The final problem of definition and classification stems
from the fact that a household is a dynamic unit that changes over time,
often in relation to consistently applied but complex rules.
It may begin as a minimal family, e.g. a single married couple, grow over
time as generations are added, and eventually subdivide as it becomes to
large to be supported by local resources or to maintain domestic harmony.
Accordingly, a researcher might record families as belonging to a
specific type without realizing that they may be occupying a stage
that will be subject to unobserved principles and unanticipated changes.
For example, many of the earliest studies of urbanization in Africa
suggested that a nuclear family pattern was replacing the
extended family households that were prevalent in traditional rural societies.
However, those carried out a generation later observed
a high incidence of large and complex domestic units.
Clearly, the first migrants to the city were young and newly married and
had formed small families with spouses and young children.
As they aged, their children married and had children of their own.
Rather than forming a second generation of separate householders,
they tended to remain resident with their parents
according to their central cultural dictates.