Origins of dual nationality
By William Wetherall
12 December 2026
Most people are born into a single nationality -- that of the country in which they are born, in accordance with its nationality laws. In most cases, the parents possess the country's nationality. And in most countries, as in Japan, the country's nationality law provides its nationality through jus sanguinis or "right of blood" -- meaning through a parent who possesses the country's nationality. In some cases, as in the United States, the country's nationality law provides its nationality through jus soli or "right of soil" -- meaning through birth within the country.
There was a time when most jus sanguinis laws were patrilineal, meaning that nationality was acquired through the father, especially in cases when the mother was married to the father. If the mother wasn't married, the child might acquire her nationality. This was the case with Japan's nationality law until 1985, since which Japan's law has been ambilineal -- through either the father or the mother, regardless of whether the mother is married. Most right of blood provisions in most nationality laws today are ambilineal.
Today, most nationality laws provide nationality through both jus sanguinis and jus soli, depending on the circumstances. In Japan, for example, children born in Japan to unknown parents, or to parents who have nationality, acquire Japan's nationality. And a child born outside the United States, to parent who possess U.S. nationality, stands to acquire U.S. nationality if the U.S. citizen or national parent has resided in the United States for a period of time that has varied historically -- now generally for 5 years, including at least 2 years after turning 14.
Birthright nationality passive
All nationality acquired at time of birth is passively acquired through automatic application of one or another country's nationality law. "Automatic" means only that nationality is acquired through provisions in the law without approval by a competent authority. Nationality is confirmed only through documentation, usually a registered birth certificate. In countries like Japan, filing a valid birth certificate facilitates acquisition of Japanese nationality only if the circumstances of the child's birth warrant its entry in a household register in municipality of Japan. The register is usually that of the child's single Japanese parent or married Japanese parents (who share the same register). Fondlings, children born to stateless parents, and aliens permitted to naturalize become Japanese upon creating a household register.
What happens, today, if a child is born in Japan to a Japanese and an American parent? The child becomes Japanese if its Japanese parent has its birth entered in his or her household register in Japan, and American if its U.S. citizen parent registers its birth while recognizing the child at a U.S. consulate. If born in the United States, the child stands to be a U.S. citizen, if its birth is duly registered.
Formalities essential
A child born to a Japanese parent, or in the United States, is potentially a national of Japan or a citizen of the United States -- "potentially" because, without valid certificates of birth, there can be no recognition of nationality, hence no issuance of a passport. One needs to establish one's legal existence, then establish the quality of one's legal existence -- whether someone affiliated with the country in which one was born, and/or with one or more other countries -- or with no country, hence stateless.
Birth to a Japanese parent, anywhere in the world, is not sufficient to qualify for acquisition of Japan's nationality. A "Japanese" parent is someone who has a household register in a municipality within Japan's sovereign dominion. It is not a racioethnic status but a purely civil status. The racioethnic "blood" of the parent does not matter. All that matters is the child is the biological child of someone who posseses Japan's nationality -- which is matter of territorial affiliation with Japan through a household register.
This what the racialists of the world have to get their heads around. "Right of blood" in Japan's nationality law is "right of lineage" from anyone who is a national of Japan by virtue of possessing a household register and therefore nationality. It's a family, not a racioethnic, right of lineage.
In any event, whether through soil or blood at time of birth, so-called "birthright nationality" is not a "grant" of nationality to a child from the state. Nor is it a matter of a parent "passing" his or her nationality on to a child. The agent of the child's acquisition of the state's nationality is state.
The nationality may be regarded as "transmitted" from the parent to the child -- if the parent qualifies as a person with the capacity to transmit the nationality. But the nationality itself belongs to the state, and as state's nationality law merely defines the conditions which, if satisfied, a child is qualified to be a vessel of its nationality.
Japan-U.S. dual nationality
Every country's nationality laws are different, and all have histories during which nationality requirements have changed. Multiple nationality has become increasing possible as right-of-blood states have adopted ambilineal principles or facilitated nationality acquisition for aliens born in the country to permanently residing aliens. Countries like the United States, which formerly denaturalized citizens who naturalized in other countries, today allows them to keep their U.S. citizenship, which may or may not be acceptable to the country in which they naturalized.
Japan-U.S.dual nationality originates several ways. The most common way is through birth in the United States to Japanese parents, or birth anywhere in the world to Japanese and American parents.
The second most common path to dual Japan-U.S. nationality is through the naturalization of an American to Japan. Under Japanese law, Japanese who naturalize to the United States, or any other country, will probably lose their Japanese nationality, especially if they try to renew it outside Japan.
Special measures
While naturalization is the usual way to acquire Japan's nationality later in life -- i.e., not through birth -- some people, like my children, became Japanese through special measures that supplemented the present Nationality Law, as revised in 1984 during their nationality law suits, and effective from 1 January 1975. The special measures allowed an alien residing in Japan, as the minor biological child of a Japanese parent residing in Japan, to acquire Japan's nationality upon filing a valid "Nationality notification" (Kokuseki todoke 国籍届) with the regional legal affairs bureau, an office of the Department of Justice.
My children were unable to become Japanese at time of birth through their Japanese mother, because she was married to me, a U.S. citizen. As I registered their births at the U.S. consulate in Tokyo, they became Americans through right-of-blood. Had their mother not been married, they would also have become Japanese through her, for until its 1984 revision came into effect in 1985, it was patrilineal for children of Japanese men, and matrilineal for Japanese women only if they were unmarried.
Multiple nationality
Children might acquire 3 or 4 nationalities through birth, depending on where they are born and the laws that govern the nationalities of their parents. A child born in the United States to a Japanese national and a France-Burkina Faso dual national will potentially have 4 nationalities -- one through the place-of-birth provisions of U.S. nationality law, another through its Japanese mother, and two more through its dual national father.
In the past, a child born in Japan to a Japanese national and national of the Republic of Korea (ROK), usually acquired its father's nationality. Today it also stands to acquire its mother's nationality. Both Japan and ROK now have nationality choice provisions. And both countries also recognize a declaration of choice of another nationality as cause to lose its nationality.
"Nationality choice" doesn't mean what it may seem to mean -- more about which in a future post.
Last revised 12 December 2026