"They believe that if a woman ascends to the throne and then gives birth to someone who ascends the throne, then the unbroken blood line would be broken," said Kenneth Ruoff, director of the Center for Japanese Studies at Portland State University in Oregon.
Although Aiko turns 3 on Wednesday, people are already thinking ahead to when she is of marriageable age.
"In today's Japan, a princess in line to inherit the throne might have a terrible time finding a spouse willing to play the secondary role," said Ruoff, author of "The People's Emperor: Democracy and the Japanese Monarchy, 1945-1995."
Finding a consort has always been a tricky business for women who wear crowns in their own right. Elizabeth I of England spent decades dodging the marriage question, perhaps because she had seen the disasters that arose when her fellow queens, Mary I of England and Mary, Queen of Scots, chose husbands who didn't meet the people's approval. (Of course, having a father who beheaded her mother and one of her stepmothers might have given the "Virgin Queen" a sour view of marriage.)
Even in modern times, prospective royal husbands have faced opposition from palace officials and the populace. Some grumbled when the future Elizabeth II married Prince Philip, whose four sisters were all married to German princelings, even though Philip himself had served in the British Royal Navy during World War II. When the future Queen Beatrix of Holland married German diplomat Claus von Amsberg in 1966, there were protests because he had served in the armed forces under the Nazis.
Princess Catharina-Amalia, Elisabeth of Belgium and Norway's Ingrid Alexandra are all too young to be worrying about marriage -- after all, Ingrid Alexandra doesn't even have hair. But at 27, Victoria of Sweden is constantly confronted with conjectures about when -- and who -- she will marry.
The princess' commoner boyfriend, Daniel Westling, is the current focus of speculation.
"But the problem is, he's not exactly [seen as] royal material," said The Local's Rapacioli. "He's a fitness instructor who allegedly doesn't speak great English and is shy about all the media attention."
Swedes have also noticed that since Westling started dating his future queen, his gym has been making a lot more money, Rapacioli said.
"In a sense he can't win," said Rapacioli. "If he's a bog-standard fitness instructor he's mocked for not being sophisticated or educated enough to be royalty, but if he's the owner of a thriving fitness empire he's accused of having used his royal connection."
Victoria herself recently admitted that the unrelenting attention can make life difficult for any man in her life.
"It's not easy to be together with me," she told Sweden's TV4, "but the situation is the same for anyone who's in the spotlight."
Which may be one way of saying that if Westling is going to stick around, he'd better get used to it.