I would argue that the difference in population growth is caused mainly by economic factors. In the developing world, it is still often the case that having more children is economically beneficial. In the developed world, children are not as economically beneficial.
Of course, there are other factors at work. People in developing countries might lack access to contraceptives. They might belong to religions that disapprove of contraception. Women might lack the status that they would need to take control of their reproductive lives.
However, I would argue that the economics are more important since essentially all countries that become developed see their population growth fall. Again, the reason for this is that children are more economically beneficial in a developing country. Here are some reasons for that:
- In a developing country, children are often needed to help work on the farm or in the family business. In the developed world, most people do not make their livings in ways that could benefit from their children’s help.
- In the developing world, people often lack pensions and rely on their children to support them in their old age. This is not typically true in the developed world.
- In the developing world, there are few costs (outside of food) to having a child. In the developed world, children are very expensive. They need everything from clothes and toys to college tuition.
For these reasons, children are more economically beneficial in the developing world and such countries tend to have higher rates of population growth.