It's not the income. It's the parent who doesn't teach the kid how to use his/her brain.
Upper income people are educated by choice, and their kids learn from example. If the parents use their brains, the kids grow up using their brains as well.
It's a family thing - not an income thing.
I blame a lot of it on food choices. Rich people are eating fresh veggies, various proteins and natural grains.
Poor people are eating canned food, low grade proteins, and boxed starches.
[The brain is a muscle. The more you use it, the bigger and stronger it gets]
Healthy diets play a contributing factor too.