Birthday-Number Effect

The birthday-number effect is the subconscious tendency of people to prefer the numbers making up their own birth date over other numbers. First reported in 1997 by the Japanese psychologists Shinobu Kitayama and Mayumi Karasawa, it has since been replicated in a number of countries and holds regardless of age or gender, and it is most pronounced for numbers greater than twelve. As with the closely related name-letter effect, with which it is significantly correlated, the explanation is that the birthday is bound up with a person's sense of self, so that people who like themselves tend to prefer their own birthday numbers while people who do not tend not to show the effect; the Number Preference Task built on this pattern is used to estimate implicit self-esteem. Some evidence suggests the effect can shape real decisions, such as one study finding people rated prices more favorably when the prices were secretly set to match their birth date, though a similar study using birth year rather than day and month found no such effect, and research into the effect's influence on larger life decisions remains contested.