On the BBC, Ruth Evans describes a widespread Japanese superstition about the relationship between blood-types and personality. Apparently, the modern junk-science belief originates with a crank called Masahiko Nomi who published a "a book in the 1970s" about it, and his son Toshitaka has continued the family tradition with more woo books and an "Institute of Blood Type Humanics." However, Evans implies that the origin of this belief is the eugenics-grounded ideology of the Imperial militarist Japanese government of the 1930s, who "formed battle groups according to blood type."
It's apparently a very widespread belief. Employers ask prospective employees for their blood-types. Dating sites use blood-type to make matches. Blood-types form part of the plot in manga, anime and games. And big companies, sports teams, politicians and teachers are all known to discriminate based on typing:
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The women's softball team that won gold for Japan at the Beijing Olympics is reported to have used blood type theories to customise training for each player. Some kindergartens have even adopted methods of teaching along blood group lines, and even major companies reportedly make decisions about assignments based on employees' blood types.
In 1990 the Asahi Daily newspaper reported that Mitsubishi Electronics had announced the creation of a team composed entirely of AB workers, thanks to "their ability to make plans".
These beliefs even affect politics. One former prime minister considered it important enough to reveal in his official profile that he's a type A, whilst his opposition rival was type B. Last year a minister, Ryu Matsumoto, was forced to resign after only a week in office, when a bad-tempered encounter with local officials was televised.