At the end of the first half of Derren Brown's current stage show, Svengali, the accomplished prestidigitator and manipulator of minds makes a plea that no one in the audience should reveal any of his act's content, lest they ruin it for others. Fair enough. A cynic might say that Brown's more concerned that no one devalue his shtick, but actually these are two sides of the same palmed coin - and besides, I happen to believe Brown is generally a good thing who adds to the gaiety of the nation.

True, he's not an illusionist with the stature of, say, the Great Lafayette (né Sigmund Neuberger), who perished in Edinburgh in the notorious Empire Palace Theatre fire of 1911 while performing his signature "Lion's Bride" illusion, wherein a smallish woman was metamorphosed into a big cat. Such was Lafayette's hold on the public that when a fault in a stage light caused it to plummet and ignite the set, the audience, assuming this was all part of the show, sat still and watched while the illusionist and ten other crew members were incinerated.

As I say, I've no desire to rain on Derren's parade, but I do think the methods he employs to obtain the raw material of his performances are worth discussing because they teach us so much about that critical component of human folly, suggestibility. Besides, I think it unlikely that the Venn intersection between New Statesman readers and potential Derren Brown audiences comprises many members - possibly I'm the sole one.

The first time I saw Brown, a few years ago, he told the massed ranks of the goggle-eyed in no uncertain terms that nothing he was doing in any way involved the supernatural. In this he was following a grand tradition of professional magicians acting as debunkers of the paranormal, the most notable example of which was Houdini himself. Nevertheless, during the interval I overheard several people saying to their companions words to the effect of: "Ooh, he says it's not real magic - but I think he's lying".

Playing the fool

Two psychological phenomena were operating simultaneously here. First, the average Derren Brown audience member must be more suggestible than most: why else would she or he be there in the first place? After all, if you don't unconsciously wish to be fooled, why go and see an illusionist? Second, Brown's impassioned assertion of the rationally explicable nature of what he was doing constituted an example of negative suggestibility - primed, by him, to disbelieve everything he did and said, the audience flatly denied this truth.

I assert that Brown primes his audience to disbelieve everything he does and says, but I should qualify this: like all adroit manipulators he wishes them consciously to question everything overt while unconsciously they absorb a great deal of covert instruction. To give one example: at the very beginning of each of his feats he throws balls or frisbees out into the audience, then asks whoever has caught one to come up on stage. This appears a completely random way of selecting his participants, but in fact he has already refined his selection to the more suggestible - because, when a frisbee flung by a magician is flying across an audience, who but someone who wishes to be manipulated would stick their hand up to catch it?

Once these gullible souls get up there, he subjects them to a further culling. In poker circles, good players become extremely adept at spotting another's "tell", the unconscious tic that reveals when someone is bluffing. Brown is a master of reading these tells: the little spasms we make when someone has hinted at a truth about ourselves we are concealing. Holding their hands, looking into their eyes, persuasively uttering their names, Brown has only to ask these already self-selectively suggestible people a few questions in order to establish whether they are what he requires for the rest of his act - namely, people who can be told what to do without being aware of it.

Hm, I wonder what other social groups exhibit the same characteristics as Derren Brown's audiences? Let's see . . . self-selecting for a willingness to suspend disbelief while also desperate to be told by a charismatic figure what they should do . . . That sounds uncannily like the psychological profile of a typical political party member, and, like Derren's dupes, party members are also fond of hand-holding and first-name-calling. Indeed, the only distinction between the audience for illusionists called Brown and the one for, say, party leaders also called Brown is that the former are unlikely to become disabused given that what they're after is purely entertainment, practised by someone they actively chose.