Because not every baby born survives to become a parent. Two children per couple would replace the parents exactly only in an imaginary country where every child lived to adulthood and exactly half of them were girls. Neither is true, so a little more than two is needed.
Two things push the figure above 2:
- Some children die before they can have children of their own. Replacement is about surviving daughters and sons, not about births. If a few per cent of children do not reach adulthood, a few per cent extra births are needed to make up for them.
- Slightly more boys are born than girls. Since it is the daughters who will bear the next generation, a couple needs a little more than two births to be reasonably sure of one surviving daughter to take the mother’s place.
Together these add roughly one-tenth of a child — hence 2.1, not 2.0.
When can the replacement rate be higher than 2.1? Whenever more children die before reaching adulthood. Use the chapter’s own figure to see how much difference this makes. In 1950 India’s child mortality was 271 per 1,000, that is, 271 of every 1,000 children died before the age of five:
Births needed to leave 2 surviving children = 2 ÷ 0.729 = 2.74
So in 1950 a couple in India needed nearly 2.75 births just to leave two children alive at age five — and more still to allow for deaths between age five and adulthood. That is one reason families were so much larger then: the fertility rate in 1950 was 5.7. By 2023, with child mortality down to 28 per 1,000, the same calculation gives
The replacement rate is therefore higher in places with:
- high child and infant mortality — poor healthcare, unsafe water, malnutrition, malaria and diarrhoea;
- high death rates among young adults, before they have completed their families;
- a sex ratio at birth tilted further towards boys, so fewer daughters are born per birth.