there is wide cross-cultural variation in the social rules governing the selection of a partner for marriage. in many societies the choice of partner is limited to suitable persons from specific social groups. in some societies the rule is that a partner is selected from an individual's own social group – endogamy, this is the case in many class and caste based societies. but in other societies a partner must be chosen from a different group than one's own – exogamy, this is the case in many societies practicing totemic religion where society is divided into several exogamous totemic clans, such as most aboriginal australian societies. marriages between parents and children, or between full siblings, with few exceptions, have been considered incest and forbidden. however, marriages between more distant relatives have been much more common, with one estimate being that 80% of all marriages in history have been between second cousins or closer.