In Japan, a long-standing social and legal tradition allows a daughter’s husband to be adopted into the family and made its son and successor. The practice is called “mukoyoshi,” which can be understood as an adopted son-in-law. It is not merely a curiosity from the past. Adult male adoption remains common in Japan, and various reports indicate that more than 90 percent of all adoptions in the country involve adult males. Typically, young men between the ages of 20 and 30 are brought into families in this way, and the reasons behind the arrangement are often practical as much as emotional.
The core idea is continuity. Many old businesses in Japan are still family-owned. If a family does not have a son, or if its own son is not considered a suitable successor to the business, the family may adopt a capable man. If that man is already married to the daughter, the process becomes smoother. After adoption, he takes the surname of his in-laws. Over time, the family business, wealth and inheritance may also come into his hands. The relationship does not stop at being a son-in-law. He becomes a member of the family and, in many cases, the next person in charge.
This tradition is used to keep three things together: the family name, the leadership of the business, and the survival of a long-standing institution into the next generation. In Japanese society, a wife traditionally takes her husband’s surname upon marriage. If a family has no son, the question of how to maintain the family surname can arise. Mukoyoshi solves this problem by adopting the daughter’s husband and giving him the family surname. In this way, a son-in-law simultaneously becomes the son of the family, the successor to the business, and the bearer of the clan name.
The history of Japanese big business bears the imprint of mukoyoshi. Suzuki Motor Corporation has seen adopted sons at the helm on several occasions. The company’s long-time head, Osamu Suzuki, was himself an adopted son-in-law who entered the family through this tradition. This type of inheritance system has also been associated at various times with companies such as Nintendo, Kikkoman and Toyota. As a result, this unusual but effective social custom is part of the background behind the long-standing family continuity of some of Japan’s most famous businesses. It has helped ensure that leadership does not pass outside the family and that the institution survives across generations.
The use of mukoyoshi is not limited to business. Adoption of an adult is also used in inheritance planning. Childless couples in particular may choose this route to decide who will inherit their assets in the future. The adoption process is registered in Japan’s official family record, the koseki. Once the adoption is complete, the person takes on the new family’s surname and receives inheritance and family responsibilities just like a natural child. However, with this comes responsibility. The duty of caring for elderly family members may also fall on the adopted child.
A question that may arise is whether foreigners can become mukoyoshi. According to reports, foreigners with long-term residence permits in Japan can also be adopted as adults if they meet certain conditions. In some cases, court approval is required. This means that a Bangladeshi person, for example, could become an adult adopted child of a Japanese family if the conditions are met. However, adoption does not grant Japanese citizenship or a visa. Those require separate legal processes. The adoption affects family registration and inheritance matters, but it is not a shortcut to nationality or immigration status.
With the declining birth rate in modern Japan, the tradition has taken on a new dimension. Many family business owners have children but no sons. Their own children may not be interested in or suitable for running the business. The family then faces the question of who will run a business that may be a hundred years old. In search of answers, some families choose a suitable son-in-law rather than waiting for a son to be born. There are even special matchmaking services in Japan to find suitable candidates. A common saying captures the logic: “You can’t choose your son, but you can choose your son-in-law.”
Over time, questions have also been raised about mukoyoshi. In modern Japan, women are leading businesses, the state and various institutions. As a result, there is debate about how necessary the old idea of seeking only male heirs really is. If a daughter can lead a company or inherit a family’s assets, the need to adopt a male successor may seem less pressing. Still, the tradition has not completely disappeared. For some, it is a story of making a son-in-law a family man. For others, it is a strategy to keep a century-old business alive. For still others, it is a historic system to pass on the family’s title, wealth and legacy to the next generation.
Japan’s mukoyoshi is therefore not just a story of adoption. It is a story of marriage, family, business, inheritance and a strange coexistence in a changing society. It reflects the importance of continuity in a country where family names, family businesses and long-established institutions carry great weight. At the same time, it shows how tradition can adapt to modern needs, including inheritance planning, business succession and the search for capable leaders. Whether viewed as a practical solution, a cultural legacy or a subject of debate, mukoyoshi remains a distinctive feature of Japanese family and corporate life.
