Origins of Blood Type Personality Theory
The idea of linking blood types to personality originated in a 1927 paper titled "The Study of Temperament Through Blood Type" by Professor Takeji Furukawa of Tokyo Women's Higher Normal School. Furukawa classified Type A as "obedient," Type B as "free-spirited," Type O as "strong-willed," and Type AB as "rational."
This theory was criticized by the academic community at the time and faded once during the 1930s. However, in the 1970s, journalist Masahiko Nomi revived it through his book "Compatibility by Blood Type," and it deeply permeated popular culture through television and magazines.
Currently, survey results indicate that approximately 70% of people in Japan believe there is some connection between blood type and personality (Yamaoka, 2009). A similar belief has spread in South Korea, but it is virtually unknown in Western countries. This suggests it is a cultural phenomenon rather than a scientific fact.
Scientific Verification Through Large-Scale Studies
The connection between blood type and personality has been repeatedly refuted by multiple large-scale studies.
Nawata (2014): A study of over 10,000 Japanese participants reported finding no statistically significant relationship between blood type and 29 personality traits. Effect sizes were extremely small (d <0.05), at a level carrying no practical significance.
Cramer & Imaike (2002): A comparative study of Japanese and American participants reported that no connection between blood type and personality was confirmed in either country.
Rogers & Glendon (2003): Confirmed no significant correlation between blood type and Big Five personality traits in a large Australian sample.
Wu, Lindsted & Lee (2005): A study of 2,681 participants concluded that no clinically meaningful relationship exists between blood type and personality.
What these studies share is that despite sufficiently large sample sizes and high statistical power, no significant relationship was found. While logically proving "no relationship exists" is difficult, given this accumulated evidence, the burden of proof lies with those claiming a relationship exists.
Why People Keep Believing - Psychological Mechanisms
Confirmation Bias: People tend to selectively remember information supporting their beliefs while ignoring contradictory information. Someone who believes "Type A people are meticulous" will think "I knew it" when a Type A friend acts meticulously, but overlook when they act carelessly.
Barnum Effect: The tendency to accept vague descriptions that could apply to anyone as uniquely applicable to oneself. A statement like "Type A people are serious, but sometimes have a bold side" effectively applies to everyone. Related books can also be found at related books (Amazon).
Self-Fulfilling Prophecy: Believing "I'm Type B so I'm free-spirited" creates a tendency to actually behave that way. This isn't blood type determining personality - it's beliefs changing behavior.
The desire for social categorization: Humans have a fundamental desire to categorize others in order to understand complex social environments. Blood types provide four simple categories that satisfy this desire. Whether it's scientifically accurate matters less than the fact that it's "easy to understand."
The Harm of Blood Type Beliefs
Blood type personality theory is often considered a "harmless superstition," but it actually causes several forms of harm.
Blood type harassment ("bura-hara"): In Japan, discrimination and prejudice based on blood type has become a social issue. Cases have been reported of blood type being asked in job interviews, romantic relationships being rejected because someone is Type B, and workplace roles being assigned based on blood type.
Distortion of self-understanding: Trying to understand yourself through the blood type framework may cause you to overlook your true personality traits and strengths. Using a scientifically valid model like the Big Five provides more accurate and useful self-understanding.
Distortion of relationship judgments: The belief that "Type B and Type A are incompatible" can cause people to give up on potentially successful relationships before they even begin. Judging human relationships by criteria without scientific basis leads to missed opportunities.
How This Site Handles Blood Type
This site claims to offer compatibility assessments based on scientific evidence. So why does it include a blood type input field?
There are two reasons. First, completely excluding blood type might feel "unsatisfying" to Japanese users. We balance meeting cultural expectations to a certain degree while maintaining scientific accuracy.
Second, by setting blood type's weight to a minimal level (5% of the total), we aim for the educational effect of helping users experientially understand that "blood type barely affects results." You can verify that changing only the blood type with the same other inputs barely changes the score.
Blood type input is entirely optional, and when left blank, it is excluded from the assessment. For those seeking the most scientifically reliable results, we recommend using only the Big Five and age difference for the assessment.