In Japan, the population sex ratio has seen slight changes over the past decades. In 2021, the number of men was around 94.6 for every 100 women, constituting a decrease from 96.1 in 1950.

What is the sex ratio? The population sex ratio is determined by the sex ratio at birth, different mortality rates between men and women, as well as losses and gains through migration. In the absence of alteration, the sex ratio in human populations is quite constant, with only minor deviations. While the sex ratio at birth is usually 105 to 107, the population sex ratio, which refers to the total number of males for every 100 females, is often below 100. The reason for the shift mostly lies in differing lifestyles and physical constitutions of men and women. In general, women tend to be more resistant to disease throughout life, while men tend to engage in higher risk behavior or violence.

Influences and consequences
The sex ratio at birth and its possible determinants such as gestation environment, climate change, chemical pollution or socio-economic factors have long been subject to scientific research. Recently the impact of natural disasters, like the nuclear disaster in Japan in 2011, was presumed to influence the sex ratio at birth. The adult gender ratio has long been recognized as a key population-level determinant of behavior. However, there are many different or competing theories in existing literature about the social impacts of gender imbalance on topics such as violence, family stability, reproduction etc.

Sex ratio at birth (male births per female births) in Japan was reported at 1.051 in 2023, according to the World Bank collection of development indicators, compiled from officially recognized sources. Japan - Sex ratio at birth (male births per female births) - actual values, historical data, forecasts and projections were sourced from the World Bank on July of 2025.

Japan JP: Sex Ratio at Birth: Male Births per Female Births data was reported at 1.056 Ratio in 2016. This stayed constant from the previous number of 1.056 Ratio for 2015. Japan JP: Sex Ratio at Birth: Male Births per Female Births data is updated yearly, averaging 1.056 Ratio from Dec 1962 (Median) to 2016, with 20 observations. The data reached an all-time high of 1.056 Ratio in 2016 and a record low of 1.056 Ratio in 2016. Japan JP: Sex Ratio at Birth: Male Births per Female Births data remains active status in CEIC and is reported by World Bank. The data is categorized under Global Database’s Japan – Table JP.World Bank: Population and Urbanization Statistics. Sex ratio at birth refers to male births per female births. The data are 5 year averages.; ; United Nations Population Division. World Population Prospects: 2017 Revision.; Weighted average;

Japan Sex Ratio: Inter Prefectural Migrants data was reported at 127.700 % in May 2018. This records a decrease from the previous number of 135.100 % for Apr 2018. Japan Sex Ratio: Inter Prefectural Migrants data is updated monthly, averaging 128.200 % from Jan 1998 (Median) to May 2018, with 245 observations. The data reached an all-time high of 146.600 % in Apr 1998 and a record low of 114.200 % in Mar 2018. Japan Sex Ratio: Inter Prefectural Migrants data remains active status in CEIC and is reported by Statistical Bureau. The data is categorized under Global Database’s Japan – Table JP.G007: Vital Statistics: Migration.

In 2023, the male population was estimated to reach approximately 60.46 million, while the female population was estimated at around 63.95 million. These figures were forecasted to shrink to about 24.1 million men and 25.64 million women by 2120.

Sex ratio theory suggests that females should bias offspring sex ratios based on maternal condition and the availability of critical food resources. Work in birds indicates that females do, indeed, bias sex ratios according to maternal condition and food quality and quantity. Yet it is unknown whether these sex ratio skews occur due to fluctuations in particular micro- or macro-nutrients, caloric content overall, or even the perception of food availability. We hypothesized that dietary fats may drive biases in offspring sex ratios, because measures of maternal condition often reflect fat reserves, and fats are critical for the process of egg-laying in birds. To test this, we provided breeding Japanese quail, a species that biases sex ratios in response to maternal condition, with either a control breeding diet or a diet supplemented with two oils (safflower oil and flaxseed oil). These oils were chosen for their high omega-3 and omega-6 fatty acid content as well as their importance in mammalian sex allocation. We then measured influences of these diets on the sex ratio of offspring, the change in maternal weight, and the laying rates of female quail. The dietary oil supplements increased weight gains in quail but decreased the number of eggs laid during the experiment. There was no influence of the oil supplements on offspring sex ratios. This indicates that fat may not be a macro-nutrient involved in the process of sex ratio adjustment in quail.

