[zsz]

Population replacement rate is 2.1 children per woman--meaning, in order for a population's size to remain the same, on average child births have to come out to 2.1 for every woman. All Western countries have been well below replacement level, for many decades at this point.

Taking the example of One brother and one sister, the sister would have to bear 2.1 children, while the brother would have to have 2.1 children with another woman, in order to merely keep the population size level.

The reason for this is obvious: two sets of parents=4 people, the same number as the 2*2.1 children (minus premature deaths, etc.).

[pyuser583]

Fertility rate is calculated as number of children per woman.

So the sister must have 2.1 children to sustain, but the brother’s “contributions” don’t matter.