There exists a common, popular belief in Japan that a person's blood type predicts his or her personality, temperament, and compatibility with others, similar to how astrological signs are used in other countries throughout the world. Blood type, however, plays a much more prominent role in Japanese society than astrology does in other countries' societies.
The belief derives from ideas of historical scientific racism. In 1926, Japan’s Rin Hirano and Tomita Yashima published the article "Blood Type Biological Related" in the Medical Journal of Army. It was seen to be a non-statistical and unscientific report, motivated by racism. By the 1930’s, Japanese interest in the theory faded as its unscientific basis became evident.
The belief 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 was unclear. Because of this, he was heavily criticised by the Japanese psychological community, although his books remain popular. His son continues to promote the theory with a series of books, and by running the Institute of Blood Type Humanics. The scientific community has dismissed such beliefs as superstition or pseudoscience.
Asking one's blood type is common in Japan, and people are often surprised when a non-Japanese person does not know his or her own blood type. Facebook in many Asian countries allows users to include their blood type in their profile.
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. It is common for video game series to allow for blood type as an option in their creation modes.
The Japanese national softball team has customized training to fit each player's blood type. Companies have given work assignments according to their employee’s blood type. Children at schools have been split up according to their blood type.
Many people have been discriminated against because of their blood type. Employers have been asking blood types during interviews despite the warnings they have been given.
After then-Reconstruction Minister Ryu Matsumoto's abrasive comments towards the governors of Iwate and Miyagi forced him to resign, he blamed his behavior on his blood type, saying "My blood is type B, which means I can be irritable and impetuous, and my intentions don't always come across."
Blood type harassment and discrimination, especially against Type B’s, has been blamed for the bullying of children in playgrounds, loss of job opportunities, and ending of happy relationships. An example can be seen in the film "My Boyfriend Is Type B" where a girl is advised not to date a man because his blood type is B.