HYPOTHESES

One of my colleagues has upset me. I get on very well with all of them at University College, but John Adams in the department of geography has made me do something rather awful. He has made me re-examine my deeply held prejudices.

I am a cyclist and a motorist. I fasten my seatbelt when I drive and wear a helmet on my bike to reduce the risk of injury. I am convinced these are prudent safety measures. I have persuaded many friends to wear helmets on the grounds that transplant surgeons call those without helmets "donors on wheels". But Adams has completely undermined my confidence in these apparently sensible precautions. What he has persuasively argued, particularly in relation to seat belts, is that the evidence that they do what they are supposed to do is very suspect. This in spite of numerous claims that seat belts save many thousands of lives every year.

There is remarkable data on the years 1970 to 1978 in which countries with seat belts have had on average about 5 per cent more road accident deaths following introduction of the law. In the UK, road deaths have decreased steadily from about 7,000 a year in 1972 to just over 4,000 in 1989. There is no evidence in the trend for any effect of the seat belt law that was introduced in 1983. Moreover there is evidence that the number of cyclists and pedestrians killed actually increased by about 10 per cent.

How are these figures, which are both puzzling and shocking, to be explained? A very likely answer is risk compensation - that is, people modify their behaviour in response to what they see as changes in risks to themselves. Thus, when we "belt-up" we drive just that much more dangerously, so that the risk of an accident increases. And while we may survive better, pity the cyclists, pedestrians and back-seat passengers. As Adams wickedly suggests, one way to prevent this is to fit all cars with a spike on the steering wheel which is directed at the driver's heart.

Risk compensation also probably works with cyclists, but the data are not easy to interpret as legislation in other countries to wear helmets results in many people no longer cycling out of fear. It is claimed that twice as many lives are lost by head injuries to elderly pedestrians than to all cyclists. Should they wear helmets?

We all take risks on a very personal basis - partly in relation to rewards. Statistics should always be treated as being open to risky interpretations. That half as many children are killed every year in road accidents than in 1922 must not be taken as evidence that there is less risk when children go and play in the street today. The reduction almost certainly reflects not that the streets are less dangerous, but the care taken by parents in keeping children off the streets.

Our common-sense approach to risk is very unreliable. The risk in losing a few pounds on the lottery is very high and most people know that; but it is hard to understand why they think that if the pot is greater it is worth risking more. And who would not think it wise to bet on red in roulette if black had just come up ten times in a row? Some perceptions are quite irrational. For example I know mothers who when taking on a new au pair are very frightened that they will run off with the child if there has been even one such case reported. If I hear that a cyclist I know has been injured I am much more cautious than normal although my risk remains the same. Statistics have little impact on such risk assessments.

A major error in personal risk assessment probable relates to our inability to make sound judgements when the amount of information is limited. For example, a little while ago there was a suggestion in the US that a game called "Dungeons and Dragons" was risky since it could lead to teenage suicide. The evidence in support of this claim was that 28 teenagers who regularly played the game had committed suicide. But the average suicide rate for teenagers nationwide is about 12/100,000 so, since some three million teenagers played the game, the number of suicides associated with it could be assigned no significance whatsoever.

Also we have an intuitively poor grasp of risk statistics. How often have we heard that so-and-so's father smoked 30 a day until he was 80, without a day's illness - as if this has any relevance to the risk of smoking damaging one's health. Individual cases are not relevant. Beware the doctor who says he knows clinical trials have shown that a particular drug or treatment is ineffective or damaging, but that in his experience it works well. The only reliable way to determine the effectiveness of a treatment is by random clinical trials.

We are all too easily persuaded of our own ability to make sound judgements. I have very few colleagues who doubt their ability to select the best medical students. How one should ask, would they know?

Stopping at traffic lights the other day, I mentioned to a fellow cyclist, a stranger, how dangerous cycling was. I had just narrowly escaped being run over. "Yes," he replied, "but I bet you don't eat beef. Yet the risk of being damaged by cycling is so much greater."

"Of course," I said, "but so are the benefits. I would be really depressed if I were to give up cycling, but giving up beef for a year or so until the evidence is clearer is no trouble at all. It's a matter of risk benefit."

"Yes," he replied, "but aren't you amazed to see mothers on TV saying they wouldn't feed their darlings beef, while puffing away at life-threatening cigarettes?"

I had to agree, and we pedalled our separate ways, my helmet firmly on my head. Risk is a risky business.