The problem lies in a 200-year-old law that obliges couples to choose one name for all family members.

Everyone in Japan might have the same family name in 500 years – Sato – which is the most popular in this Asian country, according to a simulation run at the Tohoku University.

Japan currently requires couples to choose between their family names when they get married and 96% prefer the man's name, says Professor Hiroshi Yoshida at Tohoku University's Research Center for Aging Economy and Society, in a new study. 

In 2023, as many as 1.5% of the Japanese population were called “Sato” – which does not seem impressive – but mind that during one year only the adoption rate for it was 1.0083 times.

The root of the problem is the marriage law, adopted in the 1800s, which makes it compulsory for couples to choose one name for all members of the family.

If this legal condition remains and the adoption rate maintains, everyone will be called Sato by 2531, the researcher said, noting that this is “just a probability.”

If, however, Japan amends the marriage law to allow for separate names, only 7.96% of Japan residents citizens will become Sato within the next 500 years. Under such scenario, Sato would conquer Japan not earlier than by 3310.

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According to the Future Population Projections, the Japanese population is expected to decline from 125 million in 2022 down to 41 million in 2120, and to 22 million in 3310, the study suggests. "In other words, even if 100% adoption of the Sato name is postponed for 800 years, there is a high possibility that the Japanese people themselves will become extinct before that due to the declining birthrate," the author claims.

The research was commissioned by the Think Name Project, which aims to pressure the government into allowing married couples to have separate names and preserve the name heritage of the Japanese people.

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