They even have a mathematics for it, with empirical formulas and symbols for "lateral pressures" and "exchange valences" and "exogamic relief.".    From Wordnik.com.  [Time For The Stars]

If a group is exogamic, the women in it were born abroad somewhere.    From Wordnik.com.  [Folkways A Study of the Sociological Importance of Usages, Manners, Customs, Mores, and Morals] Reference

The clans were exogamic as to their own members, as were also the phratries in primitive times.    From Wordnik.com.  [Indians of North Carolina: Letter from the Secretary of the Interior, Transmitting, in Response to a Senate Resolution of June 30, 1914, a Report on the Condition and Tribal Rights of the Indians of Robeson and Adjoining Counties of North Carolina] Reference

The particular exogamic customs vary considerably among early tribes, the differences following, in general, differences of social organization.    From Wordnik.com.  [Introduction to the History of Religions Handbooks on the History of Religions, Volume IV] Reference

The cause of Ahura's being troubled at the feast is not certain, but the king evidently supposes that she has been pleading to be allowed to marry her beloved brother, and when taxed with it she only expresses her willingness to give way to his exogamic views.    From Wordnik.com.  [Egyptian Tales, Translated from the Papyri Second series, XVIIIth to XIXth dynasty] Reference

However, this exogamic system could not prevent inbreeding, as there was always the possibility that uncles and nieces might marry, so that a "horizontal" system was superimposed across this "vertical" one, forbidding all marriages between different generations.    From Wordnik.com.  [Two Years with the Natives in the Western Pacific] Reference

Later there was a taboo on exogamic marriage.    From Wordnik.com.  [Folkways A Study of the Sociological Importance of Usages, Manners, Customs, Mores, and Morals] Reference