Many attempts have been made to identify the essential nature of marriage and to list its purposes, a project often as revealing of the observer's assumptions as of the observed practices. Across cultures, the ceremonial and social phenomena conventionally defined as marriage assume countless forms and serve varied purposes, yet marriage is usually defined as the formal ideological recognition of a sexual relationship between one man and one woman (monogamy) ; among one man and two or more women (polygamy: polygyny) ; or among one woman and two or more men (polygamy: polyandry). Because sexual intercourse is approved in this relationship, the children of a marriage usually possess a status superior to children born beyond its boundaries.
In an argument against such essentialism, the anthropologist Edmund Leach rejected universal definitions and instead approached marriage as a ‘bundle of rights’. Among the classes of rights allocated by institutions ‘commonly classed as marriage’,
Leach noted that in different societies ‘marriage’ may serve: (i) to establish the legal father of a woman's children; (ii) to establish the legal mother of a man's children; (iii) to give the husband a monopoly in the wife's sexuality; (iv) to give the wife a monopoly in the husband's sexuality; (v) to give the husband partial or monopolistic rights to the wife's domestic or other labour services;
(vi) to give the wife partial or monopolistic rights to the husband's labour services; (vii) to give the husband rights over the property of his wife; (viii) to give the wife rights over the property of her husband; (ix) to establish a joint fund of property, a partnership, for the benefit of the children of the marriage; and (x) to establish a socially significant ‘relationship of affinity’ between the husband and his wife's brothers.
Leach's essay, and the debate it provoked in the late 1950s, had a seminal influence on approaches to marriage as an ethnographic problem, as a culturally specific set of beliefs, practices, and institutions. Because marriage did not establish all of these types of rights in any known society, Leach concluded that the ‘institutions commonly described as marriage do not all have the same legal and social concomitants’ and that the meaning of marriage in any society could emerge only from detailed investigation of its ethnographic context. At the same time, Leach's essay typified an approach that has focused on how marriage may structure relationships between individuals and among groups, and has stressed the interrelationship of principles of descent, rules of residence, and issues of power over property .
Incest is sexual intercourse between individuals related in certain prohibited degrees of kinship. In every society there are rules prohibiting incestuous unions, both as to sexual intercourse and recognized marriage. The two prohibitions do not necessarily coincide. There is no uniformity as to which degrees are involved in the prohibitions. The rules regulating incest must be investigated in every society by means of the Genealogical Method.
For example, Trobriand Islanders prohibit both sexual relations between a man and his mother, and between a woman and her father, but they describe these prohibitions in very different ways: relations between a man and his mother fall within the category of forbidden relations among members of the same clan; relations between a woman and her father do not. This is because the Trobrianders are matrilineal; children belong to the clan of their mother and not of their father. Thus, sexual relations between a man and his mother's sister (and mother's sister's daughter) are also considered incestuous, but relations between a man and his father's sister are not. Indeed, a man and his father's sister will often have a teasing relationship, and a man and the daughter of his father's sister may prefer to have sexual relations or marry.