Japanese blood type theory of personality

The blood type theory of personality is a popular belief in Japan that a person's ABO blood type or ketsueki-gata (血液型) is predictive of their personality, temperament, and compatibility with others, similar to the Western world's astrology. This belief has carried over to some extent in other parts of East Asia such as South Korea and Taiwan. This theory is completely dismissed by many scientists as superstition or pseudoscience.

History
The discovery of blood types in 1901 has been hailed as one of the greatest advances in medical history. Ethnic studies did show different blood group distributions across the world (e.g. Asian people having a higher percentage of Type B). This fact was used by early Nazis to further ideas of supremacy over different races. Those distortions were debunked before Nazi Germany invoked race laws like the Nuremberg Laws, where the wording "German blood" is figurative for Aryan lineage.

The theory first reached Japan in 1927 in Takeji Furukawa's paper "The Study of Temperament Through Blood Type" in the scholarly journal Psychological Research. He was a professor at Tokyo Women's Teacher's School. The idea quickly took off with the Japanese public despite his lack of credentials, and the militarist government of the time commissioned a study aimed at breeding the soldiers. The study used no more than ten to twenty people for the investigation. The breeding program therefore ended up with miserable results - most of the army selected by the project lost their lives. In another study, Furukawa compared the distribution of blood types among two different ethnic groups, the Formosans in Taiwan and the Ainu who lives in Northeast Asia, especially Hokkaidō. His motivation for the study appears to have derived from a political incident. After the Japanese occupation of Taiwan following Japan's victory over China in 1895, the inhabitants tenaciously resisted their occupiers. Insurgencies in 1930 and in 1931 killed hundreds of Japanese settlers. The purpose of Furukawa's studies was to "penetrate the essence of the racial traits of the Taiwanese, who recently revolted and behaved so cruelly". Based on the finding that 41.2% of a Taiwanese sample had type O blood, he assumed that their rebelliousness was genetically determined. The reasoning was supported by the fact that among the Ainu, whose temperament was characterized as submissive, only 23.8% had type O. In conclusion, Furukawa suggested that the Taiwanese should intermarry more with the Japanese in order to reduce the number of individuals with type O blood.

The craze faded in the 1930s as its unscientific basis became evident. It was revived in the 1970s with a book by Masahiko Nomi, a lawyer and broadcaster with no medical background. Nomi's work was largely uncontrolled and anecdotal, and the methodology of his conclusions is unclear. Because of this he has been heavily assailed by the Japanese psychological community, although his books are phenomenally popular.

Scientific research
While scientific study has shown that people of specific blood types may be more prone to certain illnesses, little research has been carried out on the correlation between blood type and personality traits. One such study, by the psychologist Raymond Cattell, examined blood samples of 323 Caucasian Australians for 17 genetic systems including 7 blood groups and 21 psychological variables. The results produced 13 significant differences at a confidence interval of p<0.05 and 11 at p<0.01, the most conclusive finding occurring in the association between the P blood group system and anxiety.

Some medical studies show variation of hormones and enzymes which other studies have associated with personality changes: Individuals have varying levels of the enzyme dopamine beta hydroxylase and catecholamine hormones according to their blood type as a result of genetic linkage of nearby genes on chromosome 9 (the locus for ABO blood group in humans). Congenital dopamine beta-hydroxylase deficiency is caused by mutation in the gene encoding dopamine beta-hydroxylase on chromosome 9q34, also the ABO locus.

The ABO blood group system and platelet monoamine oxidase (MAO) activity are known to be two genetic markers for affective disorder. In researching a connection between the two markers, one study found that the platelet MAO activity of subjects with blood type O was significantly lower than that of subjects with other blood types, criminal behaviour, alcoholism, antisocial personality and impulsivity.

Type A blood is the most common in Japan at about 40%, and the proportion of blood type A is the largest in Hawaii, while type O is the most dominant in Bolivia, and type B is most common in Central Asia.

Persons having Rh negative blood types are less common in the world population with a recent survey placing the percentage at around less than 20% of the total population, and no particular significance is attached to the Rh type.

Applications
Blood type theory is widely popular in women's magazines as a way to gauge relationship compatibility with a potential or current partner. Morning television shows feature blood type horoscopes, and similar horoscopes are published daily in newspapers. Though there is not a proven correlation between blood type and personality, it has still remained in the many matchmaking services that cater to blood type. In this way, it is similar to the use of astrological signs in the west, which is also popular in Japan. Asking about a blood type is common and it is often a surprise if a foreigner is unaware of their own blood type.

Many Japanese idols, tarento and other Japanese celebrities include a blood type statistic in their profiles, in addition to other facts as their hobbies and star sign.

It is common among anime and manga authors to mention their character's blood types, and to give their characters corresponding blood types to match their personalities. Some video game characters also have known blood types; an example being the Soul Calibur and Street Fighter series, which lists character blood types in both the manual and in-game bios. In addition, it is common for videogame series, such as Gungriffon, Tekken, Animal Crossing and Princess Maker to allow for blood type as an option in their creation modes.