It is common belief among the Japanese that blood type can determine a person's personality, and that they take it as a fact. This belief affects Japanese people's lives in various ways.
The majority of Japanese peoples' blood type is A. People of blood type A display many traits of a typical Japanese – responsible, reserved, patient, etc. It is highly probable that this fact accounts for the widespread belief in Japan that blood types determine people's personality.
Compared to A blood type, there is a much lower number of Japanese people with blood type B. People of this blood type are considered to have an individualistic streak, and are outspoken.
Each blood type consists of positive and negative traits, but the B blood type is considered the worst in Japanese society. This is because the characteristics of a person with blood type B are not typical of the average Japanese, and therefore do not fit in the group mentality of Japanese society. Hence, it's considered a “bad” blood type in Japan.
While people of AB and O blood types do exist in Japan, they are usually not the topic of discussion among the Japanese – probably they are not that interesting compared to people of blood type B.
The Japanese are extremely fond of drinking parties, called 飲み会“nomi kai”. In such parties, they tend to have more fun drinking with people of blood type B, compared to those of blood type A. This is because that, due to their personality, the words and actions of blood type Bs when they get drunk are usually interesting. In fact, drunk blood type Bs tend to start a brawl when they get teased for their blood type. On the other hand, drunk blood type As supposedly remain relatively reserved and controlled – in other words, boring.
Elementary and junior high school students in Japan all believe in the relationship between blood type and personality, and this is highly likely due to them being constantly fed this doctrine by the adults around them. Even when they become adults, this belief remains staunch in their minds and hearts. A Japanese person can form a strong impression of others simply based on their blood type alone.
While the belief in blood type and its relationship with personality is shared among all Japanese people, the degree in which they believe in it varies from person to person. In particular, women are more likely to believe in it compared to men. In fact, blood type compatibility is a major factor for Japanese women who are looking for a partner.
In spite of the blood type beliefs that exist in Japan, blood type discrimination in the workplace is not really widespread in Japan. Small companies may tend to look for those of a certain blood type when looking for new employees, but such discrimination does not exist in the bigger ones.
The Japanese do know that this belief in blood types' relationship with personality is superstitious, but they believe in it anyway. Try to argue to them that it has no scientific grounding, and you could get a sympathetic look that says, “Ah, poor thing” in return. In the best case scenario, you would get no response at all. Trying to convince a Japanese person that such a belief is flawed is like moving mountains. Such is the wonder of a Japanese person's mind.
As with their contradictory beliefs in religion highlighted in a previous article, it is highly probable that the Japanese people's belief in blood type and personality relations stem from the simple fact that it's cool and interesting. When this belief was advocated in the 1970s in Japan, the Japanese must have thought along the lines of, “Wow, cool! This idea makes sense." Someone might even have, right on the spot, glared at a friend accusingly, saying "Now I know why you're so K.Y. 'Coz your blood type is B!” (K.Y. is an acronym for 空気よめない“kuki yomenai”. Literally, it means “can't read the air”. It's a common expression among young Japanese people, referring to people who are unobservant of their surroundings, and hence act or say things that are inappropriate)
In any case, such a belief does not really have an overly serious impact on Japanese lives.