>>82249419
Race mixing results in dysgenic hybrids that suffer from outgroup depression.
>inb4 incest
Nope. Too much of the same also results in dysgenia, albeit of a different type. The solution is slight difference. There was a time period in Europe (the details escape me right now) that saw a massive leap in intelligence in the European population.
>Why did this happen?
Because there was a series of wars that caused a great mixing of European peoples. This genetic recombination made better organisms because the genes were just different enough.
>TLDR: Genes too same/too different=bad. Genes very slightly different=good.