The primary function of the procreative families (e.g., families built around the pursuit of parenthood) is to reproduce society, biologically through procreation, socially through socialization, or in both ways. Given these functions, one's experience of one's family shifts over time. From the perspective of children, the family is a family of orientation: the family functions to locate children socially, and plays a major role in their socialization. From the point of view of the parent(s), the family is a family of procreation: the family functions to produce and socialize children. In some culturesmarriage imposes upon women the obligation to bear children. In northern Ghana, for example, payment of bridewealth signifies a woman's requirement to bear children, and women using birth control face substantial threats of physical abuse and reprisals.
In other cases, procreative families utilize marital privileges, rights, and laws (if they have access to these opportunities legally) to establish legal parenthood of a child, gain control over sexual services, labor, and / or property, establish a joint fund of property for the benefit of children, and / or establish relations between partner's larger familial networks. No society does all of these; no one of these is universal, and many people different societies lack access to whatever marital and family privileges available in their given social context. In societies with a sexual division of labor, marriage, and the resulting relationship between a marital members, increasing economic opportunities and decreasing tax burdens, which can aid the establishment of financially stable families. In modern societies marriage entails particular rights and privileges - for those allowed to marry - that encourage the formation of new families even when there is no intention of having children.
The primary functions of non-procreative families (e.g., families that are built around pursuits and desires that do not involve parenthood) is to facilitate social, economic, emotional, and interpersonal support networks, combine resources for the pursuit of financial gain and / or stability, formalize long term commitments to one another and to larger familial and social networks, claim some of the rights, benefits and privileges granted to procreative families in many countries, and / or adhere to religious / spiritual beliefs about emotional-sexual commitment, trajectory, and purpose.
In almost all societies, marriage between brothers and sisters is forbidden, with Ancient Egyptian, Hawaiian, and Inca royalty being the rare exceptions. In many societies, marriage between some first cousins is preferred, while at the other extreme, the medieval Catholic church prohibited marriage even between distant cousins. The present day Catholic Church still maintains a standard of required distance (in both consanguinity and affinity) for marriage.
These sorts of restrictions are a form of exogamy. One exception to this pattern is in ancient Egypt, where marriage between brothers and sisters was permitted in the royal family — as it was also permitted in Hawaii and among the Inca. This privilege was denied commoners and may have served to concentrate wealth and power in one family. The consequence of the incest-taboo is exogamy, the requirement to marry someone from another group. The incest taboo may serve to promote social solidarity.