Sexual activity is a key component of human fertility and well-being [9,10,11,12]. In the face of low fertility rates and a shrinking population, sexual inexperience among young adults in Japan has been subject to concern and speculation [3,4,5,6]. Our study expands on available data on heterosexual inexperience in Japan by providing national estimates of prevalence by age group and by assessing socioeconomic and regional factors associated with having no heterosexual experience.
Between 1992 and 2015, age-standardized prevalence of heterosexual inexperience had increased among Japanese adults aged 18–39 years. In 2015, prevalence was 11.9% (women) and 12.7% (men) in those aged 30–34 years, and 8.9% (women) and 9.5% (men) in those aged 35–39 years; these numbers translate to 1.56 million adults in their thirties who had never engaged in heterosexual intercourse.
While national surveys on sexual behaviour in other high-income countries in Asia are scarce, data on sexual inexperience are available for some non-Asian high-income countries, e.g., the UK,  the US,  Australia,  Denmark  and New Zealand . Compared to adults in these countries, [18,19,20,21,22] our results indicate that Japanese adults tend to become sexually active later in life and that a substantially larger proportion remain heterosexually inexperienced into their thirties. For example, in the nationally representative Natsal-3 study in the UK, (2011, 19] the proportion of women who reported no sexual partners of the opposite sex (defined as someone with whom the survey participant had engaged in vaginal, oral or anal intercourse) over the lifetime was 19.8% (16–24 years), 2.6% (25–34 years) and 0.5% (35–44 years); the corresponding numbers for men were 19.8, 5.2 and 1.5% . In the 2006–2008 National Survey of Family Growth in the US,  the proportion of women reporting no sexual partners of the opposite sex after the age of 18 years were 12.6% (20–24 years), 3.4% (25–29 years), 1.9% (30–34 years) and 0.9% (35–39 years); the corresponding numbers for men were 14.4, 3.8, 3.1 and 1.4% . In a nationally representative survey in Australia (2012–2013), the proportion of the population reporting no experience of vaginal intercourse was 40.0% (16–19 years), 10.9% (20–29 years) and 1.2% (30–39 years) for women and 35.0% (16–19 years), 9.6% (20–29 years) and 1.8% (30–39 years) for men . The reasons for the substantially larger proportion of Japanese adults who report no heterosexual experience, as compared with those in other high-income countries, remain to be investigated.
Some of those who reported no heterosexual experience in our study most likely have experience from same-sex sexual activities. Studies from other countries show that between 2 and 5% of the population identify as having a sexual orientation other than heterosexual, [19, 20] although some of these individuals may have engaged in heterosexual intercourse and not all of them have same-sex sexual experience. Nonetheless, even if 5% of the young adults in Japan were to have engaged in same-sex sexual activities without ever having had heterosexual intercourse, around one in twenty 30-to-39-year-old women and men, according to our findings, would still lack sexual experience.
While positive sexual experiences are important contributors to quality of life, [9,10,11,12] some individuals may not consider the absence of sex as a source of dissatisfaction. In the Natsal-3 study in the UK, a substantial proportion of those with previous experience of sexual activity who had been sexually inactive in the year preceding survey participation did not report dissatisfaction with their (lack of) sex life . Similarly, in a US study, those who had been sexually inactive for a year reported similar levels of happiness as their sexually active counterparts . In Japan, it has been reported that some adults do not consider intimate relationships as being of high priority in their lives, with financial insecurity and long working hours possibly contributing to the purported trend [3, 24, 25]. The choice of not pursuing intimate relationships is gaining attention and acceptance, being described using terms such as “soshokukei-danshi”(herbivore men) and “sekkusu-banare” (drifting away from sex) [3, 24, 25]. However, difficulties in finding sexual partners may also explain sexual inexperience. In fact, around 80% of women and men aged 25–39 years who reported no heterosexual experience in our study responded that they wished to get married in their lifetime, indicating that their lack of sexual experience may be involuntary. The associations between several socioeconomic/regional variables and sexual inexperience in our study further supported this notion. Importantly, while the negative association of low income and unemployment with sexual inexperience among women aged 25–39 years could be explained by the relatively high proportion of married women who are housewives,  lower income as well as unstable or no employment were strongly associated with sexual inexperience among men. Although low income and the lack of sexual opportunity may share common determinants, these findings are in line with studies showing that high [27, 28] and stable  incomes are important predictors of attractiveness in the mating market, especially for men [8, 29]. Indeed, income is associated with marriage among Japanese men,  and it has been suggested that the decrease in stable employment opportunities over recent decades has contributed to low marriage and fertility rates in Japan [7, 31]. Japan’s low fertility rates persist despite the government’s efforts to encourage marriage, pregnancy and child rearing, including matchmaking events, fertility education programs and policies to improve work-life balance and childcare . The situation of individuals who remain sexually inexperienced due to difficulties in finding a partner, as highlighted by our findings, may be considered in future policies aimed at increasing Japan’s birth rate.
Our study has limitations. First, as our analyses relied on self-reported data, results might be affected by under- or over-reporting of heterosexual inexperience due to social desirability bias;  however, the risk of such a bias may have been mitigated by the survey’s use of self-administered questionnaires . Second, although the response rate in the National Fertility Survey was high (70.0 to 83.8% among unmarried persons and 85.7 to 92.5% among married couples)  and the sample was weighted to ensure that it was broadly representative of the Japanese population with respect to sex, age and marital status, non-response may have introduced bias into our findings. Third, data on stratification and sampling units used in the surveys were not available. While this did not affect the point estimates, [34, 35] the standard errors of the estimated prevalence and odds ratios could potentially have been influenced: the lack of stratification data may have led to an overestimation of the standard error, and the omission of sampling units may have underestimated the standard error [34, 35]. Confidence intervals in our analyses should therefore be interpreted with caution. Fourth, although the term seikosho used in the question regarding sexual experience is commonly used to refer to vaginal intercourse, the definition of this term was not specified in the survey and may have been interpreted differently by some participants. Fifth, our analyses underestimate the level of sexual inactivity in the population as those who have previous sexual experience may still be sexually inactive. Sexual inactivity may be more informative to assess than sexual inexperience with respect to the implications for fertility and public health. Sixth, as individual-level data were not available for the 2015 survey, we assessed factors associated with heterosexual inexperience in the 2010 survey. Seventh, as the question regarding sexual inexperience was only asked to unmarried individuals, we assumed that those who were or had been married had heterosexual experience; this may have led to an underestimation of sexual inexperience. Finally, as the information provided in the National Fertility Survey was limited to heterosexual experience, we could not assess same-sex sexual experience. There is a dearth of data on gay, lesbian, bisexual and transgender groups in Japan,  and questions targeted to the experiences of these groups should be included in future surveys.