This system of succession will be on public display this week, when the 85-year-old Emperor Akihito abdicates Tuesday. Akihito will pass his title to his eldest male heir, 59-year-old Crown Prince Naruhito. Behind the crown prince in the line of succession is his younger brother, Prince Akishino, and after him, it’s the prince’s 12-year-old son, Prince Hisahito.

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But after him? No one. Of the 18 members of the Japanese royal family, 13 are female. There is no path to the throne for any of them.

That the Japanese royal line of ascension is so thin is a worry for many in Japan — in one incident this week, two kitchen knives were found on Prince Hisahito’s school desk, raising alarms about his security. There is now renewed debate about whether the system needs to be changed, with the government due to debate changes to the royal line of succession.

“Because this is an extremely important issue related to the foundation of the nation, we have to give careful consideration to the matter,” Prime Minister Shinzo Abe told a parliamentary committee session in March.

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It’s a modern-day problem caused by a confluence of historical factors, including not only a shrinking royal family — partially after a societal move away from concubines — but also the influence of foreign powers — in particular, a 19th-century Prussian legal scholar — on Imperial Japan.

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After Suiko, Japan had seven more female rulers, from Empress Kogyoku in the 7th century to Empress Go-Sakuramachi in the late 18th century. The possibly mythic Empress Jingu preceded Suiko by more than 300 years, though historians do not have firm evidence for her reign.

It was a desire to be more Westernized that prompted Japan to formally block female heirs in the late 19th century. After the Meiji Restoration of 1868 overthrew the feudal system, Japan’s new ruling class began looking toward Europe and the United States for reforms, hoping to remake the country as an “advanced” power while entrenching the traditional power of the Japanese emperor.

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Politician Ito Hirobumi studied various political systems before settling on the Prussian constitution of 1850 as a model. The Japanese recruited a German legal scholar, Hermann Roesler, to help devise a similar system. Prussia’s was a conservative model of government, with considerable powers placed in the king and female heirs all but a last resort for the throne.

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The royal system stood in contrast to others, such as the ones in Britain, where Queen Victoria was in the midst of her 63-year reign. But legal experts argued that it was appropriate because the new Japanese constitution would make the emperor the head of the country’s military.

The ensuing 1889 Meiji Constitution formalized this male-dominated system of government, while an accompanying Imperial Household Law made clear that male heirs were given priority over female ones in a system of agnatic seniority.

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Ironically, the Prussian system didn’t last much longer: After assuming the throne in 1888, William II was both the last King of Prussia and the last German Emperor when he abdicated after Germany’s defeat in World War I.

And after Japan’s defeat in World War II, the U.S.-occupied nation rewrote its constitution and removed most of the emperor’s powers, making the position almost entirely ceremonial — a system more closely modeled on the British monarchy. The rewritten Imperial Household Law made significant changes to the royal family, in particular limiting the size of the imperial family, but kept the rules about male succession.

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Japan isn’t the only country that still favors its male royals — many Persian Gulf royal families such as Saudi Arabia’s keep similar systems. However, many of these countries have large royal families because of a history of polygamy, a practice that fell out of favor in Japan during the Meiji era.

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During a previous period of discussion about the succession system in 2005, the emperor’s cousin mooted a return to a system of concubines to ensure a male heir. “Using concubines, like we used to, is one option,” Prince Tomohito wrote in a private letter. “I’m all for it, but this might be a little difficult considering the social climate in and outside the country.”

Indeed, polls suggest that many Japanese citizens would prefer to simply allow the imperial family to be led by a woman: Sixty-eight percent of the country was in favor, according to a 2017 poll by the newspaper Mainichi Shimbun.