Why do starving people in 3rd world countries keep having babies?

I got into an debate today with a friend of mine; and she brought up that people should be sterilized in 3rd world countries because they are making too many children. I dont believe in "one child policy" because it limits personal freedoms and isnt the best solution to the supposed over population. BUT I didnt really have much of answer to why exactly if youre family is starving would you want to have 5 children? It seems to defeat all logic and instinct- so i know there has to be some socio/economic response or reason to this that i am ignorant of? Please, anybody chime in onto what they think the reasoning is?