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 the 20 percent seen more than two decades ago, a team of Japanese and Swedish researchers said Monday.

Researchers at the University of Tokyo and Karolinska Institutet wrote in an article in U.K. medical journal BMC Public Health that the percentage of people with no such experience had risen 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 (NIPSSR).

The study polled people at random and covered only heterosexual experience, so presented no data on same-sex experience.

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

The team found that around 80 percent of women and men aged 25 to 39 who reported having had no such experience in the study responded that they wished to get married at some point in their lives.

“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.

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 of heterosexual intercourse — nearly double the figure seen in 1992.

Analyzing data from 2010, the team also found that inexperience correlated with unemployment, temporary or part-time work and low income among men between 25 and 39. In particular, the proportion without experience jumped when annual income fell below ¥3 million ($27,000).

Japan’s total fertility rate stood at 1.43 in 2017 — among the lowest in the world — and NIPSSR has predicted that the country’s population will fall to 88 million in 2065 from the current 126 million.