What Your Blood Type Says About You: Nothing, But Let's Talk About Why That's Not the Point

In Japan, it is entirely normal to be asked your blood type on a first date. Dating profiles list it next to height and occupation. Some employers have, at various points, sorted job candidates by it, on the theory that Type O makes a better leader and Type A makes a better accountant. There is a specific term for this: ketsuekigata, blood type personality theory. It has no biological mechanism, no supporting evidence at scale, and roughly fifty years of cultural staying power [1].

Where it started

The theory traces to 1927, when Japanese psychologist Takeji Furukawa published a paper linking ABO type to temperament, describing Type A individuals as gentle and anxious, Type B as cheerful and self-centered, and so on [2,3]. Furukawa's data were thin, and his own academic community pushed back at the time [3]. The theory should have died there.

It got a second wind in 1971, when journalist Masahiko Nomi, who had no medical or psychological training, published a book reviving and expanding it, adding claims that blood type predicted not just personality but disease susceptibility [1,3]. Nomi's work drew the same methodological criticism Furukawa's had. It didn't matter. By the 1980s, blood type personality theory was a fixture of Japanese popular culture, and it has stayed one since, with related belief systems taking hold in South Korea and Taiwan [4].

What the evidence shows

Unlike the blood type diet, this one doesn't even offer a mechanism worth debunking. Nobody claims to know how an antigen expressed on the surface of a red blood cell would shape whether you're anxious or gregarious. There's no lectin story here, no agglutination metaphor. It's association without a proposed pathway, which makes it a cleaner test case for pattern-seeking on its own terms.

And the pattern isn't there. Large-scale surveys using validated personality instruments, run in both Japan and the United States, have found no meaningful correlation between ABO type and standard personality traits [1]. This isn't a case of a single small study failing to find an effect. It's the specific finding, repeated across populations that supposedly believe in the theory most strongly, that belief and reality have parted ways.

Why it took hold where it did

The interesting question isn't whether ketsuekigata is true. It's why a country with excellent scientific infrastructure and health literacy adopted a folk taxonomy this thoroughly, while most of the world shrugged. Part of the answer is historical: Furukawa's early work was entangled with period ideas about national and ethnic character, giving the theory an early institutional foothold it didn't earn on data [3].

Part of it is more mundane. Once a categorization system is culturally legible, media reinforces it, and confirmation bias does the rest. If you're told Type B people are unpredictable, you notice the unpredictable things your Type B friend does and forget the rest. The Barnum effect, the same mechanism that makes horoscopes feel accurate, doesn't need a real correlation to feel like one.

The diet's cousin

If you read last week's post on the blood type diet, you'll notice the throughline: ABO type is one of the only pieces of biology most people can name about themselves, and that legibility makes it an appealing anchor for a story, regardless of whether the story holds up. The diet claims a mechanism and fails on evidence. The personality theory doesn't even bother with a mechanism, and fails just as completely. Both persist anyway.

Maybe that's the actual finding here: mechanism was never the load-bearing part of either belief. The load-bearing part is wanting a story.

References

1. Nawata K. No relationship between blood type and personality: evidence from large-scale surveys in Japan and the US. Jpn Psychol Res. 2014;56(2):163-173.

2. Furukawa T. A study of temperament and blood-groups. Psychol Res. 1927;2:612-634.

3. Cha J. Blood type and personality. Hektoen International. 2020.

4. Relationship between ABO blood type and personality in a large-scale survey. Int J Psychol Behav Sci. 2021;11(1).