Japanese adults have their first heterosexual sexual experience later than their counterparts in the US and UK, according to a new study.

Public health experts at the University of Tokyo found that sexual inexperience was on the rise in the country, with the percentage of women aged 18 to 39 who'd never had sex rising to 24.6% in 2015 up from 21.7% in 1992.

The change was greater for men of the same age, with 25.8% virgins in 2015 up from 20% in 1992.

"Sexual inexperience has become a national concern in Japan, but previous reports did not examine the trend across different age groups and socio-economic backgrounds," Peter Ueda, a public health researcher at the University of Tokyo and an author of the study that was published Monday, said.

By comparison, surveys from the UK, US and Australia suggest that rates of heterosexual inexperience are between 1% to 5% of adults in or around their 30s, said the paper, which was published in the journal BMC Public Health.

The authors said while it had been suggested that an increasing number of Japanese adults were sexually inexperienced, their study was the first to address this issue using nationally representative data.

The paper defined heterosexual sexual experience as vaginal intercourse between men and women. It did not include responses from those identifying as LGBTQ or asexual, and assumed that all married couples had had sex.

The researchers based their nationwide estimate on data from seven national fertility surveys conducted over three decades. Each drew on the responses of between 11,553 and 17,859 adults 18 to 39 years old.

'Money talks'

The report found that a higher percentage of men on lower incomes remained sexually inexperienced compared to women.

"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. Simply put, money talks," said Cyrus Ghaznavi, the lead author of the study.

The issue of birthrate is important in Japan because its population is aging rapidly.

Japan is already considered a "super-aged" nation -- meaning that more than 20% of its population is over 65 years old. Just 946,060 babies were born in 2017, a record low since official records began in 1899.

The decline means a shrinking cohort of workers is left supporting an increasingly elderly population in need of healthcare and pensions, explained Kukhee Choo, a media studies professor at Sophia University in Tokyo, who wasn't involved in the research. Choo added that putting a spotlight on the rise of sexlessness is part of a domestic agenda to rectify the population decline.

In recent years, local governments have boosted their efforts to pair off heterosexual singles in a bid to reverse the country's declining fertility rate.

"There seems to be a national push to get people to really think about this issue and feel a sense of urgency," Choo said.

Furthermore, despite Japan's multibillion-dollar porn industry, Choo said discussions around porn and sex remain taboo. "Sex is seen as dirty and corrupting in Japan," Choo said.

"My students can't use the word 'penis' or 'vagina,' and if a woman expresses any knowledge or interest in sex, you're seen as a fallen women. Men don't talk about it either."

Economy link

Conservative attitudes around sex haven't always been the norm in Japan, according to Choo. Many Japanese embraced Western notions of free love and sexual liberation following Japan's defeat in WWII in 1945 and the introduction of US values, said Choo.

As Japan underwent an "economic miracle" from the late 1940s to the the late '80s during which it became the world's second largest economy after the US, Japan's male white-collar workers increased.

Though expected to work long hours, corporations rewarded them with stable middle-class lifestyles and lifetime employment. Such men became the dominant idea of masculinity.

During those decades, companies in cities and densely populated towns became the breeding ground for new romances as they looked favorably upon marriages between staff.

"They'd actively hire people who they thought would be a good match for one another and organize sports days and company holidays to facilitate matches between people," said Shigeru Kashima, a Japanese studies professor at Meiji University.

But by the '90s, Japan's economic boom went bust, resulting in a long period of financial insecurity and economic stagnation that came to be known as the "lost decade."

Kashima argues that men's loss of economic clout following the country's period of financial stagnation affected their confidence when it came to pursuing women.

"Over the past two decades, some Japanese men have found it hard to face external hardships and fear rejection," said Kashima. "There's also an attitude of men devoting themselves more to their hobbies compared to women dedicating themselves to work."

Redefining virginity and sex

Choo said that while some Japanese heterosexuals may be inexperienced sexually, relationships mean different things to different people.

"Some people are in a relationship and they're sexless. Some people don't want to be in a relationship because they don't want to be sexual. This is all going on at the same time among young people in Japan," said Choo.

Ueda, the co-author of the study, said the heterosexual virgins surveyed in the report expressed an interest in marrying later on in life, suggesting their sexual inexperience wasn't by choice.

And while the reasons behind rising heterosexual virginity rates are complex, virtual sex and porn consumption have also been blamed for the decline of heterosexual relations in Japan.

"You could say that there is a demand for alternative sexual or romantic experiences because people are finding it difficult to find partners, or you could hypothesize that the rise in these services are because some people prefer to express their sexuality in these ways," Ueda said.

Ueda's team also suggested that Japan could be leading a global trend toward sexual inexperience between heterosexual couples, with sexual inactivity rising among young adults in Germany and the US, suggesting a global shift in attitudes on sex.