Japanese Blood Types and Personality
Every Japanese celebrity writeup and every anime character description with any claim to authority gives the person's (or character's) blood type. Why? Because, thanks to a remarkable bit of Science! by a gentleman named Furukawa Takeji, many Japanese believe that a person's blood type affects their personality.
Each blood type has a personality profile. These profiles—which I took from a site whose location is long since lost in the mists of time, my apologies—are:
People with blood type A have a deep-rooted strength that helps them stay calm in a crisis when everyone else is panicking. However, they tend to avoid confrontation, and feel very uncomfortable around people. A types are shy and sometimes withdrawn. They seek harmony and are very polite, but all the same feel that they never really fit in with others. A types are very responsible. If there is a job to be done, they prefer to take care of it themselves. These people crave success and are perfectionists. They are also very creative, and the most artistic of all the blood types, most likely because of their sensitivity.
People with blood type A are also likely to be considered classic "type A's": stressed and conscientious. In anime, people like Hotohori are type A's.
The Third Hokage, Haruno Sakura, Hotohori, Heero Yui, Keiko
People with blood type B are the most practical of the blood groups. They are specialists in what they do. When they start a project, they spend extra time understanding and trying to follow directions than others might. When they are doing something, all of their attention is focused on it. They tend to stick to a goal and follow it through to the end, even if it seems impossible. They tend to be less than cooperative, as they like to follow their own rules and their own ideas. They are individualists. B type people pay attention to their thoughts a little more than their feelings, and therefore can sometimes seem cold and serious.
People with blood type B are often considered more relaxed, freewheeling, and unconventional than other types, although not necessarily to an unacceptable degree. In anime, the genki, off-the-wall types are type B, along with any kind of well-intended character who's ruled by their impulses.
Naruto, Duo Maxwell, Miaka, Nuriko, Tasuki, Sagara Sanosuke
People with blood type O people are outgoing, energetic and social. They are the most flexible of the blood types. They easily start up projects but often have trouble following through because they give up easily. They are flighty and not too dependable. O types always say what's on their mind. They value the opinion of others and like to be the center of attention. Also, people with O blood are extremely self-confident.
Type O, the most "average" blood type, is considered the best type in Japan.
Quatre, Sailor Moon, Saito Hajime, Kamiya Kaoru, Tamahome
People with blood type AB are hard to categorize. They can have characteristics on both ends of the spectrum at the same time. For instance, they are both shy and outgoing. They easily switch from one opposite to another. AB people are trustworthy and responsible, but can't handle it when too much is asked of them. They don't mind doing favors or helping out, as long as its on their own conditions. People with this blood type are interested in art and metaphysics.
AB is considered the worst blood type. In predictability-loving Japan, they're loose cannons. They also like to set their own conditions and reserve the right to drop out when things don't meet their expectations. They're known to be sensitive and considerate—at times—but it just isn't enough to balance out the flaws in this blood type. For a while, some companies tried dividing their employees into work groups based on blood type, and no one wanted to work with the AB group. Anime villains are likely to be type AB.
Uchiha Sasuke, Hongo Yui, Tomo, Trowa Barton, Kenshin
A is most compatible with A and AB.
B is most compatible with B and AB.
AB is most compatible with AB, B, A, and O.
O is most compatible with O and AB.
Blood Type Distribution
If you grew up in the English-speaking world, the Japanese blood type system might not seem to make much sense. You've been taught that Type O and Type A are both common, at about 44% and 39% of the population, respectively; Type B is rare, at about 10% of the population; and a tiny 4% or less is Type AB. So why does the Japanese blood type system give Type B such an important profile? Why all the hate for Type AB's? Isn't that a lot of weight to put on a little sliver of the population?
The system makes more sense when you realize the Japanese have a more even distribution of blood types than Caucasian-majority populations. 30% of Japanese are Type O, 40% are Type A, 20% are Type B, and 10% are type AB.link That's more than twice as many people with "rare" blood types. Type B's and AB's are a prominent part of the system because they're a prominent part of Japanese society.
So the Japanese are still being jerks to Type AB's. They're just being jerks to enough Type AB's to fight back.
Personality and Blood Type in the West
The idea that blood type determines personality has been dying out slowly in Japan. At the same time, the West has finally noticed the potential of blood typing. We haven't quite cottoned onto the idea that blood type is related to personality, but there's a growing belief in fad diets that teach that each blood type evolved at a certain point in human history, so people with that blood type should eat the diet most common among humans at that point. Instead of balanced, nicely average O's, we have savage cavemen hunching around the fire and tearing at hunks of meat; solid-citizen type A's become "docile vegetarians" as well; bouncy type B's get to indulge in ice cream and cheese; and type AB's, as always, are half one thing and half another, and get to eat vegan food or dairy-and-everything-else—IF it suits them. Blood type diets are worth looking into if only to imagine your favorite characters' reactions to their new diets.
Sailor Moon, cavewoman. That image puts me in a happy place.
ABO Fan Home Page/Hypothesis - A scientific defense of Blood Type Humanics.