Lately I’ve been on an anime kick, and it’s heightened my awareness of the idea that blood type affects your personality. More precisely, it’s made me aware of my own prejudices and socialization.

Some background: as noted in the above link, many people (especially in Japan) believe that your blood type has an effect on your behavior. The Wikipedia article compares this to the Western belief in astrology. And the first time I heard about the blood type belief, I immediately dismissed it as bullshit.

Now here’s my question: did you feel the same way? Did you say, “That’s a ridiculous superstition”? If so, why?

After all, blood types like A-positive and O-negative are based on substances in your blood…and we know that substances in our blood can affect mood and behavior. (See alcohol and other drugs.) So why would we immediately think it’s nonsense that a particular blood type might slant our personality in a specific direction? The chemicals involved in blood type pass through our brain and all other tissues. Why is it ridiculous to think they might have an effect? The possible chain of cause-and-effect is obvious (unlike, say, astrology where a cause-and-effect mechanism is difficult to imagine).

There’s only one reason I can see why I immediately rejected the idea: because I wasn’t brought up with it. Therefore it felt weird to me. And perhaps I have an unconscious bias against unfamiliar notions from different cultures.

Now as it turns out, there’s no evidence to support the blood type personality theory. If the theory were true, it would be relatively easy to detect: just give personality tests to a bunch of people and see if there’s any correlation between blood type and personality scores. No such correlation has ever been found.

So the model isn’t true. But I still contend that the idea isn’t ridiculous, it’s just incorrect. I shouldn’t have rejected it until I saw actual data. The fact that I did say, “That’s bullshit,” makes me wonder how many other ideas I’ve rejected reflexively: not because they had to be wrong, but purely from my socialization.

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