Anecdotal evidence?

sneezingThe plural of anecdote is not evidence. Anecdotes are unreliable for all sorts of reasons as discussed in The Role of Anecdotes in Science-Based Medicine.

However beliefs about cause and effect are rife in our society. Many of the health-related ones became planted in the minds of our parents and grandparents at a time when medicine was largely ineffectual and rigorous testing of drugs was unknown. Here are just a few:

  • Starve a fever; feed a cold
  • Cracking your knuckles will cause arthritis or big knuckles
  • Gum that is swallowed takes seven years to digest
  • Wear plenty of warm clothes in winter or you’ll catch a cold
  • If you have a cold don’t drink milk as it makes you phlegmy
  • If you go to bed with wet hair you’ll get sick
  • Food quickly picked up from the floor is safe to eat
  • Don’t go swimming right after you eat or you’ll get a cramp
  • Wear a hat because you lose most of your body heat through your head

My favourite is one that is prevalent in the Philippines: if it’s raining, don’t get your head wet or you’ll catch cold. Despite defying common logic, it is almost universally regarded as sound advice throughout the archipelago. Note that it’s OK to get your head wet swimming or showering, but get a few drops of rain on your head and you’re in big trouble.

It’s easy to imagine how these myths gained credibility. Conversations might have gone like this:

Person 1: “My son went out in the rain yesterday and got his head wet. Today he’s in bed with a cold.”
Person 2: “That’s interesting. I recall last year my husband got his head wet when we got caught in a sudden downpour. Just a day or two later he had the flu.”
Person 3: “Yes, I’ve always thought it was a good idea to keep your head covered when it was raining.”

Now if we were able to look at every case of a cold, and ask whether a wet head preceded it, we would not find any statistical link. However memory is selective, any before you know it a myth is born. The myth is reinforced every time someone gets a cold within hours or days of getting their head wet, but the myth is not weakened every time someone gets their head wet and does not catch a cold.

In fact scientists have conducted studies where they placed viruses in the noses of volunteers, and then compared the results of patients kept warm and dry versus patients exposed to cold, wet conditions. There was no difference in the outcome.


The Role of Anecdotes in Science-Based Medicine
Acute cooling of the body surface and the common cold.
Does being cold give you a cold?
Transmission of the Common Cold to Volunteers Under Controlled Conditions
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