Descriptions of non-nuclear family systems in terms of their characteristic rules of residence often seem to imply very large and complex households. For example, in a classic patriarchal family system, a household containing a father, his wife, two married sons, and their wives and children would number ten or more persons. Such large households are seldom encountered as modal or average, however. A recent compilation of census materials reported in the U.N. Demographic Yearbook showed no bona fide case of a national average household size larger than six.

Several explanations have been offered for this discrepancy between the ideal and the actual. Lang has stressed economic limitations on household size and composition, arguing that only the better-off segments of any society can realize the ideal, because only they have the required land, housing and other material goods.

Hsu has stressed the social psychological difficulties of maintaining large, complex households, and argued that relatively few male or female heads will have the social and administrative skill to hold these households together in the face of centrifugal forces (e.g., mother-in-law or sister-in-law problems).

Others have stressed demographic limitations, pointing out in particular the role of high mortality in hindering realization of the ideal household form. Some quantitative data on this point were presented by Collver in his study of the Indian family life-cycle.