Twins Day

What Is Twins Day?

Every December 13th is "Twins Day" (双子の日, Futago no Hi) in Japan. On December 13, 1874, the Meiji government issued a legal ordinance establishing that when twins are born, the child delivered first shall be legally registered as the elder sibling (兄・姉, ani/ane) — settling a question of birth order that had practical implications for inheritance, family succession, and social status in Meiji Japan. The day commemorates this legal clarification, which brought twins' birth order in line with the conventions applied to non-twin siblings.

Birth Order and Japanese Society

The question of which twin is "older" may seem trivial, but in Meiji Japan — where eldest sons had specific inheritance rights, family succession obligations, and social status — birth order had significant legal and social consequences. The government's 1874 ruling that first-born means first-delivered provided a clear rule for civil registration and settled potential disputes about succession and family rights between twins and their families.

Twins in Japanese Culture

Twins (双子, futago) have always attracted special attention in Japanese culture — their identical appearance was considered both remarkable and sometimes associated with supernatural connotations in folk belief. Modern Japanese popular culture features twins prominently in manga, anime, and drama — the visual potential of identical characters with distinct personalities is a recurring narrative device. Real-world twins in Japanese entertainment (particularly idol groups and comedy duos) frequently leverage their identical appearance as a performance asset.

Multiple Births and Modern Medicine

Japan's twin birth rate — approximately 2% of all births — has been influenced by the same factors as in other developed nations: increased maternal age at first birth, and the rise of assisted reproductive technology (ART) that increases the likelihood of multiple eggs being fertilized. Japan has developed sophisticated neonatal medicine to support the higher-risk births often associated with twins and higher-order multiples, improving survival rates dramatically compared to earlier generations.

Fun Facts

• Reversed birth order: Some cultures associate twins' birth order with reversed social significance — the second-born twin being considered "older" because they entered the birth canal second (having been positioned "on top"). Japan's 1874 ruling definitively settled this question in favor of delivery order.
• Twins research: Japan participates in international twin studies — scientific research that uses twins, particularly identical twins raised apart, to investigate the relative contributions of genetics and environment to human traits and disease susceptibility.