Prince Hisahito turned 20 on Sunday, a milestone that in most families would pass without constitutional consequence. In his it means that Japan’s imperial house now has exactly one adult male heir of his generation, and that the survival of a 1,500-year-old hereditary institution rests on the marital and reproductive future of a second-year biology student.
The son of Crown Prince Akishino and Crown Princess Kiko is second in line to the Chrysanthemum Throne, behind his father. He studies at the College of Biological Sciences in the School of Life and Environmental Sciences at the University of Tsukuba, where his research subject is dragonflies – which supplied the theme of the poem he submitted to this year’s imperial New Year poetry-reading ceremony, the utakai hajime.
A year of public duties
Hisahito completed his coming-of-age ceremony in September 2025, the first held for an imperial prince in 40 years, since his father’s. The year since has functioned as an introduction to the working half of the role: he appeared at the New Year’s greetings at the Imperial Palace in early January, attended the banquet for Philippine President Ferdinand Marcos Jr. and his wife during their state visit in May, and made regional visits to six prefectures – Hokkaido, Mie, Kyoto, Osaka, Nara and Hiroshima.
That schedule is light by the standards of a working royal, and deliberately so. He remains a full-time student, and the Imperial Household Agency has paced his public life to leave the degree intact.
The law that changed in July
The birthday arrives seven weeks after the most consequential change to Japanese royal law since the Allied Occupation. On July 17, parliament enacted the first major substantive revision of the Imperial House Law in nearly eight decades, aimed squarely at the arithmetic problem of a shrinking family.
It does two things. First, it creates a legal pathway for the imperial family to adopt unmarried males from eleven former branch families – lineages that surrendered their royal status in 1947. Second, it allows princesses to retain imperial status after marrying commoners, reversing a rule that had steadily drained the family as women married out and left.
What it does not do is change who can inherit. The children of those princesses remain excluded from the succession, and the throne remains male-only.
Princess Aiko and the gap with public opinion
The person that rule most visibly excludes is Princess Aiko, Emperor Naruhito’s daughter, who is popular in a way few members of the family are and whom many Japanese would accept as his successor. Opinion polling has consistently shown high support for allowing a woman to take the throne.
Parliament went the other way, and Prime Minister Sanae Takaichi has said she opposes changing the male-only rule. The result is a settlement that answers the question of how to keep the family populated while leaving untouched the question of who may reign – which is the one the demographic data actually poses.
Why the adoption route is contested
Reviving the branch families is an elegant solution on paper and a strange one in practice. The eleven houses lost their imperial status 79 years ago; their descendants have lived as private citizens for three generations, and their claim rests on a male line that in some cases diverged from the main house centuries ago.
Royal watchers and constitutional scholars have raised the obvious objections. An adopted adult with no public life becomes a potential emperor by administrative act. Whether the institution’s legitimacy – which rests on continuity and public affection rather than on any political power – survives that kind of engineering is a question nobody can answer in advance. Several experts have warned that insisting only men can be emperor may doom the institution it was designed to save.
What the next decade looks like
The practical horizon is narrow. Emperor Naruhito is 66; Crown Prince Akishino is 60. Barring the adoption provision producing candidates quickly, the line after Akishino consists of Hisahito alone. Every ordinary decision of a young man’s twenties – what he studies, whether he continues to postgraduate work, whom he marries and when, whether that marriage produces a son – now carries institutional weight that no 20-year-old should reasonably be asked to bear.
That is the substance beneath the ceremonial coverage of his birthday, reported by Nippon.com. Japan has spent two decades debating whether to let a woman inherit the throne, and has repeatedly chosen instead to make the male-only rule work a little longer. The July law bought time. It did not resolve anything, as AP reporting on the revision made clear.
