"The truth is that domestication makes any animal dumb. You name the species — dogs, cats, cows, horses, sheep, pigs — human selection on “tameness” for thousands of years accumulates a wide array of traits, including floppier ears, shorter snouts, hair color variability and the like, most likely based on more basic inherited alterations in certain stem cell and stress hormone production patterns (see Domesticated: Evolution in a Man-Made World, by Richard Francis, for a great read on all this). Different species show these external traits to different degrees. But the trait that ALL domesticated species demonstrate relative to their wild species is a smaller brain. I’d bet it’s happening with humans, too, but that’s just an observation for another day."

http://epsilontheory.com/too-clever-by-half/