In 2023, the total population in Japan slightly decreased to just below 125 million people compared to the previous year, with the female population reaching around 63.9 million, as compared to 60.5 million men. The oldest population in the world  Japanese society is facing severe demographic problems such as decreasing birthrates, remaining under one million births annually recently, and a thereby aging population. The country’s average age lies at around 48 years, making its population the oldest in the world. Elderly people aged 65 years and older accounted for about 29 percent of the population in 2023. According to a forecast, the age group 65 years and older would make up approximately 39 percent of the Japanese population by 2070. Challenges with the demographic shift The rapid aging of the society poses significant economic and sociopolitical challenges to the country, as the workforce will continue to shrink while increasingly more elderly will receive long-term support. Currently, close to seven million Japanese require long-term care, leading to national benefit expenses of over 14 trillion yen annually, including in-home and community-based services.

Japan Sex Ratio: Annual: Intra Prefectural Migrants data was reported at 104.700 % in 2017. This records a decrease from the previous number of 104.800 % for 2016. Japan Sex Ratio: Annual: Intra Prefectural Migrants data is updated yearly, averaging 102.600 % from Dec 1958 (Median) to 2017, with 60 observations. The data reached an all-time high of 107.200 % in 2009 and a record low of 99.600 % in 1959. Japan Sex Ratio: Annual: Intra Prefectural Migrants data remains active status in CEIC and is reported by Statistical Bureau. The data is categorized under Global Database’s Japan – Table JP.G006: Vital Statistics: Migration.

Japan Sex Ratio: Internal Migrants data was reported at 113.700 % in May 2018. This records a decrease from the previous number of 125.100 % for Apr 2018. Japan Sex Ratio: Internal Migrants data is updated monthly, averaging 113.500 % from Jan 1998 (Median) to May 2018, with 245 observations. The data reached an all-time high of 131.100 % in Apr 2009 and a record low of 104.400 % in Nov 1999. Japan Sex Ratio: Internal Migrants data remains active status in CEIC and is reported by Statistical Bureau. The data is categorized under Global Database’s Japan – Table JP.G007: Vital Statistics: Migration.

In 2024, the total labor force in Japan was composed of approximately **** million people, among which around ** million were men and close to **** million were women. While the number of men in the workforce declined, the number of women in the labor force increased, resulting in an increase in the total labor force.

JP: Population: as % of Total: Female: Aged 0-14 data was reported at 12.256 % in 2017. This records a decrease from the previous number of 12.304 % for 2016. JP: Population: as % of Total: Female: Aged 0-14 data is updated yearly, averaging 18.671 % from Dec 1960 (Median) to 2017, with 58 observations. The data reached an all-time high of 29.143 % in 1960 and a record low of 12.256 % in 2017. JP: Population: as % of Total: Female: Aged 0-14 data remains active status in CEIC and is reported by World Bank. The data is categorized under Global Database’s Japan – Table JP.World Bank: Population and Urbanization Statistics. Female population between the ages 0 to 14 as a percentage of the total female population. Population is based on the de facto definition of population.; ; World Bank staff estimates based on age/sex distributions of United Nations Population Division's World Population Prospects: 2017 Revision.; Weighted average; Relevance to gender indicator: Knowing how many girls, adolescents and women there are in a population helps a country in determining its provision of services.

Population, female (% of total population) in Japan was reported at 51.19 % in 2023, according to the World Bank collection of development indicators, compiled from officially recognized sources. Japan - Population, female (% of total) - actual values, historical data, forecasts and projections were sourced from the World Bank on July of 2025.

