Turns out a lot of Japanese people still believe that your blood type can tell a lot about your character!
The whole thing started in 1927, when Takeji Furukawa, a professor at Tokyo Women's Teacher's School published a paper purportedly showing particular characteristics of people with specific blood type. The government seized the idea and tried to use it to (what else?) breed soldiers.
Link - via The Yin Yang Report
By the way I'm an O blood.