About a quarter of people aged 18 to 39 in Japan were estimated to have had no experience of heterosexual intercourse as of 2015, higher than 20 percent more than two decades ago, a team of Japanese and Swedish researchers said Monday.

Researchers at the University of Tokyo and Karolinska Institutet said in an article in the British medical journal BMC Public Health that the percentage of people with no such experience rose among men to 25.8 percent in 2015 from 20.0 percent in 1992, and among women to 24.6 percent from 21.7 percent, based on data from a fertility survey conducted by the National Institute of Population and Social Security Research.

The researchers said "their lack of sexual experience may be involuntary," citing unstable job and income conditions among men as potential reasons behind the trend.

The team found around 80% of women and men aged 25 to 39 who reported no experience in a study responded that they wished to get married at some point in their life. The study only covered heterosexual experience, so there was no data on same-sex experience, it said.

"Further research is needed on the factors contributing to and the potential public health and demographic implications of the high proportion of the Japanese population that remains sexually inexperienced well into adult age," the team said.

Japan's total fertility rate stood at 1.43 in 2017, among the lowest in the world, and the National Institute of Population and Social Security Research predicted the Japanese population will fall to 88 million in 2065 from the current 126 million.

The proportion grew smaller with age, but among people aged 35 to 39, 9.5 percent of men and 8.9 percent of women had no experience, nearly doubling from 1992.

Analyzing 2010 data, the team also found that inexperience had a correlation with unemployment, temporary or part-time work and low income among men between 25 and 39 and the proportion jumped when their annual income fell below 3 million yen ($27,000).