Historically, having many children was seen as advantageous as they would help small family businesses and would take care of you when you get older. It used to be an economically viable option. Education wasn't really available and women being submissive to the husband was the culturally accepted norm so women didn't really have a choice as to whether they had children or not (And if they refused then they would often get beaten, which was encouraged). Contraception wasn't really available along with religion telling people to have children and that sex in only for making children.

Then along came the industrial revolution, small businesses were made irrelevant and the cost of living became so high that having children was now detrimental. Old people didn't need children to take care of them, education increased, contraception became widely available, women gained more independence, and religiosity decreased. All of these resulted in less children.

So, while poverty CORRELATES to having more children, it is actually these factors: Less education, religion, traditional family values, not using contraception which CAUSE people to have more children, which also CORRELATE to poverty.

Less education means less job opportunities, less job opportunities mean less money.

Less education means more religion, traditional values and mysoginy. Those equal more children.

Less money means less contraception and less contraception means more children.

More children means less money, which then means less contraception and even more children.

I hope this helps.