Human sex ratio
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[1] or 1.06[2] or within a narrow range from 1.03 to 1.06[3] males per female.
The sex ratio for the entire world population is approximately 101 males to 100 females (2020 est.).[4] The sex ratio of the total population is affected by various factors including natural factors, exposure to pesticides and environmental contaminants,[5][6] war casualties, effects of war on men, sex-selective abortions, infanticides,[7] aging, gendercide and problems with birth registration.[1]
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 108,000 males and 100,000 females the ratio of males to females is 1.08 and the proportion of males is 51.9%. Scientific literature often uses the proportion of males. This article uses the ratio of males to females, unless specified otherwise.
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Factors affecting sex ratio in humans[edit]
Fisher's principle[edit]
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.[33][34] 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.[35]
Natural factors[edit]
The natural factors that affect the human sex ratio are an active area of scientific research. Over 1000 articles have been published in various journals. Two of the often cited reviews of scientific studies on human sex ratio are by W. H. James.[36][37] The scientific studies 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. Remarkably, 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.
In a scientific paper published in 2008,[9] James states that conventional assumptions have been:
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Countries:
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