Answer: Japan

Japan has one of the highest adoption rates in the world with over 81,000 adoptions per year. While that alone would be of interest and worth a trivia bite for you to mull over, what makes the entire thing even more curious is that the majority of these adoptions, over 90 percent of them, are adult adoptions: specifically the adoption of men ages 20-30 years old.

Why would you adopt a grown man? It’s a cultural practice known as Yōshi-engumi (adoption of an heir) wherein a man or a family adopts an adult male as a way of bringing him into the family with both legal and cultural standing. The most common form of this adoption is when a family adopts the husband of their daughter to create a male heir in the family while still protecting their daughter and her inheritance.

Less common, although still of interest, is the practice of adoption standing in for same-sex marriage because there is no other legal recourse in Japan for same-sex couples. The elder of the pair adopts the younger of the pair and as such they can extend legal protections to each other (like inheritance without excessive taxation) without penalty.