In Japan, your blood type (A, B, AB, O) determines your personality and relationship compatibility.
Ketsueki-gata (blood type personality theory) is a popular belief in Japan, South Korea, and parts of East Asia claiming ABO blood types predict temperament and romantic compatibility. Scientific research consistently finds no correlation between blood type and personality. The belief emerged in 1920s Japan, gained prominence after a 1971 bestseller, and became culturally embedded through television, dating services, and workplace dynamics. Some Japanese still consult blood type compatibility when dating or hiring. Unlike superstitions dismissed as folklore, ketsueki-gata persists as quasi-scientific belief maintained by otherwise rational people. This illustrates how pseudoscience thrives within cultures despite scientific consensus.