Poverty as a indicator of family size - just a couple of issues I thought about...
One thing people aren't mentioning here is large families seem to make more large families and the children of these grow up without any sense of consequence or responsibility for their actions. Having babies seems part of life - with little decision-making or planning involved.
If one is trying to survive and just keep the family fed and clothed - there is not much room for teaching the societal ramifications of love, lust, and pregnancy. These are accepted as natural as finishing school, getting a job, and having babies - and worse now it doesn't even seem to include marriage as a requisite.
Our first teachers are our parent(s) and extended family members and if they tend to have large families, some of the children will follow that pattern.
I don't know that I can agree with that entirely. Within three generations our family size has shrunk... 14 children by my great grandma, 8 children by my grandma, 3 by my mom, 2 by me. Not one of my aunts or uncles has had more than three kids either, and the only set of cousins to exceed 2 kids, did so merely due to lucking into twins with their last pregnancy. Each generation was richer sooner, and more educated than the last. While their parents struggled to make ends meet, public school also gave each new generation better access to education, and each has seemed to develop its own social standing and views, independent of the way the last generation did things.
I can't argue that our parents bear a large impact on much of our behavior, but it doesn't seem to be such a huge impact in family size, in my mere experience.