How does poverty affect cognitive development?

There would only be an impact on the IQ if the family was so poor that the kids would not be fed enough or not enough nutrition or there would be physical changes in the brain. There are places like that on this planet of course and famine is one reason that can happen. If you live in a poor neighborhood with a bad school they might not realize how smart you are but the basic IQ is already there and can not be changed.

Behavior is influenced by many things such as parents, siblings, environment etc. This is the same regardless of where you live, rich or poor. Good behavior can be taught to anyone and is not linked to money. If the child however live in a environment where they feel they have no choice than to join a gang for survival for instance that would change his behavior because before that he probably did not dream about hurting someone and now he has to. If there are no jobs and people around you basically have given up that will also have an impact.