Does Smoking Prevent Lung Cancer?

In 1964 the US Surgeon General released a famous report showing that smokers were much more likely to die of lung cancer.

This report came under attack from statisticians all over the world, since it had reported a correlation and inferred a causation.

The legendary Hans Eysenck, who coincidentally at the time was receiving a very large amount of research money from the tobacco companies, proposed that there might be some sort of hidden factor which caused both smoking and lung cancer.

This argument was greeted with the derision it deserved, and served mainly to discredit Eysenck in the eyes of the public, and cement the reputation of the tobacco companies as weaselly amoral murderers ( I speak as a happy customer of many years. )

But ridicule and cries of "come off it" shouldn't be enough to demolish an argument. I believe that they laughed at Christopher Columbus, when he said the world was round[1]

Can we make Eysenck's argument look good?

Well, suppose that there were a vitamin deficiency disease, associated with the currently undiscovered Vitamin N.

Primarily it affects the lungs. If you're deficient in this vitamin, your lungs don't work so well, and you have a tendency to develop lung cancers in later life.

As it happens, one of the hundreds of ingredients in tobacco smoke is chemically close to Vitamin N. When people with the deficiency smoke, they find that they enjoy it! A bit like if you haven't eaten fruit for a while, Vitamin C tablets taste extra-nice.

If this was the whole story, then we'd expect to see that non-smokers died of lung cancer much more often than smokers. Which we obviously don't see.

But what's a vitamin and what isn't is genetically determined. Almost all animals can make their own vitamin C. For some reason, the fruit-eating great apes, who have a lot of it in their diet anyway, have lost the ability. Humans are a type of great ape. So we don't have the ability either, but we've moved away from the diet of fresh fruit without regaining the ability.

Without a source of vitamin C, we die of scurvy. Luckily we can get it from fruit or fresh meat. If we start trying to live on preserved foods, without taking precautions, we die.

But there must have been a point in the history of the great apes where there were some apes who had the ability to make vitamin C, and some who didn't. It just didn't matter very much when they lived on fruit.

So suppose there's a gene for making vitamin N. Some people have it, and some don't.

Let's say that half of people have the gene.

They don't tend to take up smoking because it doesn't cure any lack they have, so they never 'get the taste'. Suppose 20% of them smoke. And they're unlikely to get cancer. Say 20% chance. But if they smoke, it does protect them a bit, so they only have a 5% chance.

But the other half of the people don't have the gene. Unaided by tobacco, they'll have a full 90% chance of spontaneously contracting lung cancer. If they're lucky enough to try smoking at school, then they'll find it very enjoyable, will likely continue to smoke all their lives (80% chance) and it will reduce their death rate to 80% (it's not a very good substitute for vitamin N, but it does some good).

What will the numbers look like in this world? ( Which I find all too plausible. Nature is always playing nasty tricks like this. )

Consider 200 people:

100 of them have the gene, 20 of those smoke, and 1 gets cancer. 80 don't smoke, and 16 get cancer.

100 of them don't have the gene, 80 of them smoke, of whom 64 get cancer. 20 don't, and 18 of them get it.

So if we don't know about vitamin N or the gene for it, and we do a cancer study, then

100 smokers, 65 cancers
100 non-smokers, 34 cancers

and the US Surgeon General issues his report, and up goes a huge public outcry against tobacco, and a few statisticians point out that correlation is not causation, and are ignored.


Exercises for the Reader:

This argument is utter bollocks from start to finish. But why?

Next time someone publishes a study saying that vitamin supplements shorten life expectancy, what will you believe?

The standard answer to this sort of question is a controlled randomized trial. Consider how the idea of an experiment where you ask people to take up smoking is likely to go down at the ethics committee meeting that decides whether you're allowed to do it.

No, you can't do it on dogs. Dogs don't get scurvy either. They can make their own vitamin C. That's why they can live on tinned meat and dry biscuits.


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1/ Actually sailors have always known the world was round. They laughed at Christopher Columbus for badly miscalculating the size of the thing and thinking he could sail to India the long way round. The error should have killed him and his men, but they got lucky and found the Caribbean Islands instead. I believe that Colombus died thinking that he had found a nice short route to India. Jamaicans today immortalize this mistake by calling themselves West Indians.