Universal features of kinship systems that have been proposed include the following:
These postulated universals are subject to extreme ranges of variation which often challenge the validity of any generalizations. For example the extension of kinship ties and the binding of individuals into kinship relationships assumes a basic theory of sex and birth. However, cultures have different views about the "facts" of life and the meaning of marriage, parentage, and birth. The Trobriand Islanders maintain that the sex act has nothing to do with a child's birth, which is the result of impregnation by the mother's ancestral totemic spirit. Accordingly kinship is determined only according to links through females in a matrilineal system. Fathers and people linked through males are technically not relatives at all, although they may assume important social roles and relationship. Similarly, the Yanomamo group people into localized patrilineages, whose members regularly marry into the same groups generation after generation. Therefore a man's wife and mother usually belong to the same lineage, creating a situation where mothers are considered as in-laws (affinal relatives) rather than biological (consanguineous) kin.
An different perspective is taken by long standing Catholic views on consanguinity and affinity. Marriage is seen as a literal union of the husband and wife, who become "one flesh" as a consequence of the wedding sacrament. The resulting network of people linked by marriage become more than mere affines; they are transformed into kin in both spirit and substance. Consequently, canon regulations, impose incest prohibitions are applied to a range of a person's spouse's relatives, which has varied over time but at one period included distant affinal cousins. In addition to this regulation, the Church applies standards of kinship to an individual's baptismal sponsors, or godparents, who are unrelated to the child by birth or marriage but who have entered into kinship through a shared sacrement. Anthropologists term this relationship fictive kinship, but this is an inaccurate designation for Catholic practice, which at one time prohibited marriage not only between godparents and godchildren, but also between a godparent and a sponsored child's parent (i.e. coparents) and between otherwise unrelated godchildren of the same godparents on the basis of shared substance.
Another example of the development of strong ties on the basis of fictive kinship is provided by the "namesake kin" system of the San peoples of the Kalahari desert. They believe that everyone who bares the same name is the descendent of a common ancestor, even when genealogical connections are not documented. Residence rights and incest prohibitions are frequently extended solely on the basis of people's names.