The BSE saga has provided the unseemly picture of a government hiding behind the white coats of science. But, reports John Durant, the passing of responsibility from politicians to scientists only compounds the problem.

This is a difficult time for agriculture and health ministers, for cattle farmers and food retailers, and for the beef-eating British public. Indeed, for any cows that may be minded to think on the question of their own future these are hardly happy days. Amid all the understandable anxiety, it might seem churlish to spare a thought for science; yet, barring the nightmare scenario of a major epidemic of beef-induced human encephalopathy (from which Heaven preserve us all), I suspect that in the long run it is science that stands to lose most from the current debacle over BSE.

The problem has been a long time in the making. Ever since BSE was first diagnosed in British livestock in the mid-1980s, scientists have been struggling simultaneously to understand this peculiar condition and to say something useful about it to the Government. The technical problems involved have been bad enough; but they have been compounded by the way in which politicians have chosen to make use of scientific expertise.

Starting out with the apparently sensible determination to obtain the best available expert advice, successive government ministers have so blurred the distinction between scientific and political judgement that at times public policy has appeared to be entirely in the hands of scientific committees.

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The problem was apparent as long ago as 1990, when BSE first hit the headlines in a big way. That was the time when the then secretary of state for agriculture, John Gummer, was publicised in the act of apparently force-feeding his daughter with a hamburger. Anxious to be seen to be doing the right thing by both producers and consumers, the Ministry of Agriculture opted for an uncomfortable and potentially unstable combination of heavy reliance on its expert advisory committee, then chaired by David Tyrell, and conventional public relations. As a result, the qualified technical judgements of Dr Tyrell and his colleagues were given great prominence in successive press statements from the Ministry of Agriculture, which were designed both to allay public fears and to avert an economic catastrophe in the meat industry.

At the height of that particular storm, when pictures of junior ministers happily clutching sausages appeared in the press alongside full-page advertisements from the Meat and Livestock Commission declaring that "British Beef is Safe", Tyrell gave a lecture in London in the course of which he expressed unease about the role of his advisory committee in the public debate. The problem, as he saw it then, was that his committee was being edged into a position of apparent responsibility for public policy-making which was more properly the job of government. The scientists could say what was and (far more significantly) what was not known about BSE; but, he suggested, they could not and should not be asked to say what ought to be done about it.

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Today, it is hard to imagine that Tyrell's successor, John Pattison, chairman of the Spongiform Encephalopathy Advisory Committee, feels any more comfortable about his and his colleagues' position in the debate. Five years on from the first big scare, the scientific picture has of course changed. Just a little more is known about BSE and its human equivalent, CJD; but both conditions continue to be very poorly understood, the economic implications of a collapse in the beef industry remain as serious as ever, and if anything the public health issues have become even more vexed. At the same time, the Government's reliance upon scientific judgement as an authoritative basis for public policy has become so great that at times it has seemed as if Seac alone is responsible for deciding what will happen next. With front-page headlines screaming the latest bans on British beef in schools, fast-food chains and the European Union, senior ministers have given interviews in which they have been so anxious to root Government policy in scientific expertise that they have refused even to paraphrase the latest Seac statements.

But listening carefully is one thing, and passing the buck is quite another. There are several important reasons why public policy on BSE should not be placed in the hands of experts alone. First, expert knowledge about BSE is incredibly limited. Still today, after ten years of research, the list of the things that are not known about BSE is far more significant than the list of things that are known. It is not obvious that the custodians of what is known are necessarily the best guides to what to do in the face of what is not known. Of course, the standard response to ignorance and uncertainty these days is risk assessment. "I do not want to hear about inconceivables or extremely smalls," rails Simon Jenkins in a typically tendentious column in The Times. "I want to know, from those more knowledgeable than I, where a steak stands alongside an oyster, a North Sea mackerel, a boiled egg and running for the bus. Is it a chance in a million of catching CJD or a chance in ten million? I am grown up. I can take it on the chin."

Would that things were quite as simple as this. Jenkins appears to be unaware of the fact that it is perfectly possible to be so ignorant that even a conventional risk assessment becomes meaningless. It is always possible to attach numbers to the holes in one's knowledge, and hence to arrive at a calculus of probable harm; but such a calculus may be little more than a compound of a series of hunches; that is, it may be simultaneously more precise and less accurate than a more straightforwardly intuitive judgement such as "inconceivable" or "extremely small". Ultimately, the pocket calculator is no substitute for proper knowledge; and even grown-ups with strong chins have to learn to cope with ignorance.

As we are all learning (probably to our collective cost), objective risk is not the only factor relevant to the determination of public policy on BSE. Of at least equal importance is that intangible but vital ingredient: public confidence. It is not clear if anyone can lay claim to genuine expertise about public confidence. It is certain that scientists have none, but it may perhaps be conceded that wise politicians possess a little. The real point, however, is that because public policy must take account of public attitudes, public expectations and probable public reactions to any particular course of action, policy-making can never be a "mere" science.

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We require public policy to be in the hands of elected politicians precisely in order that public confidence in the policy-making process can be maintained. Passing the responsibility to scientists (or any other unelected body, however well-intentioned) can only undermine confidence, not just in the political process but, I fear, in science as well. Professional scientists would be well advised to consider how best to avoid being compromised by the apparent willingness of politicians to hide behind their expert coat-tails when the cold winds blow.

John Durant is assistant director, Science Museum.