Sociological theories of exogamy include:
Murdock also observed that no society prohibits sexual relations between nuclear family members only, but all cases in the sample applied prohibitions to relatives in some other categories. However the range and type of prohibitions vary considerably from case to case.
Red circle marks range when first cousin marriage is allowed.
Yellow extension marks range when first cousins marriage is prohibited.
Other societies have even more extensive bilateral restrictions. The Ju/'hoansi prohibit marriages within a range extending to second cousins. The Catholic Church once banned unions within a sixth cousin range but has progressively narrowed prohibitions to first cousins.
Unilineally organized societies tend to apply a different logic, frequently banning marriage between members of the same descent group, regardless of the biological degree of relatedness, or prohibiting parallel cousin marriage while permitting or even encouraging unions between cross cousins.
Permissible relationships are indicated in green
The ban on marriage within the group reinforces the identification of group members since they are all consider "too close" to marry. The cross cousin marriage preference encourages a pattern of consistent alliances between component lineage groups to form a larger social matrix. (See: More about Cross Cousin Marriage Rules)
Ancient Hebrew society formulated a completely different set of marriage restrictions from either the bilateral extensions of European systems or the cousin marriage dynamics of lineage group alliances. The incest prohibitions listed in Leviticus 18 suggest a very narrow range of prohibitions within the extended biological family and an elaboration of restrictions on certain categories of affines.
Patrilineally related men are indicated in blue
As the above diagram indicates, restrictions on marriage within the family were extended primarily to nuclear family members. Cousin marriages of any type were allowed, and in fact there was a preference for patrilateral parallel cousin marriage, i.e., a union between the children of two brothers, in some circumstances. (This arrangement involved marriage within a patrilineal group and indicates that descent groups in this instance were not exogamous.) Moreover, there was no explicit rule against sexual relations or marriages between uncles and nieces, although aunt/nephew prohibitions are specified. This asymmetry seems to be related to a preference for men to marry women of lower generations, as exemplified by the legends of the Hebrew patriarchs and matriarchs in the book of Genesis.
While the restrictions on relations between biological kin were even less extensive than those of contemporary Western societies, affinal restrictions were more comprehensive. Above and beyond the drastic penalties for adultery, condemnations of incest were applied to sex or marriage between a man and his son's wife, brother's wife, stepmother, father's brother's wife, wife's mother, or wife's sister. In these cases prohibitions were in force only during the lifetime of the male relative. In fact other marriage provisions within the Old Testament favor levirate marriage to a brother's widow, thus lifting the affinal ban subsequent to a brother's death. The pattern of affinal prohibitions reveals a second instance of asymmetry in the system, this time based on lineage membership. While relations with a father's brother's wife are not allowed, there is no converse restriction concerning mother's brother's wife. Accordingly, we can understand some of the extensive affinal prohibitions as supporting the cooperation among close patrilineal kin by forestalling competition over women.
These biblical prohibitions have of course had an influence on Western marriage customs and legislation. However, in the course of applying scriptural principles to the realities of social life many reinterpretations, additions, deletions, and controversies have occurred because of the different cultural contexts and social orders of the societies that adopted the Judaic tradition. References to Leviticus in the formulation Western marriage regulations carry over only part of the body of original restrictions and never incorporate the generational or lineal asymmetries, thus revealing different logics of Hebrew and contemporary models. Furthermore, affinal restrictions so dominant in the biblical text have been continually balanced against a European concern over cousin marriage that originated with Catholic canon law in the early middle ages. This ambivalence is still expressed in the curious patchwork of marriage prohibitions that are currently present in the United States. (See Variations in U.S. Marriage Restrictions)