Industrialization and urbanization create many specialized jobs which tend to scatter family members among different employers and thus to seperate residences as soon as they become wage earners.
The small family which has only one or if the wife works also, two employed members, is better able to adapt to rapid change and to move when the job moves.
The nuclear family is almost universal, and the nuclear group of father, mother and their children is recognized even when it is part of an extended family.
They are cases, however, which strain the definition.
Polygamy, for example, bring several wives and their children into the picture. But polygamous households are not common in any society.
More difficult to explain are the cases of divided residences.
Among the Ashanti people of Africa, where the wife and husband do not reside together, the child gets training and affection from the mother's brother and learns that his mother's husband is not his family.
And even stranger situation existed with Nayar of India before being changed by outside influence.
There the household consisted of brothers and sisters and the sisters' children.
The sisters were not married and the brothers simply took care of whatever children their sisters had.
Inheritance customs also have an influence on the structure of the family.
In England, the farm was passed on to the eldest son in order to keep the family land intact.
Younger sons had to go out and start a new farm or join the army or move the town and take up a trade.
They provided a large part of the labor supply during the England's industrialization process.
In many areas of the European continent, all of the sons share equally in the inheritance and more extended households were common.
Although the exact form varies place to place and time to time, we can say that the family is the orginal and most natural social group.
The ties we develop by long intimate association with the small group of persons who are biologically related to us cannot be matched in any of the forms of communal living which are tried every now and then.