Why are mixed race people more likely to be superior in terms of physical appearance and intelligence?

Please do not answer if you do not accept this as fact, or at least Research the reports that confirm this as fact. I think it's because it's like the opposite of inbreeding. That's why North USA is superior to South more (specifically the bible belt). I mean any mix of race and it is proven that the more diverse your genes are the more likely you are to be successful in life.