Historically cultures and tribes that do not practice exogamy are usually wiped out by their healthier, more socially-connected neighbors who attacked and took their land, enslaved their women, and killed the men.
exogamy - word of the day!
n. The custom of marrying outside the tribe, family, clan, or other social unit.
n. The fusion of gametes from individuals that are not closely related, as in outbreeding.
n. The custom among certain tribes which prohibits a man from marrying a woman of his own tribe.
Hybrid vigor?