There's common sense
It's highly unlikely that identifiable differences evolved in isolated human populations in almost every biological function except brain function.
But then even in monoracial populations, we can see drastic and obvious physical differences between people. There are probably inherited mental differences there, too, and the spread of the individual differences may override any spread in racial ones.
These are, of course, guesses. Again, who's going to study it when the wrong answers will kill them?