Stop and search the whole gang

Suppose it is known that England supporters are more likely to behave violently at international football games than supporters of other national teams. If other countries impose a ban on English people watching football games, does this treat the law-abiding majority of England supporters unjustly? Suppose that in a civil lawsuit it is established that the plaintiff was injured by an unidentified taxi, and 75 per cent of the taxis in the area are owned by the defendant. Given that the standard of proof in civil cases is "the balance of probabilities", should that entitle the plaintiff to damages? If statistics show that possession of illegal drugs is more common among young black males than in the general population, is it legitimate for the police to use a person's youth, blackness and maleness as reasons for stopping and searching him?

Frederick Schauer discusses many issues of this kind and tries to find general principles to govern how we treat them. His thesis is that "decisions by categories and by generalisations" are defensible, and that people who are disadvantaged by the categories they fall into have no grounds for complaint.

He marshals some strong arguments, mostly pragmatic rather than principled.

There is no way of avoiding generalisations in decision-making and legislation because lines have to be drawn somewhere. No evidence is absolutely reliable; thus, all evidence derives its significance from statistical generalisations about reliability. If each case is to be treated on its merits, someone has to be entrusted to judge those merits; as errors are unavoidable, the overall result may be more arbitrary than a procedure based on clearly defined categories.

Schauer writes clearly, with the slightly studied informality one might expect of an attorney's address to a jury or an introductory lecture from a law professor (which he is). The book covers many issues and addresses a wide readership, but Schauer has more interest in, and a surer grasp of, the legal aspects of probability and stereotyping than the economic, political or statistical. In discussing the use of generalisations in stop-and-search procedures, he fails to consider something that would occur immediately to a game theorist - that genuinely random selection is a defence against strategic thinking by opponents. Thus, he recommends that airport security staff, when choosing which passengers to search, should focus on categories that have been disproportionately represented among previously identified terrorists - young males of Middle Eastern appearance with one-way tickets and no check-in baggage. But how difficult would it be for al-Qaeda to respond to this policy by choosing mature, light-complexioned female operatives and instructing them to buy return tickets and carry large bags?

Schauer also misses the significance of Bayes's theorem in interpreting statistical evidence. Suppose a defendant's DNA matches a sample taken at the crime scene, and it is known that the probability that a randomly selected person has matching DNA is one in a million. Does that establish the defendant's guilt beyond reasonable doubt? Not if he was arrested only because he was the first person to be found in a search for someone with matching DNA.

Schauer's book might have been more interesting if he had been braver.

Whenever he seems on the point of reaching a particularly provocative conclusion, he draws back. If decision-making by generalisation would disadvantage a group that is already discriminated against (women, African-Americans or Jews, but not English football fans) or if the harm imposed on any individual is "grave", Schauer deems it unacceptable. While he endorses the use of statistical evidence in civil cases, he does not extend the argument to criminal cases, even though it seems equally applicable to them.

Curiously, one of Schauer's most controversial claims is orthogonal to his main argument. Endorsing Charles de Gaulle's remark about the impossibility of governing a country with 246 varieties of cheese as if it had been meant literally as a hypothesis in political science, Schauer takes the promotion of uniformity for its own sake to be a proper objective of national law. He says the suppression of diversity - even superfluous types of cheese - is a legitimate policy instrument for building a sense of common citizenship in the European Union. Is this an American constitutional lawyer's perspective on nationhood? Or just a sign that Schauer lacks a sense of irony?

Robert Sugden is professor of economics, University of East Anglia.