In which countries in the world does everyone know their blood types? The answer has to be Japan and South Korea. The considerable interest in blood types in the two countries has become something of a running joke  [ko], especially when compared to Western countries in which many people don't know – and don't really care about – their blood group.
Whilst evident across Asia, nowhere is the ‘blood type personality theory’ more strongly adhered to than in Japan and South Korea, with books, comics, songs, and products on sale, all based on the idea. On South Korean Facebook, people can even add their blood type to their profile along with other important personal information.
Although this fad has somehow faded over the recent years, its popularity is still strong, somewhat equivalent to that of horoscopes in Western countries.
Blood type personality theory
According to the theory,  people's ABO blood type is predictive of their personality, temperament, and compatibility with others. The adjectives which best describe the traits of each blood type are as follow: Type A is considerate but shy, B is creative but whimsical, O is sociable but unorganized and AB is rational but calculating.
The images below, created by cartoonist Park Dong Sun, exhibit how each blood type behaves in a certain situation. Park's cartoon scripts on blood type personalities are published on South Korea's largest portal site, Naver.com  on a regular basis.
Even though there are criticisms that his works often over-simplify the complex human personalities, they are widely loved by Koreans. (The images below is from his blog  [ko], not from his official cartoon page on Naver  [ko]. Most of his cartoon images circulating online were once posted on and paid by Naver and therefore are protected by copyright.)
To find out more details about the each type's trait, read this Global Voices post  written by Hyejin in 2007.
Harmless fun or dark relic?
Although harmless, cute images are adopted to explain the blood type personality theory in cartoons, the theory itself is a relic of the dark past. It is often traced back to the scientific racism used first by Nazis  and later by Japanese imperialists  to promote ideas of supremacy over different races or countries during the 1920s-30s.
When Austrian scientists first found the four different blood types, it was a truly ground-breaking research which later saved countless human lives by preventing them from receiving mismatched blood transfusions. However, later studies revealed that the different blood groups are distributed disproportionately across the world and this prompted some racist, imperialist researchers to publish non-statical reports on the theory of blood type personalities.
One famous piece of research was done by Japanese scholar Furukawa , who focused on the racial traits of the Taiwanese who revolted against Japanese imperialism. The researcher concluded that since more than 40 percent of Taiwanese had type O blood, which is believed to be the least submissive type, they needed to dilute the country's ‘rebellious blood’ by increasing intermarriages with Japanese.
Even after many Japanese academics discredited the theory for its lack of scientific basis, it somehow managed to survive through several sensational publications, with the help of media's unyielding interest. Starting from late 1990s, this fad spread to South Korea, gaining huge popularity in early 2000, and has been famous ever since.
South Korean blogger Age of Mass Production Type Romance reflected  [ko] Korean public sentiment toward the theory. Although the blogger admitted that the blood type personality lacks scientific evidence, his experience has convinced him that it is a sure-fire way to categorize people:
그런데 문제는 “실제로 겪어보면 맞는데 어떡해” 라는 거다. 물론 안 그런 예도 전무한 건 아니다. 에이형이라도 되게 직설적이고 제할말 다하는 사람 물론 있고 오형이라도 꽁생원에 좀팽이인 사람 없는 건 아니다. 하지만 […] 사람 다루는 입장에서는 사실 혈액형만큼 편한 잣대가 별로 없다. […] 과학적 근거 같은 걸로 잴 수 있는 건, 잴 수 없는 것보다 훨씬 적다 이말이다.
Blogger Unalpha was once a strong believer of the blood type personality but not any more. The blogger analyzed  [ko] that among several tools which make the blood type theory sounds believable is the Cold Reading method , which is a series of techniques used by psychics, fortune-tellers and con artists in order to convince people that they understand them:
대범한 척하지만 은근히 마음이 깊고 소심한 구석이 있어서 이미 결정한 일에 대해서도 괜히 “정말 이 말이 맞나?” 하고 혼자서 되돌아보는 날들이 있을 거에요. 타인에게 상냥하고 친구들관계도 나쁜 편은 아니지만 실제로 속내를 다 보여주는데는 꽤나 오랜 시간이 걸리죠[…] 위의 글을 읽으시면서 끄덕끄덕 하시게되지 않나요? 사람에게는 누구에게나 양면성이 있습니다. 늘 대범한 사람에게도 사실은 말못할 소심함이 있고, 털털하다고 느끼는 사람에게도 나름 세심한 구석이 숨겨져있기 마련이죠.
These sentences are defined in the Barnum statement , an open-ended statement that seems personal, yet applies to many people. Blogger Wawoooo, using an example of a school kid who refuses to speak in front of class because he is the shy A blood type, blamed  [ko] collectivism's role in Korean society in bolstering public belief in the blood type personality:
우리나라와 일본과 같이 집단주의 사상을 교육받고 있는 나라에서는 주위의 의견과 서로 어울림을 중요시하는 공동체 의식이 강하기 때문에 나이많은 어른들이나 주위의 집단의견을 무시하지 못하고 받아들여야만 하는 성향이 강해 이런 혈액형 성격론의 여파가 상대적으로 큰 문제를 불러일으키고 있다.