Do the dim die young?

Scots psychologist Ian Deary claims that clever people live longer than thickheads. Sure, some bright people die young and many thickheads live into old age but if you measure a large bunch of people the statistics point that way. Dr Deary and his team looked at more than 2000 Scottish children given IQ tests in 1932 when they were 11 years old. He traced most of these people again in 1997 and found that those still living at age 76 had average IQs of 102 but those who had died had average IQs of 98.

Dr Deary says more evidence comes from IQ tests on large numbers of young men recruited into the Australian Army at the time of the Vietnam War and nearly a million 19-year-olds inducted into the Swedish Army. Twenty years after the tests, those who had died in the meantime had lower average IQs than those who remained alive. Several other surveys point in the same direction.

Some critics find Dr Deary's claims insulting. "So, you're saying that the thick die quick?" "Anyway", they challenge, "haven't IQ tests been discredited"? "Well, no," says Dr Deary. IQ tests have a predictive value unequalled in psychology. Hundreds of data sets since 1904 show that IQ remains almost unchanged over a lifetime, can predict educational achievement, occupational success, propensity to sickness and age of death with some confidence. It's a better predictor of life expectancy than body mass index, total cholesterol, blood pressure or blood glucose.

But why IQ should be a good predictor of life expectancy remains a mystery. Some epidemiologists suggest that intelligent people get the easy jobs, leaving the heavier, dangerous, life-threatening work to dumber people. Or, they suggest, most people with high IQs behave better. In early life people with higher IQs are more likely to have better diets, do more exercise, avoid accidents, give up smoking, do less binge drinking and put on less weight in adulthood.

But Dr Deary has checked all that stuff, and finds it does not wash. Rather, he thinks, intelligence causes the association between education, social class and health. He favours the theory that IQ tests in youth reveal a well-wired body better able to respond effectively to environmental insults.

Some supporting evidence comes from the finding that simple reaction speed - the time taken to press a button when a stimulus appears - can replace IQ test scores as an even better predictor of an earlier death. Reaction-time tasks don't demand complex reasoning, so are unlikely to improve by education. Dr Deary hopes his findings will explain the connection between childhood IQ, sickness and earlier deaths and help to tackle problems of health inequalities.

In Christchurch, David Fergusson leads a team studying the behaviour and fates of 1265 children born there in 1977. He has already shown that those with higher IQs did better at school. If his study continues long enough, it may throw light on the connection between IQ and life expectancy of Christchurch kids.

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