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But why do the Japanese want to know your blood type?
The Japanese want to know your blood type not because they want to hold a blood drive in the event you get hurt and need a blood transfusion or to play family detective, but because they believe your blood type reveals your personality and prospective talent. For example, a Type-B person could be a good romantic partner but is also predicted to be lazy and irresponsible; Type-0 people would be good at baseball; Type-AB people could be great actors or singers. In Japan, they call it blood personality theory. The upside is that people are empowered to achieve their dreams, given the right blood type. The downside is outright discrimination.
You love acting but your blood type is not a right fit, so you have to pursue a career as an accountant instead. I can't marry you because our blood types are incompatible.
Although blood discrimination is not a widespread problem in Japan, it probably contributes to school bullying, social alienation and suicide.