Recall Bias

Human memory is not a video recorder. It is a creative writing exercise.

Recall Bias happens when participants in a study remember past events differently because of their current condition.

The Mothers of Sick Children

Imagine a study on birth defects.

You ask mothers of healthy babies: "Did you drink alcohol during pregnancy?" They say: "No." They might have had a glass of wine, but they forgot. It doesn't matter.

You ask mothers of babies with defects: "Did you drink alcohol?" They rack their brains. They feel guilty. They remember every sip. "Yes, I had a glass of wine at a wedding."

The Result

The data shows a link between one glass of wine and birth defects.

But the link is fake. The healthy mothers drank too. They just didn't report it.

The Case-Control Problem

This destroys case-control studies, which rely on history.

If you have cancer, you will remember every chemical you ever touched. If you are healthy, you won't.

It looks like chemicals cause cancer. Really, cancer causes memory.

The Diagnosis

Prospective studies are better. Follow people forward in time. Ask them what they are doing now.

Do not trust the past. It changes every time we look at it.