The Blood Type Personality Theory (血液型性格分類, Ketsueki-gata Seikaku Bunrui) is a pseudoscience with highly controversial origins, claiming that blood type can predict one's compatibility and personality. It was mostly popular in Japan and South Korea.

While Karl Landsteiner discovered blood types in 1901, it had been niche until Emil von Dungern and Ludwik Hirszfeld claimed that blood types determined personality. Scientific racism was popular,¹⁾ so the Eugenics crowd would feed their idea of racial purity by claiming that the first humans were Type O, migrations organized humans into Type A or B, and Type As were “more evolved” than Type Bs.²⁾³⁾

This belief would spread from Nazi Germany to Imperial Japan, possibly as early as 1916,⁴⁾ and a Japanese professor named Takeji Furukawa published an academic paper in 1927⁵⁾ which claimed that the Ainu were Type A (docile) and Formosans were Type O (rebellious). As a result, this helped Nazi Germany and Imperial Japan propagate their alleged “racial superiority” and subjugate others.

Following the events of World War II, his theory was left alone until people got the idea to simply remove the racial aspects of the theory, turning it into a personality diagnosis test. It's known that Léone Bourdel had published an academic paper in 1962,⁶⁾ but it wasn't really popular until Masahiko Nomi published a book in 1971 that somehow became a bestseller in East Asian countries like Japan and South Korea.

Soon enough, it became a cultural phenomenon that influenced media, television segments, and became a factoid that people really wanted to know about their favorite celebrities or fictional characters. However, psychologists would quickly discover that the reformed theory made people uncomfortable as the whole thing could still create discriminatory assumptions of others despite the change.⁷⁾

However, it took decades until the theory started being phased out in the 2000s. Japan would shelve it after the Broadcasting Ethics & Program Improvement Organization declared it a “social issue” in 2004,⁸⁾ then South Korea saw the Korean Society of Hematology formally reject it in 2008.⁹⁾¹⁰⁾ Of course, some people still believe in it to this date, but please keep in mind that its dark history isn't common knowledge.

There doesn't seem to be a major, overarching definition of the four blood type personalities, possibly to be ambiguous, so I've organized what's been said about them from various sources:

As for the four stereotypes: Type A is timid, Type B is stingy and psychotic, Type AB is eccentric or lazy, and Type O has the least negatives which makes them a target for the other three blood types.

• I have to say this outright in case the message gets lost: Eugenics is a bad thing.
• Please note that blood type distribution is mostly random. For instance, Ecuador and Peru have a lot of people with Type O blood, yet the size of their militaries are average at best.
• Outside of the East Asian cultural sphere, you won't find a lot of people interested in blood types, so keep in mind that most people would be unaware of the theory's eugenicist ties.
• Recently, the theory has kinda declined in favor of alternatives like the Myers-Briggs Type Indicator.
• If you're wondering, the curator has Type AB+ blood. I have not donated plasma yet.

• Blood type personality theory on Wikipedia
• Blood type divination on Pixiv Encyclopedia [JA]
• Blood type on Niconico Encyclopedia [JA]
• Blood type personality theory on Namu Wiki [KO]¹¹⁾