Families of procreation set up their own, independent households independent of parents. Bands are fluid with changing membership. In harsh areas, like Great Basin deserts occupied by Shoshone Indians, bands split into nuclear families to forage independently and reunite with band when seasons and resources permit. Like industrialized societies, mobility is main factor in deciding emphasis on nuclear family. Foragers not tied to one territory or single resource.
Descent groups provide mechanism for inheritance of property.
In non-industrial societies, marriage often serves to join two kin groups in an economically, socially or politically relationship beneficial to both. Most importantly in tribal societies, marriage serves to assign offspring to kin groups, especially descent groups. Polygyny serves to incorporate widows into households. First wife may request help of second. May increase household productivity.