You're probably all too familiar with the widely recognized correlation between the stars and our lives and character traits. Interestingly, the stars are not the only place people look for that same information. In fact, a popular belief in Japan is that our personality, temperament and compatibility with others is determined, or greatly influenced, by our ABO blood type.
It's not uncommon to find blood-type 'horoscopes' and compatibility tests in the Japanese media, in much the same way that astrology has become a widely visible phenomenon in the western media. Of course, the big question is, "Is there any truth or basis to the system?"
It all began with the discovery of blood types in 1901, and in the following years, the distribution of different blood types across the globe was widely studied. While the discovery was a major leap forward in medical science, blood type statistics were often used to fuel generalizations about entire races, even being cited by Nazi Germany as an argument for 'race supremacy'. As medical science advanced, however, these beliefs became less and less popular, until they were more or less forgotten.
The idea cropped up again in Japan in the 1970's, though in a much less threatening way. Japanese journalist and author Masahiko Nomi wrote many best-selling books on the subject, proposing that there is indeed a correlation between blood type and personality, and that people of each personality type (A, B, AB and O) interact with each other in different ways. The idea spread throughout the media, and became widely popular, partly due to the natural curiosity people have to learn about themselves and their compatibility with others.
Type A's are thought to be reserved, introverted, patient and meticulous, though stubborn and self-conscious. B's are thought to be creative, passionate and optimistic, though irresponsible. AB's are considered rational, sociable and popular, though critical and unforgiving. Finally, O's are thought to be natural leaders, ambitious, self-confident, though insensitive and arrogant.
Is there science to back it up? The scientific community widely regards 'blood-type personality theory' as mere superstition, or pseudoscience, though blood-type does, naturally, have a minor effect on a person's physiology. Whether or not this influences personality is uncertain, and of course even if it does, it may simply be one of many factors playing a small contributing role. The one thing we do know is that enough people believe in it for it to remain a widely publicized and talked about phenomenon.
Perhaps the biggest down side to the system is that it is extremely general. With only four 'types', each one must be general enough to apply to an entire quarter of the population. This also means that, even if the system was completely random, 1 in 4 people would still have a personality which matches their blood-type's description, and that's more than enough people to keep any belief system alive. It's naturally very difficult to prove or disprove a system which has statistics like that. Why not compare your friends' and family's blood types, and make up your own mind?