. Orchestrated lies depend on the connivance of a wide layer of people, and there are always those who break ranks. And then, especially when a war is as unpopular as the one in Iraq, millions become sceptical of the Government's statements. So the case for the war was exposed, which added to the movement against it.

What we know they do for certain is bad enough. George Bush justified the war every day by saying Saddam was responsible for the attack on the twin towers when he knew that wasn't true. Blair and Bush both exploited the atrocity by using it to justify a war that would have happened anyway.

There's no need to add extra bits in that to sound crazy. It would be like if Robert Mugabe were arrested, and someone started screaming: "Never mind what he's done to Zimbabwe, I've got evidence that he's Voldemort and tried to murder Harry Potter."

The conspiracy theory can only make people feel helpless, as if we're under the control of an unbeatable sinister machine, capable of orchestrating every massive event and manipulating the whole population, except for a handful who see through them. So they can end up helping the governments they're supposed to be exposing.

So the more you think this through, the more obvious it seems that conspiracy theories and their websites are all started by MI6 and the CIA, the cunning bastards.