Thought of the Day 06.09.10

“Lunacy” is a word that comes from the idea that the full moon (luna) causes people to behave strangely. Unfortunately, it turns out not to be a real phenomenon. So why might millions of people (including me until just today) believe it is? Well, once you have the idea in your head (from any source) that the full moon makes people weird, you become prone to a thing called “Confirmation bias.” Here’s how it works.

When you are out and about on a day when you see a lot of strange stuff, if you also notice there’s a full moon, you think to yourself, “Aha. That’s why.” But if you happen to notice strange stuff on a random day without a full moon, you think, “Well, it’s just a weird day.”

On the other hand, if you happen to notice that it’s a full moon on a day when nothing really strange happens, you never think to yourself, “Gosh, I guess that means there’s no effect.” You simply don’t notice or think about it, both because the absence of weirdness doesn’t fit your bias and because it’s notoriously difficult to notice a lack of something. There’s just nothing memorable about nothing happening.