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 .
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,Very high sex ratios were common in even late medieval Europe, which may indicate sex-selective infanticide. Josiah Cox Russell, 1958, Late Ancient and Medieval Population, pp. 13–17. 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 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,Sen, Amartya (1990), More than 100 million women are missing, New York Review of Books, 20 December, pp. 61–66 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 1.00Michel Garenne, Southern African Journal of Demography, Vol. 9, No. 1 (June 2004), pp. 91–96 (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.Michel Garenne, Southern African Journal of Demography, Vol. 9, No. 1 (June 2004), p. 95
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.For example, the number of births reported to family planning and other administrative agencies has been significantly lower than the number determined in population surveys and the census. See Zhang, Guangyu (April 2004) "Very Low Fertility in China in the 1990s: Reality or An Illusion Arising from Birth Underreporting?" Paper presented at the annual meeting of the Population Association of America. 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.For studies reveal underreporting or delayed reporting of female births in China, see and
History The human sex ratio at birth has been an object of study since early in the history of statistics, as it is easily recorded and a large number for sufficiently large populations. An early researcher was John Arbuthnot (1710), who in modern terms, performed statistical hypothesis testing, computing the p-value (via a sign test), interpreted it as statistical significance, and rejected the null hypothesis.
Human sex at birth was also analyzed and used as an example by Jacob Bernoulli in Ars Conjectandi (1713), in which an unequal sex ratio is a natural example of a Bernoulli trial with uneven odds. Willem 's Gravesande (1774) also studied it. Pierre-Simon Laplace (1778) used human sex ratio as an example in his development of probability theory. He considered the statistics of almost half a million births; the statistics showed an excess of boys compared to girls. He concluded by calculation of a p-value that the excess was a real, but unexplained, effect.
Factors affecting sex ratio in humans
Natural factors Scientific studies on human sex ratio are based on extensive birth and death records in Africa, the Americas, Asia, Australia, and Europe. A few of these studies extend to over 100 years of yearly human sex ratio data for some countries. These studies suggest that the human sex ratio, both at birth and as a population matures, can vary significantly according to a large number of factors, such as paternal age, maternal age, multiple births, birth order, gestation weeks, race, parent's health history, and parent's psychological stress. The trends in human sex ratio are not consistent across countries at a given time, or over time for a given country. In economically developed countries, as well as developing countries, these scientific studies have found that the human sex ratio at birth has historically varied between 0.94 and 1.15 for natural reasons. While considering the race, human sex ratio at birth is about 1.03 for Blacks and 1.06 for Whites.
In a scientific paper published in 2008, the author states that conventional assumptions have been: there are equal numbers of X and Y chromosomes in mammalian sperm X and Y stand equal chance of achieving conception therefore equal numbers of male and female zygotes are formed therefore any variation of sex ratio at birth is due to sex selection between conception and birth.
The author cautions that available scientific evidence stands against the above assumptions and conclusions. He reports that there is an excess of males at birth in almost all human populations, and the natural sex ratio at birth is usually between 1.02 and 1.08. However, the ratio may deviate significantly from this range for natural reasons.
A 2015 study showed that human sex ratio at conception is roughly 50%, but rises due to total mortality surplus of female embryos. A dataset of 139,704 embryos derived from assisted reproductive technology showed a male sex ratio of 50.2%. A dataset of 4,999 embryos from induced abortions showed a rate of 51.1% for the first trimester and 55.9% for the last two trimesters.
Multiple birth A 1999 study reported the sex ratio for 815,891 children born in Denmark between 1980 and 1993. They studied the birth records to identify the effects of multiple birth, birth order, age of parents and the sexes of preceding siblings on the proportion of males using contingency tables, chi-squared tests and regression analysis. The secondary sex ratio decreased with increasing number of children per plural birth and with paternal age, whereas no significant independent effect was observed for maternal age, birth order, or other natural factors.
Length of gestation A 2009 study reported the sex ratio derived from data in United States birth records over a 25-year period (1981–2006). This paper reports that the sex ratio at birth for the white ethnic group in the United States was 1.04 when the gestational age was 33–36 weeks, but 1.15 for gestational ages of less than 28 weeks, 28–32 weeks, and 37 or more weeks. This study also found that the sex ratios at birth in the United States, between 1981 and 2006, were lower in both black and Hispanic ethnic groups when compared with non-Hispanic white ethnic group.
A 2010 study found that sex ratio does not seem to change significantly with either maternal or paternal age. Neither gravidity nor parity seem to affect the male-to-female ratio. However, there is a significant association of sex ratio with the length of gestation. These conclusions have been disputed, due to suggestions that the results are based on some demographic variables and a small data set, a broader study of variables and larger population set suggests human sex ratio shows substantial variation for various reasons and different trend effects of length of gestation than those reported.
Environmental factors