"One of the most well-documented differences between domesticated animals and their wild counterparts is their brains: across every species that has been looked at, the brains of domesticated animals are smaller. Why does domestication lead to shrinking brains? And what does this mean in terms of intelligence" source
other articles of interest:
Can humans be considered domesticated?
Effects of domestication on variation
If modern humans are so smart why are our brains shrinking?