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There are different skull shapes between races, so we we know there isn't a barrier. The brain is just another organ, why wouldn't it be susceptible to evolution?
Black men have 20% more testosterone than whit men.
Testosterone is related to aggression and violence.
Evolution is slow paced. Such a change couldn't have happened in the last centuries or milennia. It'd take a longer time for it to cause a perceivable change.
First of all, it's measurable. Testosterone levels are not a theory.
Second, evolution is slow, correct. But that hasn't stopped there from being obvious change. The brain is an organ, like any other organ, it evolves.
Your argument that it would take much longer is flying in the face of reason. If it were to take so long, then why do we have races?