Japan's unique law requiring married couples to share the same surname has raised concerns about the future diversity of surnames in the country. A study by Hiroshi Yoshida, a professor at Tohoku University, predicts that if this law persists, all Japanese people will eventually share the same surname, Sato, in approximately 500 years.

The study highlights that the number of surnames in Japan is declining annually due to this law. Currently, the most common surname in Japan is Sato, accounting for 1.5% of the population. The study found that the proportion of people with the surname Sato increased by 0.83% between 2022 and 2023.

If the current law remains in place and the number of people named Sato continues to grow at this rate, half of the Japanese population will be Sato by 2446, and eventually, 100% of the population will have the surname in 2531. This would not only be inconvenient but also undermine individual dignity and lead to the loss of family and regional heritage associated with surnames.

However, if the option of separate surnames is introduced, the increase in people named Sato would slow down significantly. The study estimates that if 60% of married couples choose to have separate surnames, the proportion of people named Sato would only reach 7.96% in 2531, preserving the diversity of surnames in Japan.