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.

The ABO blood types were first discovered by Karl Landsteiner in 1901, then further research was done by Emil von Dungern and Ludwik Hirszfeld on heritability. During this time period, scientific racism was popular,¹⁾ so the Eugenicists obsessed with racial purity attempted to warp the research to fit their future slogans of blood and soil, asserting that certain types were “move evolved” than others.²⁾³⁾

In 1916, Kimata Hara, a Japanese doctor that studied under Durgen, would bring their research to Imperial Japan and briefly touched on linking blood types to personality.⁴⁾ This was taken seriously by the military around 1926,⁵⁾ eventually leading up to Takeji Furukawa publishing his paper in 1927 that formally linked blood types to personalities, enabling their military to discriminate by blood type.

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 deemed 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 want to be clear that, in case the message somehow became lost, Eugenics is a bad thing.
• The truth about blood types is that blood type distribution is essentially random. You can see that Ecuador and Peru has a lot of Type O blood, but it doesn't affect their military personnel.
• Recently, the theory has kinda declined in favor of alternatives like the Myers-Briggs Type Indicator.
• For those curious, 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⁾