In particular, women tend to give birth to no fewer than three children in countries where GDP per capita is below $1,000 per year. In countries where GDP per capita is above $10,000 per year, women tend to give birth to no more than two children.

Why is fertility so much higher in poor countries? There are several possible reasons:


Time is relatively cheap in poor countries, so spending time away from work to take care of a child is not as costly as in a rich country. If this effect is strong enough, it can (and probably does) offset the fact that it is difficult to afford a child on a low income.


A child may require more education to be successful in a rich country. Thus, a child may be more costly there, so families may opt to have fewer, more educated children.
Infant mortality can play a role. More births might be needed to achieve a desired number of surviving children when infant mortality is high, as it tends to be in poor countries.


Children can take care of their parents when they are old. However, this is not necessary in rich countries with a well-developed social security system and functioning financial markets.
@m_mamajonov