The idea that conspiracy theories bubble up from below, like bad smells from a dank drain, and infect previously level-headed intellectuals is absurd, Aaronovitch argues. Paranoid ideas often begin among the educated and move on to dominate the supposedly ignorant masses, as the history of communism proves. Nor are conspiracies always dangerous. The Vatican may be exasperated by the phoney histories, which culminated in The Da Vinci Code, but Dan Brown's readers are generally harmless and occasionally regular churchgoers. Aaronovitch shows that the paranoid do not always believe in a gigantic conspiracy as the motive force of history. They can nod along with wholly irrational ideas about the death of David Kelly or Princess Diana, without thinking that scheming Jesuits, Jews or multinational corporations secretly control the planet. When people think "that the British Royal Family executes its more awkward members, that Robert Kennedy had a poisoned suppository inserted into Marilyn Monroe before being assassinated by a Manchurian Candidate, or that the Roman Catholic Church has for two millennia been suppressing the truth about the secret bloodline of Christ" they are driven as much by a psychological need as a totalitarian ideology. Their delusions impose a comforting coherence on the mess of life and randomness of death. By "suggesting that there is an explanation, that human agencies are powerful and that there is order rather than chaos," the conspiracy theorist places himself in a sophisticated elite that discerns connections where the multitude sees only happenstance.

As a conclusion, Aaronovitich's notion that the paranoid, like the poor, will always be with us is both convincing and unsatisfactory. His dissection of psychological states fails to say when we need to stop analysing and start worrying. More pertinently, it fails to say whether we need to start worrying now.

In the past century, there have been two lethal upsurges in paranoid fantasies. Both produced their damage in global recessions. Everyone learns the story of the 1930s at school. Francis Wheen makes a good case for including the disasters of the 1970s on the curriculum.