Strike up a conversation at a bar in Japan and the chances are good that it will eventually turn to blood type. Japanese people's enduring and pervasive fascination with blood type as an indicator of personality, as well as a predictor of behavior suitability for jobs and even of amorous compatibility, has always mystified me. Common knowledge in Japan has it that people with type A blood are serious busybodies, those with type B are curious and creative, folks with type O are easy-going and positive, and people with type AB have dual personalities and are calm.

I was unsure of my blood type before coming to Japan. Asking around, it turns out that many foreigners are in a similar state of ignorance. The topic can confound foreigners in Japan. Asked what type of blood he had, one British friend responded tongue-in-cheek: "It's red."

Why in the world do Japanese get so hung up on blood type? Not long after blood type was discovered in Austria in 1901, researchers in Japan began investigating it as a means of pigeonholing people's personality traits and aptitude. Later, it was even taken up by the military, but the topic largely lay dormant until the 1970s when Nomi Masahiko published a series of easy-to-read books on blood type and compatibility.

"There has been some research abroad but it is all very piecemeal. Nowhere else in the world has this much data about blood type (and personal characteristics) been made public through books, magazines, television and radio for the public to be able to check against reality," said Nomi Toshitaka, who has carried on in his father Masahiko's footsteps.

Aside from the mountains of data, a big reason that blood type has proved popular could be the diversity here of blood types. Most countries are dominated by only two types, but in Japan the four blood types are fairly evenly distributed, Nomi explains.

I suppose just two types might take away the allure of the guess-my-blood-type game. "The Japanese love classifying everyone into tidy pigeonholes by age, region, university, sex and so on. Blood type is just another category," said longtime Japan commentator, Mark Schilling.

"There is something comforting about being able to label or identify yourself and others in a common way that is socially acceptable and harmless," confided one Japanese friend.

Ultimately, while blood type is a conversational lubricant of sorts, it seems to boil down to a safe and socially acceptable way of being different. It is also like a game as people try to guess blood types by personality.

Being of blood type A myself, and supposedly having a penchant for particulars, I would like to elaborate … but I think I will just leave it at that.