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In 1930s, the Japanese military investigated the connection between blood types and personality in an attempt to create ideal soldiers.
The connection between blood type and personality of an individual has been a topic of interest in several Asian countries. However, the origins of this concept lie in the Japanese culture, and is termed as the 'Blood Type Theory of Personality', which emphasizes that a person's blood type can be useful in predicting his personality, general disposition and his rapport with others.

The relation between ABO blood types and personality was studied and brought to life by Mashiko Nomi, a Japanese journalist, in the 1970s. Nomi's interest was sparked by a paper titled The Study of Temperament Through Blood Type, published by Takeji Furukawa in 1927, in the journal Psychological Research. Although lacking a strong scientific explanation, and being dismissed by members of the scientific community, popular belief subscribes most wholeheartedly to this concept.

Various personality traits associated with the four different blood types under the ABO system, have been provided below.
Blood Type A - The Farmers
Referred to as farmers, people with blood group A, are patient, peace-loving and loyal. These individuals may be overly sensitive, and care a lot about social standards and etiquette. They are sensitive to others feelings, but take time to share their own. They are very careful while making decisions, and are more comfortable handling one task at a time.
Blood Group O
Famous People with Blood Type A
George H. W. Bush, Britney Spears, Adolf Hitler, Richard Nixon, Ringo Starr, and Jet Li.
Blood Type B - The Hunters
Classic Bs are considered analogous to hunters, and they tend to choose the less beaten or unconventional tracks. (Not to be confused with Peter D'Adamo's attribution of 'hunter' to blood type O.) They are quick decision makers, but cannot take orders easily. These are the people who are likely to focus on something they have set their minds on, and never let go of it, no matter how unachievable the goal. They are highly respectful of scientific works and practical findings.
Blood Group B
Famous People with Blood Type B
Leonardo DiCaprio, Jack Nicholson, Luciano Pavarotti, Tom Selleck, Paul McCartney, and Vince Young
Blood Type AB - The Humanists
Individuals with this relatively rare blood type, are perceived as humanists. They have excellent analytical and logical reasoning skills. Yet, they may represent a mixture of traits observed in those with blood types A and B. They may be outgoing and confident at times, but may act shy sometimes. Although calm and cool, an unsolved problem can make them highly anxious. It is hard to categorize them because of their tendency to display an amalgamation of the traits of differing blood groups.
Blood Group AB
Famous People with Blood Type AB
John F. Kennedy, Marilyn Monroe, Thomas Edison, Jackie Chan, Mick Jagger, and Barack Obama.
Blood Type O - The Warriors
The people with blood type O, may often be trendsetters, and are very strong and enduring like warriors. They are not just honest but also dislike people who hide the truth. These individuals are born leaders, but give up the task easily if they find it meaningless. They generally tend to overlook the finer points and details.
Blood Group O
Famous People with Blood Type O
Elvis Presley, Queen Elizabeth II, Prince Charles, Al Capone, John Lennon, and Paul Newman
Although the correlation between blood types and personality has no scientific evidence, the idea had become the basis for scientific racism. Those in the West labeled individuals with the blood type B, the most common blood type amongst Asians, as inferior. Moreover, during the early 1900s there was widespread propaganda suggesting that there existed a 'superior' blood line characterized by A type of blood.

Today, the relation between blood type and personality is followed closely in Japan, much like the use of sun signs to determine personality and compatibility for matchmaking.