The Yanomamo are organized into named localized lineage groupings on the basis of patrilineal descent. Lineage groups are quite shallow, seldom extending beyond two adult generations, and small, seldom reaching as many as 100 members. Group dimensions are limited by the frequent segmentation and relocation of lineages because of internal conflicts, usually over women, which plague larger units. The marriage system also acts to construct regular relationship between pairs of lineages who regularly intermarry through a system of bilateral cross cousin marriage.
Intermarrying units tend to pair off and exclusively occupy the same village, thereby generating a moiety system. Members from other lineages may also reside in the village and marry within it, but there will always be two intermarrying moieties which dominate the settlement both numerically and socially. Yanomamo lineages do not take on substantial corporate functions, such as land ownership, that they frequently assume in other patrilineal societies.
Yanomamo marriage patterns are structured on the basis of four main principles: lineage exogamy, bilateral cross cousin marriage, village endogamy, and polygyny. Lineage Exogamy incest prohibitions among agnates (patrilineal relatives) mark the patrilineage as a significant unit in the exchange of marriage partners with other lineages. The Yanomamo follow a bilateral cross-cousin marriage system whereby marriage partners are doubly related to one another as matrilateral and patrilateral cross cousins as a consequence of similar marriages among their parents.
The bilateral cross cousin marriage system and co-settlement of intermarrying lineages establishes a pattern of village endogamy. This pattern is variable, however, since conditions of warfare create the need to form marriage alliances outside of the immediate settlement. Accordingly, the rate of endogamy varies with village size and power. Larger groups have less need for outside alliances and can maintain high levels of endogamy. Smaller, weaker units, must limit inmarriage and provide brides to neighbouring settlements in return for military support.
The Yanomamo follow a practice whereby men can and normally do marry more than one wife as a benefit and marker of social status. Polygyny of this sort is sometimes explained as a result of male and female demographic imbalances, but the Yanomamo have significant more men than women in their populations because of female infanticide. Polygyny is possible in such a situation only if some men do not marry or, more usually, if they marry at a much later age than women.