The relationship between personality and blood type is professed to be an actual predictable relationship according to my source. She first heard this from her friends in middle school. It is a widely held belief in Korean culture that blood type determines one’s personality. Blood type A represents uptight, more meticulous people, blood type B represents more relaxed and easygoing personality traits, blood type O is ambitious and athletic, but more vain, and finally blood type AB represents cool and controlled, but more critical people. She asked me my own blood type and analyzed my own personality traits in accordance with my blood type.
I find that the fact that this folk belief is still widely held, while lacking any real scientific or empirical evidence behind the fact is interesting. Much like how the Chinese calendar and your zodiac animal is said to describe your persona, this folk belief is one in which blood type designates personality. The notion of predetermined pathways in life is a common aspect of many east asian cultures.