The representation of women in Japanese higher education continues to grow, with female students comprising **** percent of university enrollments in 2024. This marks a steady increase over the past decade, reflecting changing societal attitudes and educational opportunities for women in Japan. Study field preferences Female students enrolled at universities in Japan exhibit a strong interest in the ***************, with the highest number of female undergraduates majoring in the subject in 2024. At the postgraduate level, the *********** field had the highest number of female students in the same year. When it comes to gender distribution, ******************************************among others, attracted a higher share of women than men in postgraduate studies. Employment prospects The rising female university enrollment is translating into positive career outcomes. In 2024, over 80 percent of female university graduates in Japan entered employment after completing their studies. It is worth noting that this proportion was much lower among women with postgraduate degrees, with below ** percent of those with master’s and doctoral diplomas securing employment after graduation.

In 2022, there were around 1.11 male births per female births in China and Vietnam, respectively. Comparatively, in Sri Lanka, the gender ratio was 1.04 male to female births that year.

Population ages 00-04, female (% of female population) in Japan was reported at 3.0888 % in 2024, according to the World Bank collection of development indicators, compiled from officially recognized sources. Japan - Population ages 0-4, female (% of female population) - actual values, historical data, forecasts and projections were sourced from the World Bank on July of 2025.

Japan Sex Ratio: Intra Prefectural Migrants data was reported at 103.300 % in May 2018. This records a decrease from the previous number of 114.400 % for Apr 2018. Japan Sex Ratio: Intra Prefectural Migrants data is updated monthly, averaging 103.300 % from Jan 1998 (Median) to May 2018, with 245 observations. The data reached an all-time high of 118.500 % in Apr 2009 and a record low of 94.700 % in Nov 1999. Japan Sex Ratio: Intra Prefectural Migrants data remains active status in CEIC and is reported by Statistical Bureau. The data is categorized under Global Database’s Japan – Table JP.G007: Vital Statistics: Migration.

The total number with data available for all parameters according to the method of pregnancy. CT (early cleavage-stage embryo transfer), BT (blastocyst transfer), smoking and alcohol consumption habits at pregnancy recognition, SF-8 score of 50 or less. Low education: defined by the highest academic background and categorized as high school graduation or lower, Low income: defined by the annual revenue of the household and categorized as an annual income of 4,000,000 Japanese yen or less. (XLS 47 kb)

