Japanese culture: Is it true adult adoption is common in Japan?

True adoption is not that common.

This is due to the fact that Japanese civil law prior to WW2 did not allow one to choose to adopt a woman's surname when they married. If he really wanted to choose a woman's surname, he had to be adopted into a woman's family and married at the same time.

There are two main current examples of this being done: firstly, because a family with a very large estate but no heirs wants to create a new person to manage the estate. The other is for gay people who want to marry in Japan, where same-sex marriage is currently not possible, to be legally recognised as a family. Neither is very common, considering the overall percentage of the population.

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However, if you read the Japanese literature with automatic translation, you will find many more references to "adopted"(養子) than this.

This derives from the way surnames are decided when getting married in Japan. In Japan, when people legally marry, they are required to unify their surnames to one of them: around 90% of people unify their surname as a couple using the male surname, while the rest adopt the female surname. This unification with the female surname when getting married is a kind of slang for 'adopted'.

This custom died out after WW2, but the word 'adopted' remained in the language. It has actually been used as the title of a manga.

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