The Japanese have a passion for filing and categorization that reaches fever pitch when it comes to the always-popular system of classifying people by their A, B, AB or O blood group — "ketsuekigata" (血液型, blood type)." Women, especially, will ask about the blood type of anyone we feel friendly toward (and will eagerly volunteer our own); the topic is regarded as a surefire ice-breaker, the veritable exchange of personality meishi (名刺, business cards), as if to say, "This is me, so what about you?"

On the other hand, after getting acquainted and telling each other about families, birth dates, etc., followed by the inevitable question of blood — "Ketsuekigata nāni?" (血液型なあに? What's your blood type?), it could be that the other person will look a little wary or mildly disappointed. Many Japanese women know, down to the minutest detail, the characteristics of their particular blood group — and more importantly, it's aishō (相性, suitability) with other types. A mismatch of blood types could ruin the buds of a beautiful friendship.

A widely known fact of Japanese blood is that, demographically, there are more "A gata" (A 型, A types) than any other. Key A traits are majimesa (真面目さ, seriousness or gravity) and hatarakimono (働き者, industriousness) — easily believable, judging from most people's behavior in this country. A types are also tayorini naru (頼りになる, dependable) and shiryobukai (思慮深い, to think before speaking and weigh the consequences of actions), which makes them excellent advisers and genuine friends.