Kin Groups and Descent. Today, Seri descent is reckoned bilaterally, and residence is neolocal with a slight patrilocal preference. The nuclear family is the core residential and economic unit. The "tribe" is recognized as a valid concept, although it functions mainly as a unit of ethnic identity. There are no formal groupings between the level of nuclear family and tribe. In the past, the Seri were divided into several geographically separate, politically independent units (bands), which differed in dialect and culture. Spanish recognition of "nations" only partly coincides with bands as recalled in Seri oral history. The Seri maintain that bands were further subdivided into geographical groupings called ihizitim, which they believe were well-integrated patrilineal, patrilocal, and exogamous units of everyday life. To what extent Seri social structure differed in the past has been a subject of intense debate. Different sources—Seri oral history, colonial documents, ecological considerations, and anthropological theory—have led investigators to an astonishing array of reconstructions. These include matrilineal, patrilineal, and bilaterial descent; clans; subclans; patrilocal bands; composite bands; and rancherías (groups of huts). The data on which reconstructions must be based are so ambiguous that it is doubtful that aboriginal Seri sociopolitical structure and descent will ever be well understood.
Kinship Terminology. Kinship terminology is bifurcate collateral, with sibling terms extended to both parallel and cross cousins. Numerous terminological distinctions are made. One quasi-kinship system that still operates is the hamac relationship of reciprocal obligations for sponsorship of burials and the girls' puberty fiesta.