A mukoyōshi (婿養子); lit. 'adopted son-in-law') is an adult man who is adopted into a Japanese family as a daughter's husband, This is a centuries-old tradition and is still widely practiced today. Many Japanese companies with household. Because the family law in Japan forbids polygamy and intermarriage between close relatives but does not forbid inbreeding, a family register recorded Aizawa's children as her father's illegitimate children.