A quarter of adults under the age of 40 have never had heterosexual sex, according to a new study.

The results showed that the number of women and men aged 18 to 39 years old who had never had heterosexual sex increased from 21.7% and 20% to 24.6% and 25.8% respectively between 1992 and 2015, which is an increase of about 4-5% in 23 years.

Broken down, the stats showed that 11.9% of women and 12.7% of men aged 30 to 34 were virgins, as were 8.9% of women and 9.5% of men aged 35 to 39.

There are more and more old people in Japan, and women aren't having enough children to maintain the population. This has led some to refer to the country as a "demographic time bomb."

People in their 30s who still live with their parents are known as "parasitic singles," and some people are turning to virtual assistants and cartoons for companionship.

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More and more young adults in Japan aren't having sex, according to a study by public health researchers at the University of Tokyo.

In the study, published in the journal BMC Public Health, the team analyzed data from seven surveys conducted between 1987 and 2015, each with several thousand participants. They found that the number of women and men aged 18 to 39 years old who had never had heterosexual sex increased from 21.7% and 20% respectively in 1992 to 24.6% and 25.8% in 2015, which is an increase of about 4-5% in 23 years.

A breakdown of the figures revealed that 11.9% of women and 12.7% of men aged 30 to 34 were virgins, as were 8.9% of women and 9.5% of men aged 35 to 39.

According to the results, men were more likely to have had sex if they had a permanent job and lived in big cities of more than a million people. Those with the lowest income were between 10 and 20 times more likely to be virgins.

Women, on the other hand, were less likely to be virgins if they made less money. The researchers said this could be because they are married homemakers.

"Although the discussion around cause and effect becomes very complex when considering who becomes sexually experienced and who remains a virgin, we show that heterosexual inexperience is at least partly a socioeconomic issue for men," said Cyrus Ghaznavi, one of the authors of the study. "Simply put, money talks."

Read more: 9 signs Japan has become a 'demographic time bomb'

Compared to other countries, the rates of people with no heterosexual experience are about 1-5% of people in their 30s in the UK, US, and Australia.

"Sexual inactivity or inexperience, whether voluntary or not, should not be exoticized, ridiculed, or necessarily considered a concern for everyone," said Peter Ueda, who led the research.

"More research is needed on reasons for sexual inactivity and how mating market dynamics might be evolving due to online dating, shifting expectations in romantic and sexual relationships, and changing values, lifestyles, and labor market trends."

Japan's population dropped by 394,373 people in 2017, leading some to refer to the country as a "demographic time bomb." It has an ageing population, and the current birth rate is 1.44 children per woman, which is far below what the country needs to maintain its population.

Business Insider reporter Valentina Resetarits lived in Japan for a year, and she noticed how the culture seems to "emphasize hard work, and de-emphasize sex and relationships."

"In addition, there is the strange Japanese relationship to sexuality," Resetarits wrote. "You would think that people are looking for one-night stands if they don't want to have a relationship, but many people are too uptight for that.

"When in doubt, they prefer to concentrate on fictional sex objects in manga porn or video games instead of chatting at a bar."

Hatsune Miku. NurPhoto / Getty

In fact, last year a man married a hologram of a virtual reality singer called Hatsune Miku. Acting as a virtual assistant, the floating hologram device wakes Akihiko Kondo up, turns on his lights, and talks to him. In return, he keeps stuffed dolls of her in his bed.

"Miku-san is the woman I love a lot and also the one who saved me," he said.

There is also an increasing population of "parasitic singles" in Japan — people in their 30s who still live with their parents, which is likely another factor in people not pairing up.

The new study didn't look into same-sex experiences, or those who are not virgins but have become celibate later in life for different reasons.

"The most informative aspect of sexual inactivity involves those who have opted out of or are, for some reason they cannot control, excluded from the mating market, regardless of their previous sexual experience," Ghaznavi said. "We'd like to investigate those dynamics in future work."

Read more: Almost half of Japanese people aren't having sex — and it could endanger the global economy

Japan isn't the only place with a population problem.The picturesque town of Locana in Italy recently offered people $10,000 to move there, and the only catch is they have to have a child.

"Our population has shrunk from 7,000 residents in the early 1900s to barely 1,500 as people left looking for a job at Turin's big factories," said Locana's mayor to CNN Travel. "Our school each year faces the risk of shutting down due to few pupils. I can't allow this to happen."