Human sex ratio

Duration: 8min 30sec Views: 546 Submitted: 19.03.2020
Category: Brunette
Two remarkably consistent and poorly understood features of human biology are the slightly male-biased sex ratio at birth and the female survival advantage throughout life. These patterns appear across geography and time wherever reliable birth and death records are available 1 , 2. Putting together the birth sex ratio bias and the female survival advantage 5 , we expect a monotonically declining sex ratio from birth to death, which is exactly what we find across cultures and across historical epochs Fig. Note, however, that until later life the sex ratio does not stray far from , an observation that would gladden the heart of evolutionary biologist, Sir Ronald Fisher, who argued that natural selection should favor equal parental expenditure—a delightfully vague phrase—in males and females 6. Fisher assumed, as have many since then, that the human sex ratio at conception is even more male-biased than the sex ratio at birth, and there were some good reasons to assume this. First, males are less likely than females to survive from birth to age 5 y in all countries with reliable records 5.

Human sex ratio

The human prenatal sex ratio: A major surprise

The sex ratio is the ratio of males to females in a population. In most sexually reproducing species, the ratio tends to be This tendency is explained by Fisher's principle. Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps such as Polistes fuscatus and Polistes exclamans , bees, ants, and termites. The human sex ratio is of particular interest to anthropologists and demographers.

The human prenatal sex ratio: A major surprise

Jacobsen, H. Analysis of the effect of multiple birth, birth order, age of parents and the sexes of preceding siblings on the secondary sex ratio was performed for children, born in Denmark, — The secondary sex ratio decreased with increased number of children per plural birth and with paternal age, whereas no independent effect was observed for maternal age, birth order, the sex of the preceding child, or the combination of sexes of previously born children in the family.
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