I don´t think there is direct connection between income and kids brain. The problem is that poor parents arent usually so smart as rich ones. So this is just cycle and it´s hard to get out of it. If your parents are poor, they are always drunk and dont go to job, you have big chance to do so as well. On the other if the parents are smart, but they were just unlucky to get good job or they just do what they want to even though they dont have so much money of it. The children are then more reasonable, they know the cost of money and they are not afraid of job.
It´s not always true, this is just my opinion. But a lot of rich parents don´t care about raising the children as much as they should.