In all the cultures and societies marriage is not entirely of free choice because the institution of marriage is socially derived and socially sanctioned. Every society places certain limitations on the range of persons from among whom spouses may be chosen. There are two major rules of marriage that are almost always present in all societies. They are exogamy and endogamy.
Exogamy is the social rule that requires an individual to marry outside a specific culturally defined social group of which she/he is a member. The universal nuclear family is always exogamous. It is even said sometimes that exogamy results from the effects of the incest prohibitions. The social group beyond which marriage is required to take place may either be a lineage or a clan or a phatry or a moiety. Thus the exogamous unit is always a subdivision of a large society. Exogamous practices serve to enhance and improve sociability among people by connecting groups of people.
Endogamy is the social rule that requires an individual to marry within a specific culturally defined social group of which he/she is member.
The occurrence of endogamy is not as common as exogamy. There is no particular universal type of social group to which the endogamous rule applies unlike exogamy. The function of endogamy is probably to regulate marriage in a way that preserves the cultural identity of a group.
A classic case of endogamy within the Indian subcontinent is caste endogamy. Persons who are members of a caste group are required to select their marital partners from the same caste group. Endogamous caste marriages are supported, reinforced and rationalized by ritual explanations that are in turn manifest in everyday behavioral patterns. Concepts of physical pollution are related to the caste endogamy. A person of a higher caste who comes into physical contact with a person of a lower caste becomes polluted the severity of the pollution being dependent upon the relative rank of two castes. Endogamy with its reinforcing concepts such as pollution helps to set one group apart from others.
Examples of endogamy can be seen among ethnic groups within larger societies.