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The availability error is responsible for a large number of irrational judgements in real life. Do you regard fairgrounds as dangerous? Certainly most people do. There is the Big Wheel with its carriages turning precariously in the air, the roller-coaster with its frightening bends and changes of speed, the Octopus subjecting you to massive centrifugal force while rocking you violently to and fro, and many other machines moving in a variety of contortions. Yet most people (including myself until I learned the facts) are wrong. According to a report of the British Health and Safety Committee, if you cycle on main roads for an hour you are forty times as likely to be killed as if you spend the same length of time riding fairground machines, and you are seven times as safe on them as when driving a car.

Fairground accidents are of course dramatic and well publicised: they are ‘available’. It is also known that people grossly overestimate the chances of dying a violent death, for example, in an air crash or in street riots. In one study10 it was found that people think they are twice as likely to die of an accident as from a stroke; in fact forty times as many people die of strokes as from accidents. The reason for this false belief is that, although most people die in their beds, air crashes and violence are constantly reported in the media and are highly dramatic: they are therefore ‘available’.

Not only do people hold irrational beliefs about the frequency of violence, but they are driven by their beliefs to wholly irrational actions. In 1986 the number of Americans visiting Europe as tourists showed a sharp drop. They had been scared away as a result of a few much-publicised plane hijackings and possibly by the American bombing of Libya. But they had failed to take account of the less-publicised prevalence of violent crime in the US: in fact Americans living in cities put themselves at greater risk of meeting a violent death by staying at home. Exactly the same irrational refusal to fly occurred during the Gulf War.

Sometimes the availability error does seemingly drive people to act rationally. In California the number of insurance policies taken out against earthquakes increases steeply after a quake, but then drops gradually until the next one. But even this behaviour is not really rational since whether you take out insurance should depend not on when the last earthquake was but on the probability of earthquakes in the future. Again, after Mrs Ford and Mrs Rockefeller developed breast cancer, large numbers of American women rushed to the hospitals to have diagnostic tests. They had hitherto been completely unmoved by government warnings that they should have tests at regular intervals.

Unusually, you can also find stories about this in the field of search engine optimisation. An associate of mine from SEO Leeds told me a story that is familiar to everyone that drives. A driver who has just passed an accident almost invariably slows down. The accident makes available the possibility that he too will crash: unfortunately the effect wears off within a few miles. The sight of a police car has the same result.