Japanese blood type theory of personality

It is popularly believed in Japan that a person's ABO blood type or ketsu eki gata is predictive of their personality, character, and compatibility with others, similar to the Western world's Astrology. This belief has carried over to certain extent in other parts of East Asia such as South Korea and Taiwan. In Japan, asking someone their blood type is considered as normal as asking their astrological sign in other countries. This theory is dismissed by many scientists as superstition or pseudoscience; much is still unknown about blood types.

History
The discovery of blood types in 1901 has been hailed as one of the greatest advances in medical history, but the breakthrough was then used by the Nazis to further their eugenics program, and claim the superiority of Germans -- mostly types A and O -- over Jews, Asians and others with a larger proportion of type B blood.

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 craze faded in the 1930s as its unscientific basis became evident. It was revived in the 1970s with a book by Masahiko Nomi, an advocate and broadcaster with no medical background.

Scientific support
While scientific study has shown that people of specific blood types may be more prone to certain illnesses, no medical studies have been found directly correlating blood type with all of the personality traits associated with them. Some studies do however 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 blood type A and with the other blood types. MAO polymorphism is associated with aggression in children, criminal behaviour, alcoholism, antisocial personality, and impulsivity.

Character trait associations
According to Nomi's theory, those with type A blood tend to be reserved, punctual, and law-abiding, while type Os tend to be more outgoing, passionate, and individualistic. Type Bs are said to be carefree and cheerful, while ABs are said to be serious and solitary by nature.

Type A blood is the most common in Japan and Germany while type O is most prevalent in the United States. 60 percent of Japanese Prime Ministers have been type O. Type Bs also said to make great cooks and restaurant guides featuring Type B chefs are popular products in Japan.

Persons having Rh negative blood types are extremely rare in the Japanese population with a recent survey placing the percentage at around less than 1% 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. The lack of a proven correlation between blood type and personality has not slowed the many matchmaking services that cater to blood type. Asking about a blood type is common and it is often a surprise if a foreigner is unaware of their own blood type.

Blood type consultants attempt to calculate how well different people work together based on blood type.

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 Final Fantasy VII where blood types of the main characters are revealed in the game's manual. In addition, the videogame series Gungriffon, Tekken and Princess Maker allow for blood type as an option in their creation modes.