author : Robyn M. Dawes

This book has confirmed my prejudices about (a certain class of) psychiatrists: they are little better than witch doctors.

Anyone who has read even a little bit about probability knows that in general people are very bad at getting the odds right, and make common mistakes. One such common mistake is to assume that if most A is B, then most B is A. For example, to assume that if most people with lung cancer are smokers, then most smokers will get lung cancer. It's easy to highlight the fallacious reasoning with a clearly silly example: most schizophrenics brush their teeth, therefore most people who brush their teeth are schizophrenic.

Dawes goes into some detail explaining this error, and others, which lead to irrational reasoning about the world. He points out that in order to discover the cause of something like an air accident, say, you have to compare it with cases when the accident didn't happen. Just because a symptom is common to most cases of interest doesn't mean it is a good symptom to use to predict them: the symptom must not be common in the other cases, too. For example, just because pilots are often tired before a crash doesn't give you any insight into whether this is important, unless you know they are often not tired before a safe landing. Otherwise we're back to schizophrenics and their teeth (or, more realistically, their reaction to ink-blots). However, most accident investigations don't look at safe landings, many psychiatric diagnostic test are not also evaluated against sane people, and so on.

Some of the more bizarre forms of irrationality lead certain people to argue that both a proposition and its negation are supporting evidence for an hypothesis (such as admitting, or denying, having committed a certain act both counting as evidence of having committed the act). His arguments are quite careful, and he goes into the (very simple) probabilistic arguments in some detail. I feel that occasionally his language gets a little too "academic", and the exposition would have benefitted from more diagrams and examples. But all the information is present.

If exposing the kinds of poor reasoning, and showing how not to fall into the traps, was all he did, that would be well enough, and the book worth reading. But he also goes on to talk about how such irrational reasoning has been used by a portion of the psychiatric community (some of whom seem not to have any qualifications in anything at all), to push their agendas on "recovered memory syndrome" and the like, and the very terrible consequences it has had for many families caught up in the hysteria. (He wryly points out that precisely the same reasoning is used to prove the existence of large satanic cults, and alien abductions.) This is heart-rending stuff, and the world needs more people like Dawes to fly the flag for sanity, and good mathematics.

• Another good book on this subject is Piattelli-Palmarini's Inevitable Illusions.