【リソース】Volume 1_6-1_Trends in infant deaths, infant mortality rates (per 1,000 live births), sex ratio and proportion of infant deaths to total deaths:Japan / Volume 1_6-2_Trends in neonatal deaths, neonatal mortality rates (per 1,000 live births), sex ratio and proportion of neonatal deaths to infant deaths:Japan / Volume 1_6-3_Infant mortality rates (per 100,000 live births) and percent distribution by sex and age (days, weeks and months):Japan, 2017 / Volume 1_6-4_Trends in infant mortality rates (per 100,000 live births) by sex and age (days, weeks and months):Japan / Volume 1_6-5_Trends in infant deaths and infant mortality rates (per 1,000 live births) by month:Japan / Volume 1_6-6_Live births, infant deaths and infant mortality rates (per 1,000 live births) by months of birth:Japan / Volume 1_6-7_Trends in infant deaths and percent distribution by place of occurrence:Japan / Volume 1_6-8_Percent distribution of infant deaths by place of occurrence:Japan, each prefecture and 21 major cities, 2017 / Volume 1_6-9_Infant deaths, infant mortality rates (per 1,000 live births) and percent distribution by type of occupation of household:Japan, 2017 / Volume 1_6-10_Trends in infant deaths by each prefecture:Japan / Volume 1_6-11_Trends in infant mortality rates (per 1,000 live births) by each prefecture:Japan / Volume 1_6-12_Infant mortality rates (per 100,000 live births) by age and proportion of neonatal deaths:Japan, each prefecture and 21 major cities, 2017 / Volume 1_6-13_Trends in infant deaths and infant mortality rates (per 100,000 live births) by causes (the list of causes of infant death):Japan / Volume 1_6-14_Infant deaths and infant mortality rates (per 100,000 live births) by age and causes (the list of causes of infant death):Japan, 2017 / Volume 1_6-15_Percent distribution of infant deaths by age and causes (the list of causes of infant death):Japan, 2017 / Volume 1_6-16_Trends in leading causes of neonatal and infant death:Japan / Volume 1_6-17_Infant mortality rates (per 100,000 live births) by causes (the list of causes of infant death):Japan, each prefecture and 21 major cities, 2017 / Volume 1_6-18_Infant deaths and infant mortality rates (per 100,000 live births) by diseases, causes (the list of causes of infant death) and birth weight:Japan, 2017 / Volume 1_6-19_Neonatal deaths and neonatal mortality rates (per 100,000 live births) by diseases, causes (the list of causes of infant death) and birth weight:Japan, 2017 / Volume 2_1_Infant deaths (under 1 year), neonatal deaths (under 4 weeks) by sex and month of occurrence:Japan, urban/rural residence, each prefecture and 21 major cities / Volume 2_2_Infant deaths (under 1 year) by age and sex:Japan, urban/rural residence, each prefecture and 21 major cities / Volume 2_3_Infant deaths (under 1 year) by age, sex and type of occupation of household:Japan / Volume 2_4_Infant deaths (under 1 year) by month of occurrence and date of birth:Japan / Volume 2_5_Infant deaths (under 1 year) from diseases, birth weight and mean birth weight by sex, plurality of birth and age of mother:Japan / Volume 2_6_Infant deaths (under 1 year) from diseases, birth weight and mean birth weight by sex, plurality of birth and birth order:Japan / Volume 2_7_Infant deaths (under 1 year) from diseases, birth weight and mean birth weight by sex and period of gestation:Japan / Volume 3_1_Infant deaths (under 1 year) by causes (the list of causes of infant death), sex and age:Japan / Volume 3_2_Infant deaths (under 1 year) and neonatal deaths (under 4 weeks) by causes (the list of causes of infant death) and sex:Japan, each prefecture and 21 major cities / Volume 3_3_Infant deaths (under 1 year) and neonatal deaths (under 4 weeks) by causes (the list of causes of infant death), sex and month of occurrence:Japan / Volume 3_4_Infant deaths (under 1 year), neonatal deaths (under 4 weeks) and early neonatal deaths (under 1 week) from diseases by causes (the list of causes of infant death), sex and birth weight:Japan / Volume 3_5_Infant deaths (under 1 year), neonatal deaths (under 4 weeks) and early neonatal deaths (under 1 week) from diseases by causes (the list of causes of infant death), sex and specified period of gestation:Japan / Vital Statistics_Vital statistics of Japan_Final data_Infant mortality_Yearly_2017 / Volume 1_6-1_Trends in infant deaths, infant mortality rates (per 1,000 live births), sex ratio and proportion of infant deaths to total deaths:Japan,Volume 1_6-2_Trends in neonatal deaths, neonatal mortality rates (per 1,000 live births), sex ratio and proportion of neonatal deaths to infant deaths:Japan,Volume 1_6-3_Infant mortality rates (per 100,000 live births) and percent distribution by sex and age (days, weeks and months):Japan, 2017,Volume 1_6-4_Trends in infant mortality rates (per 100,000 live births) by sex and age (days, weeks and months):Japan,Volume 1_6-5_Trends in infant deaths and infant mortality rates (per 1,000 live births) by month:Japan,Vo

Supplementary Material 3

In anurans, female polyandry under male harassment is distributed across taxa because of external aquatic fertilisation. According to the sexual selection theory, male–male competition for access to females is affected by the operational sex ratio (OSR) and population density. The Japanese common toad, Bufo japonicus, is widespread in mainland Japan, and like the European common toad, B. bufo, it engages in explosive breeding. We observed the breeding behaviour of B. japonicus in isolated local populations for three years in two breeding ponds with different population sizes and densities: large-low (L-pond) and small-high (S-pond). We analysed the relative polyandry ratio in egg clutches laid by females and estimated the size-assortative mating pattern to be an indicator of male–male competition in the two ponds. Both ponds tended to exhibit a size-assortative mating pattern; however, the frequency of polyandry was different in the two ponds (L-pond = 20% and S-pond = 90%). We also found that polyandry could occur without multiple amplexus with a high population density, i.e. eggs were often fertilised by free-swimming sperm in the small shallow pond. We propose that high female polyandry ratios without continuous male harassment are generated because of a male-biased OSR and a high population density in the small pond.