I assume you are being sarcastic, but there are plausible hypothesis that could be tested to answer this question. For example the genes that have caused a reduction in human pigmentation in some populations have occurred very recently (within historical times) and under very strong selection. Genes that influence the propensity for impulsiveness or violence may have been carried along with the selection for low pigmentation.

Another possible hypothesis is that since melanin and dopamine have an overlapping biochemical pathway that the genes influencing melanin production may also influence dopamine levels.

A third possible hypothesis is that since different populations have historically lived at different densities this may have influenced how well people interact peacefully at high population densities (i.e. how domesticated people are in the same way cows are much calmer than wild cattle).

All of these are testable hypothesis that no sane scientist will ever study - in the end we just don't know the answer and most likely never will.