Outbreeding Enhancement Or Depression

Perhaps hybrid vigor (also known as outbreeding enhancement and heterosis) resulted for us from interbreeding (also known as crossbreeding and outbreeding) with Neanderthals (who had bigger brains than did archaic humankind *prior to* the arrival of the Cro-Magnons) - that is, we inherited improved biological traits as a result of interbreeding in this case - but that does not mean hybrid vigor resulted from *every case* of interbreeding. "Not all outcrosses result in heterosis. For example, when a hybrid inherits traits from its parents that are not fully compatible (contradictory instinctive patterns, for example), fitness can be reduced. This is a form of outbreeding depression." There is also such a thing as inbreeding depression.

This is why, historically, and even today among many ethnic groups there are strong feelings that children should "marry within the ethnic group" as the norm for the majority of people, and the group or tribe will only accept bringing people of other groups "into the family" (so to speak) who have traits likely to eventually benefit the group as a whole - in other words, those who will contribute good genes to the gene pool of the group. Rare but occasional inclusion of other people into the gene pool is aimed to minimize inbreeding depression and to maximize the possibility of outbreeding enhancement. Especially for ancient peoples, acquiring mates was not about romantic love (a fleeting human experience), it was a social responsibility toward ensuring and improving the survival odds for the group's descendents.

It's also possible that the outbreeding enhancement which gave rise to the Cro-Magnons was not due to interbreeding with Neanderthals at all but from interbreeding with some other archaic hominid that existed on a continent that now no longer exists (like Atlantis) or in Doggerland (now submerged under the North Sea).