The human sex ratio is the ratio of males to females in a population in the context of anthropology and demography. In humans, the natural sex ratio at birth is slightly biased towards the male sex. It is estimated to be about 1.05 worldwide or within a narrow range from 1.03 to 1.06 males per female at birth. The sex ratio for the entire world population including all ages is approximately 101 males to 100 females as of 2024. The sex ratios at birth and of the total population are affected by various factors including natural factors, exposure to pesticides and environmental contaminants, war casualties, effects of war on men, sex-selective abortions, infanticides, aging, gendercide, problems with birth registration, and sex differences in life expectancy. Human sex ratios, either at birth or in the population as a whole, can be reported in any of four ways: the ratio of males to females, the ratio of females to males, the proportion of males, or the proportion of females. If there are 105,000 males and 100,000 females, the ratio of males to females is 1.05 and the proportion of males is 51.2%. Scientific literature often uses the proportion of males. This article uses the ratio of males to females, unless specified otherwise.
Demographic observations
Fisher's principle and biology Fisher's principle is an explanation of why the sex ratio of most species is approximately 1:1. Outlined by Ronald Fisher in his 1930 book, it is an argument in terms of parental expenditure. Essentially he argues that the 1:1 ratio is the evolutionarily stable strategy. Many species deviate from an even sex ratio, either periodically or permanently. Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps, bees, ants, and termites. In the evolutionary biology of sexual reproduction, the operational sex ratio is the ratio of sexually competing males that are ready to mate to sexually competing females that are ready to mate, or alternatively the local ratio of fertilizable females to sexually active males at any given time. This is different from the physical sex ratio because it does not take into account sexually inactive or non-competitive individuals (individuals that do not compete for mates).
Observed ratio
The sex ratio at birth is estimated to be about 1.05 males/female worldwide as of 2024 according to the CIA. In a study around 2002, the natural sex ratio at birth was estimated to be within a narrow range of 1.03 to 1.07 males/female. Some scholars suggest that countries considered to have significant practices of prenatal sex selection are those with birth sex ratios of 1.08 and above (selection against females) and 1.02 and below (selection against males). This assumption has been questioned by some scholars. Infant mortality is significantly higher in boys than girls in most parts of the world. Often this is explained as due to biological and genetic sex differences, with boys more biologically vulnerable to premature death and disease. Numerous preconception or prenatal environmental factors affect the probabilities of a baby being conceived male or female. It has been proposed that these environmental factors also explain sex differences in mortality. In the United States, the yearly sex ratios at birth ranged from 1.046 to 1.059 between 1940 and 2002, with an overall decreasing trend. Among Western European countries around 2001, the ratios ranged from 1.04 in Belgium to 1.07 in Switzerland, Italy, Ireland and Portugal. In the aggregated results of 56 demographic and health surveys in African countries, the ratio is 1.03, but with considerable country-to-country variation.
There is controversy about whether sex ratios outside the 1.03–1.07 range are due to sex selection, as suggested by some scholars, or due to natural causes. Some scholars, including Amartya Sen, argue that strong socioeconomic factors such as the dowry system in India and the one-child policy of China are responsible for prenatal sex selection. Others argue that an unbalanced sex ratio should not be automatically held as evidence of prenatal sex selection; Michel Garenne reports that many African nations have, over decades, had birth sex ratios below 1.00 (that is, more girls are born than boys). Angola, Botswana and Namibia have reported birth sex ratios between 0.94 and 0.99, which is quite different from the presumed "normal" sex ratio, meaning that significantly more girls have been born in such countries.
Data sources and data quality issues For most of the 20th century in Russia (and the Soviet Union), extremely premature newborns (less than 28 weeks gestational age, or less than 1000 grams in weight, or less than 35 centimeters in length) were not counted as a live birth until they had survived for seven days; if that infant died in those first 168 hours it, would not be counted as an infant death. This led to serious underreporting of the infant mortality rate (by 22% to 25%) relative to standards recommended by the World Health Organization.
Some researchers have, in part, attributed the high sex ratios reported in mainland China in the last 25 years to the underreporting of the births of female children after the implementation of the one-child policy, though alternative explanations are now generally more widely accepted, including, above all, the use of ultrasound technology and sex-selective abortion of female fetuses and, probably to a more limited degree, neglect or in some cases infanticide of females. In the case of China, because of deficiencies in the vital statistics registration system, studies of sex ratios at birth have relied either on special fertility surveys, whose accuracy depends on full reporting of births and survival of both male and female infants, or on the national population census from which both birth rates and death rates are calculated from the household's reporting of births and deaths in the 18 months preceding the census. To the extent that this underreporting of births or deaths is sex-selective, both fertility surveys and censuses may inaccurately reflect the actual sex ratios at birth.
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