TOKYO — Japan’s parliament enacted Friday a historic revision to the 19th-century Imperial House Law by insisting only paternal-lineage men can become emperor, sparking fear that it could doom the already shrinking imperial family.

The revisions include adoption of distant male relatives to father future heirs and allowing princesses to keep their royal status after marrying commoners.

Royal watchers and experts fear the new measures could doom the 1,500-year-old hereditary institution by insisting that only men can be emperor, sparking worry about the shrinking, fast-aging imperial family.

Emperor Naruhito ’s 24-year-old daughter is hugely popular, and many Japanese want her to be his successor, but Princess Aiko is ineligible because she is a woman. Japan’s male-only succession rule means the line must move to the emperor’s younger brother, then to his 19-year-old nephew Prince Hisahito. Next in line after him is the emperor’s 90-year-old uncle.

In an imperial family that places a premium on male royal babies, Hisahito is the first such boy to be born in four decades. Only five of the 16 adults in the imperial family — there are no children — are men.

This matters, as Prime Minister Sanae Takaichi and other conservatives insist the male bloodline is “the only source of the emperor’s authority and legitimacy,” which will be the basis for the upcoming measures.

While an emperor’s mother can be a commoner, as is the case with the current one, only boys born to men with royal blood can be heirs to the throne, according to the Imperial House Law.

The revision passed Friday to the antiquated law is meant to solidify the principle of that crucial bloodline by allowing the adoption of distant royal male relatives to father future heirs.

The new measures will also allow princesses to keep their royal status if they marry a commoner.

“It’s a declaration to prevent female monarchs … and to defend the male-lineage at all costs,” said Hideya Kawanishi, a Nagoya University expert on monarchy. “They cannot say it’s male chauvinism, so they call it tradition.”

There have been eight female monarchs. The last was Empress Gosakuramachi, who ruled from 1762 to 1770.

The paternal-line male succession was stipulated for the first time in the 1890 Imperial House Law, when Japan promoted patriarchal systems. That law was largely carried over to the current 1947 version.

Friday’s revisions have led to protests from Japanese who see the government efforts as meant to eliminate Aiko from ruling and to justify discrimination against women and a patriarchal system.

“It’s very ironic that the first female prime minister herself is the leading proponent of the obsession with male-succession,” Chizuko Ueno, a prominent feminist scholar, wrote recently referring to Takaichi.

Ueno said the new measures “treat male royals as stallions and put female royals under pressure as ‘childbearing machines’ to produce male offspring.”

After Aiko’s birth, her mother Empress Masako, a Harvard-educated former diplomat and a commoner, developed a stress-induced mental condition, apparently over criticism for not producing a male heir.

Because of the male-only succession rules and the dismissal of princesses who marry commoners, the monarchy after Hisahito is “extremely unstable,” former Imperial Household Agency chief Shingo Haketa told Kyodo News recently.

Historians say the male-only system is unworkable today, as Japan more broadly faces a fast-aging, dwindling population.

It only worked in the past because concubines produced half the emperors until about 100 years ago, when the practice ended under Naruhito’s great-grandfather, Emperor Taisho.

There was a government proposal in 2005 to allow female monarchs, but it was scrapped following Hisahito’s birth.

Naruhito’s two male heirs are his 60-year-old brother, Crown Prince Akishino, who is only six years younger than the emperor and has reportedly said he would be too old to serve, and Hisahito, Akishino’s 19-year-old son. Third in line is Naruhito’s uncle, Prince Hitachi, who is 90.

The more controversial of the two measures allows unmarried male descendants, aged 15 or older, of distant imperial relatives — but only of paternal lineage — to be adopted into the royal family.

Fifty-one members from 11 branch families renounced their royal status in 1947, mainly to ease the postwar financial burden on the monarchy, Imperial Household Agency official Yoshimi Ogata told a recent parliamentary session.

These people are at least 36 generations removed from Naruhito because they split from a common male-line ancestor 600 years ago, Ogata said.

There is criticism of what some see as the government’s extraordinary efforts to make sure that male royals are producing male emperors.

“Who wants the son of an adoptee who nobody knows to be emperor instead of Aiko?” asked Yoshinori Kobayashi, a cartoonist campaigning for Aiko’s succession.

It may also be unrealistic to ask former royals to reenter a very strict family known as “an enclave without human rights.” Royals cannot choose their jobs or homes, and must follow other serious constraints.

“I wonder if anyone would raise a hand,” 81-year-old Asahiro Kuni, whose family renounced its royal status when he was 3, told TBS television. “I imagine many people, by age 15, have some idea about their future. It’s cruel to tell them … to change the course of their life.”

Kuni, who worked as an engineer at a major Japanese company, said he would tell his family to decline if asked by the palace. “You are asked to sacrifice your life for the happiness of the people. I can’t tell my family to choose such a difficult life.”

He also expressed support for female monarchs in interviews with other Japanese media.

Aiko, known for her engaging smile, enthusiasm and witty conversation, is a public favorite.

Five single princesses, including Aiko and her popular cousin Kako, 31, may be affected by the other main revision to the Imperial House Law, which would allow them to keep their royal status and continue serving official duties if they marry commoners, although their spouse and children wouldn’t be accepted as royals.

Aiko’s elder cousin Mako renounced her royal status and moved to New York after marrying her college boyfriend, a commoner who now is a lawyer. The move was largely seen as her attempt to flee from the restrained imperial life.

Ueno calls the system inhumane and urges the princesses to follow Mako’s example and leave when they can.

Hisahito, possible adoptees and their future wives will face enormous pressure to produce male offspring, Kawanishi said.

“The emperor is a symbolic figure, and I don’t see why women cannot serve in the role,” said 78-year-old Junichiro Tsujimaru, a sushi chain founder.

Yoshio Iwase, also 78, says Aiko, as the daughter of the emperor, is the legitimate successor. “I think it’s fine because there used to be female emperors in the past.”

There is worry that the government’s push will upset former Emperor Akihito’s legacy, which included making amends for the victims of World War II, fought in his father’s name.

Akihito, who abdicated in 2019, also tried to bring what was seen as an aloof monarchy closer to the people, an example followed by his son, Naruhito, and his family.

Akihito reportedly supports Aiko’s succession. He avoided directly answering a question about the 2005 government proposal but said female royals served a major role in the monarchy and that its role was to work for the happiness of the people — a remark interpreted as his support for female monarchs.

Naruhito also said in June that he hoped discussions about the measures would reach a conclusion that “will gain understanding of the people,” a comment palace watchers said was his nuanced displeasure.

Japan on Friday also enacted a controversial new law prohibiting desecration of its national flag, a key right-wing agenda pushed by Takaichi. Opponents see it as an attempt to intimidate the public and silence criticism against her government.