A popular belief in Japan is that a person's ABO blood type is predictive of their personality, character, and compatibility with others.

A popular belief in Japan is that a person's ABO blood type is predictive of their personality, character, and compatibility with others. This belief is also widespread in South Korea and Taiwan. Deriving from ideas of historical scientific racism, the theory reached Japan in a 1927 psychologist's report, and the militarist government of the time commissioned a study aimed at breeding better soldiers.The fad faded in the 1930s due to its lack of scientific basis and ultimately the discovery of DNA in the following decades which it later became clear had a vastly more complex and important role in both heredity generally and personality specifically. No evidence has been found to support the theory by scientists, but it was revived in the 1970s by Masahiko Nomi, a broadcaster with a background in law who had no scientific or medical background. Despite these facts, the myth still persists widely in Japanese and South Korean popular culture.