Three words we should like to hear: 'I don't know'

In ignoring uncertainty, evidence-based policy distorts the expert voice and misleads the public, argues Jack Stilgoe

BSE, the disease that changed the way the UK thinks about its experts, has its 21st birthday this year. After mad cow disease ravaged the country, the Government could no longer pretend that experts had all the answers. They had to take seriously uncertainties such as those that John Gummer, then Agriculture Minister, famously swept aside with the help of his daughter and a burger. But the onus of that episode also falls on experts. They must learn to speak up about what they do not know.

For although the Government claims to recognise the growing importance of uncertainty, it has sought comfort in the mantra of "evidence-based policy". It appears a straightforward, good idea - basing policy on evidence seems sensible. And there are plenty of policymakers who quietly think that, in these irrational times, technocracy is a way of saving policy from public opinion, media manipulation and political whim.

But "evidence-based" language mutes the voice of the expert. It accentuates the positive - what we know - and it often becomes a way of justifying decisions rather than making them. The temptation for civil servants and politicians is to look past the uncertain expert to the comfortable certainties. But experience tells us that it is uncertainty - the absence of incontrovertible evidence - that comes to define our problems. New issues raise new questions; and while policy looks forwards, evidence looks back.

With BSE, Gummer was not the only one to claim false certainty. In 1990, the Meat and Livestock Commission declared: "Eating British beef is completely safe." In private, the message was very different. An official from the now-defunct Ministry of Agriculture, Fisheries and Food told his boss: "We do not know where this disease came from; we do not know how it is spread; and we do not know whether it can be passed to humans. The last point seems to me the most worrying aspect of the problem."

Erik Millstone, professor of science and technology policy at Sussex University, described BSE as "the most serious failure of UK public policy since the Suez invasion of 1956". Yet Tony Blair's speech on science in November demonstrated that lessons have still not been learnt. Blair talked about the "brilliant light of science" as though it was an unproblematic source of authority.

The army of external experts that advises the Civil Service on policy questions, mostly drawn from universities, has mushroomed. The US academic Sheila Jasanoff has called them a "fifth branch" of government. Their discussions are a crucial element of 21st-century policymaking. They are a resource, we are told - "on tap, not on top".

Since BSE, there have been great strides towards openness in expert advice.

The smoke-filled rooms where the men in Whitehall once spoke to the men in white coats have had their doors flung open. Committees have started to meet in public, and newer bodies such as the Food Standards Agency, created from the post-BSE rubble, run their business with a presumption of transparency.

But recent flashpoints tell us that openness is not enough. With the combined measles, mumps and rubella vaccine, a sceptical public were all too willing to question the voice of government experts. The evidence did not speak for itself. The controversy over the triple jab was about more than knowledge. It was about credibility and the space that had been allocated for public debate. While the experts and the Government were noisily talking about "the facts", parents were quietly asking about uncertainty.

A collaboration between the think-tank Demos and academics from Liverpool University looked at how expert advice within the Department for Environment, Food and Rural Affairs works now. How do expert committees deal not only with the need to open up, but also take seriously scientific uncertainties as they apply to everyday life? We concluded that we needed to refresh and enrich the idea of expertise. The language of evidence-based policy does the public few favours - and it disempowers experts.

The physicist Werner Heisenberg defined an expert as "someone who knows some of the worst mistakes that can be made in their subject and who manages to avoid them". Expertise is not just about evidence. It is also about wisdom, reminding people in power what we still might not know, in addition to what we think we know, and cautioning against complacency. For the Government to get the most from its experts, it should encourage them to be brave about uncertainty.

Jack Stilgoe is senior researcher at think-tank Demos. He is co-author, with Alan Irwin and Kevin Jones, of The Received Wisdom - Opening up Expert Advice , published by Demos, £10.00.