the peoples of the americas are a wrinkle. it's difficult to pin down when they arrived. they may have been out and about twice as long as europeans and they certainly were not more complex than the chinese. pre contact they were in an asian haplogroup (even now many latinos look asian) related to the ket people found in siberia. their complexity was not uniform, some groups remained in or near the late stone age. did some develop complexity or did they bring it with them? did they come in waves with later ones bringing advancement? they either did not shrug off natural aggression or re-acquired it. it's all difficult to sort. it does not help that early contact brought rabid christians who destroyed anything that didn't agree with their worldview. in modern times latin america sports some of the most murderous countries in the world. regardless of if mt hypothesis is correct, good parenting may mitigate this for them.

note: one of the less violent countries murder wise is found in the far south, chile, where it is cold and less lush.