Liberalism...
This theory could help explain why people living in large cities and sharing a common group-think are increasingly stupid. Their brains shrink because they rely on their mindless herd.
This appears to also occur with those who spend too long in higher education.
Those who are more spread out across the heartland and likely to be more self-reliant, perhaps growing their own food, fixing things, building sheds and other independent activities likely have larger brains.