Human sex ratio in the context of "Sex ratio"

Sub-replacement fertility is a total fertility rate (TFR) that (if sustained) leads to each new generation being less populous than the older, previous one in a given area. The United Nations Population Division defines sub-replacement fertility as any rate below approximately 2.1 children born per woman of childbearing age, but the threshold can be as high as 3.4 in some developing countries because of higher mortality rates. Taken globally, the total fertility rate at replacement was 2.33 children per woman in 2003. This can be "translated" as 2 children per woman to replace the parents, plus a "third of a child" to make up for the higher probability of males born and mortality prior to the end of a person's fertile life. In 2023, the global average fertility rate was around 2.2 children born per woman.

Replacement-level fertility in terms of the net reproduction rate (NRR) is exactly one, because the NRR takes both mortality rates and sex ratios at birth into account.

The gross reproduction rate (GRR) is the average number of daughters a woman would have if she survived all of her childbearing years, which is roughly to the age of 45, subject to the age-specific fertility rate and sex ratio at birth throughout that period. This rate is a measure of replacement fertility if mortality is not in the equation. It is often regarded as the extent to which the generation of daughters replaces the preceding generation of women. If the value is equal to one that indicates that women will replace themselves. If the value is more than one that indicates that the next generation of women will outnumber the current one. If the value is less than one that indicates that the next generation of women will be less numerous than the current one.

The gross reproduction rate is similar to the net reproduction rate (NRR), the average number of daughters a woman would have if she survived her lifetime subject to the age-specific fertility rate and mortality rate throughout that period.

The returning soldier effect is a phenomenon which suggests that more boys are born immediately after wars. This effect is one of the many factors influencing human sex ratio. It was especially noticeable worldwide during and right after both of the World Wars.

The phenomenon was first noticed in 1883 by Carl Düsing of the University of Jena, who suggested that it was a natural regulation of the status quo. Writing in 1899, an Australian physician, Arthur Davenport, used Düsing's findings to hypothesize that the cause was the difference between the comparative ill-health of the returning troops compared to the good health of their partners.