Mistaking the Cause - A Common Thinking Error

Did you know that women who wear lipstick are much more likely to suffer from breast cancer than women who do not wear lipstick? It is true. Does this mean that the wearing of lipstick should be banned on health grounds? No it does not. To do so would be to commit a common thinking error - mistaking the cause. We tend to think that because two things are correlated then one must cause the other; in this case assuming that wearing lipstick leads to breast cancer. The explanation will come later.

In the 1930s an eminent medical journal published a report showing that cancer was much more frequent in New England, Minnesota and Wisconsin than it was in the Southern States. It was also more common in Switzerland and England than in Japan. It was known that people in New England, Minnesota and Wisconsin drank more milk than people in the Southern States. Also people in Switzerland and England drank much more milk than people in Japan. There was a strong statistical correlation between drinking milk and dying from cancer. The article concluded that drinking milk led to cancer. Many people saw the correlation as proof but it was not. A deeper investigation showed that the milk-drinking regions were more prosperous and that people in general lived longer there than in the regions where less milk was drunk. Cancer is a disease that mainly affects older people so it is not surprising that it has a higher incidence in places where people live longer.

Similarly it has been shown that people who get University degrees earn significantly more over their lifetimes than people who do not have degrees. The conclusion that is drawn is that the degree directly leads to higher earnings. This conclusion has been used to justify loans rather than grants for students and even to proposals for special 'graduate taxes.' But is the conclusion justified? The people who get into Universities have shown themselves to be intelligent, diligent, articulate, and good at exams, at solving problems and at remembering things. These are all qualities that would help them to succeed in the modern workplace whether they had a degree or not. It is not the degree itself that marks them out for success and higher earnings, it is the qualities they showed to get selected for the degree course.

We need to constantly guard against wrongly assigning causes. Consider what other possible causes might lead to the same outcome. Could a common factor lead to both cause and effect - e.g. to the degree and the higher earnings? Could an entirely different factor be the cause - e.g. cancer resulting from longer life expectancy and not from drinking milk?

Now let's return to the lipstick and breast cancer. Across the world as a whole women who wear lipstick are more likely to suffer breast cancer - but inside the USA or Canada or Germany they are not. Breast cancer is much more common in the Western World than in Asia or Africa. Women in the West are much more likely to wear lipstick than women in Asia or Africa so that is why there is a strong correlation between wearing lipstick and suffering from breast cancer but there is absolutely no evidence that one leads to the other. Beware of mistaking the cause whenever there is a correlation.

This article is based on an extract from the book How to be a Brilliant Thinker by Paul Sloane published by Kogan Page.