
Manga and anime advertising posters on a building in Akihabara, Tokyo
Tokyo is the world's largest metropolis and home to more than 35 million people, so on the face of it, it is hard to believe there is any kind of population problem at all.
But Akihabara, an area of the city dedicated to the manga and anime subculture provides one clue to the country's problems.
Akihabara is heaven for otaku.
They are a generation of geeks who have grown up through 20 years of economic stagnation and have chosen to tune out and immerse themselves in their own fantasy worlds.
Kunio Kitamara, of the Japan Family Planning Association, describes many young Japanese men as "herbivores" - passive and lacking carnal desire.
It seems they no longer have the ambition of the post-war alpha males who made Japan such an economic powerhouse and no interest in joining a company and becoming a salary man.
They have taken on a mole-like existence and, worryingly, withdrawn from relationships with the opposite sex.
A survey by the Ministry of Health, Labour and Welfare in 2010 found 36% of Japanese males aged 16 to 19 had no interest in sex - a figure that had doubled in the space of two years.
I met two otaku, who believe themselves to be in relationships with virtual girlfriends.
This girlfriend is actually a Nintendo computer game called Love Plus, which comes as a small portable tablet.
Nurikan and Yuge take their girlfriends, Rinko and Ne-ne, on actual dates to the park, and buy them cakes to celebrate their birthdays.
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