Cobra Effect · Evidence in medicine
The doctors who tracked smoking
How two doctors showed smoking causes lung cancer without a single experiment.
7 cards, read aloud in 3:08, with a test and sources.
After the Second World War, deaths from lung cancer in Britain were climbing fast.
Nobody knew why. Popular suspects included fumes from cars, smoke from coal fires and the tar laid on roads. Smoking was so ordinary that few people thought to blame it. Richard Doll, a young doctor who smoked himself, set out to find the cause with the statistician Austin Bradford Hill.
In the late 1940s, they questioned lung cancer patients in London hospitals.
For each one, they also questioned a similar patient of the same age and sex who did not have lung cancer. Everyone was asked the same questions about their lives and habits, including smoking. Then they compared the two groups. Whatever was more common among the cancer patients was a clue.
Smoking stood out, and the more people smoked, the higher their risk.
Of the men with lung cancer, almost every one was a smoker. Heavy smokers were far more likely to have the disease than people who had never smoked. Doll and Hill published their findings in the British Medical Journal in 1950. Doll gave up smoking while the study was under way.
But two things turning up together does not prove that one causes the other.
People who are ill might remember their past habits differently from people who are well. The famous statistician Ronald Fisher, himself a pipe smoker, suggested an inherited tendency might lead people both to smoke and to get lung cancer. Doll and Hill wanted a test that looked forward instead of back.
In 1951 they wrote to the doctors of Britain and asked how much they smoked.
More than forty thousand doctors sent back answers. Now their habits were on record before anyone fell ill. Then Doll and Hill waited, and kept track of every death. Their first report, in 1954, showed the same pattern. Doctors who smoked were dying of lung cancer far more often.
The doctors were followed for fifty years.
In 2004, the final results showed that smokers who kept smoking died about ten years earlier, on average. Smoking raised the risk of many diseases besides lung cancer, including heart disease. Those who stopped at around thirty avoided almost all of the lost years. Doll himself lived to ninety two.
So when two things go together, ask which came first.
Doll and Hill could not make people smoke to see what happened. Instead they compared groups with care, checked that more smoking meant more risk, and recorded habits before illness struck. Each check ruled out another way the link could have been a mistake. When an experiment is impossible, careful watching can still find the truth.
Sources
- British Doctors Study, University of Oxford, Clinical Trial Service Unit. The fifty year study of smoking and death among British doctors.
- 50-year tobacco study shows smokers lose a decade of life, Cancer Research UK. What the final results showed in 2004, including the benefit of stopping.
- The British Doctors’ Study (1951–2001), The Embryo Project Encyclopedia, Arizona State University. How the study was designed and what it found over fifty years.
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