Kennedy conspiracyWe are such party-poopers, we rationalists. It actually matters to us if something is true, and we are possibly too prepared - verbally, at least - to deny people the easy consolations of faith or quackery.

But often making such a challenge really matters, as it does in the case of bad science, such as that behind the MMR scare, bad education as in "Intelligent Design", and as it can do in challenging monstrous prejudice masquerading as fact, as in the case of racial theory.

Having spent six years researching and writing a book on the subject I would add conspiracism to the list of potentially dangerous false consciousnesses. Not only can a tendency to accept conspiracy theories lead, as with the Protocols of the Elders of Zion, to the creation of scapegoats, but it may also create a generally unhelpful delusion about the way the world works. Here I have to be honest and add that, when confronted with a slightly pompous person who reiterates conspiracist nonsense as if it were culled from the peer-reviewed pages of Scientific American - while describing themselves as "sceptical" - then, as Oscar Wilde says, the duty of correction becomes a positive pleasure.

Of course, if there's one thing that Jesus (whoever he was) was right about, it was motes and beams. There will be many humanists and rationalists who have, in a relaxed way, bought into conspiracy theories, partly because these concepts have inhabited the same intellectual territory as we have and we have grown up together. Lazily, I passively bought into several theories that were widely believed by the people I came of age amongst: JFK, the Reichstag fire, the Gulf of Tonkin, to name three.

This week, interviewed for a magazine about the book, I was asked - rather hopefully, I thought - whether there were any theories which I had previously dismissed but now thought to be true. No, I replied, exactly the reverse. Almost without exception, when you begin to look at the evidence, conspiracy theories - theories which are more complex than the conventional explanations - evaporate. Occam's Razor rules.

We shouldn't really be surprised. It was far simpler for a lone gunman acting in conformance with his beliefs and needs to shoot a President, than for some vast, complicated inter-agency matrix, acting entirely against its stated mission, to do it. Think of Lincoln, McKinley, Garfield, Reagan, George Wallace, Huey Long and Anton Cermak, all shot at by barely connected mavericks. After all, JFK has been dead 45 years; Fidel Castro - target of numerous sophisticated plots - is still alive.

One of the things that distinguishes conspiracism is its dedication to complexity. Another is its faith in organisation. Put together these require a belief in a world largely freed of accident and contingency, where virtually everything is planned and therefore plannable. And in a funny way this is reassuring. Deaths which might seem random and pointless, like that of Princess Diana in a ridiculous car accident, are given shape and agency by conspiracists.

What I found wonderful, in a way, was the lengths that people are prepared to go to, intellectually, to create and believe in such tales. There are plenty of examples in my book, some of them (I think) hilarious, and some even poignant. The 9/11 Truth movement, almost entirely a product of the professional classes of America, has come to believe in a plot of impossible complexity and deviousness - a plot which has not even the remotest precedent in world history. The Twin Towers were brought down by controlled demolitions somehow timed to fit in with the crashing planes; the Pentagon was hit by a missile and not by the aircraft that hundreds of eye-witnesses thought they saw; phone calls to relatives were created using voice morphing technology, and so on. Not only are such confabulations believed, but they are believed indignantly.

One of my chapters looks at the way in which the Liberal Democrat front-bencher, Norman Baker - the epitome of the indignant believer - constructed his theory that Dr David Kelly had been murdered, steadfastly during the course of an entire book, abnormalising the normal and normalising the abnormal, until nothing could become more unnatural than suicide and nothing more natural than hit squads of anti-Saddam Iraqis questing through the Oxfordshire countryside.

Suicide is very unsatisfactory and in that sense - the sense in which there is a narrative, a story that improves upon reality - conspiracism is not unlike religious belief. Lewis Wolpert speculated that our evolutionary development of tool-making may have required the habit of causality, which we necessarily extended to the question of why we existed. Hence the universality of religious belief. This principle could extend, too, to the need to construct better stories than reality apparently furnishes.

Of course, to rationalists reality is the better story. But it has to be admitted (though I hope that sufficient book-buyers prove me wrong) that we often seem to be in a minority in thinking this way.