Final answer:
The focus on the nuclear family in modern nations is due to geographic mobility that isolates individuals from extended families, the formation of smaller households especially among the urban poor, and the achievement of higher incomes that support the ideal of a nuclear family.
Step-by-step explanation:
The focus on the nuclear family in many modern nations can be attributed to several factors characteristic of industrialized societies. One reason is the geographical mobility required by modern economies, which tends to isolate individuals from their extended family units. As individuals move in search of employment or better living conditions, they often form nuclear families, which consist of one or two parents and their immediate offspring. A consequence of this mobility is that it facilitates a more individualistic society where caring for extended family members becomes a voluntary obligation rather than a societal expectation.
In the context of the modern urban poor, smaller and exclusive household units are formed partially due to economic constraints. Furthermore, with higher incomes and industrial advancements, adults are more capable of realizing a lifestyle that embraces the cultural ideal of a nuclear family model, where parents and their children form an economic and emotional unit separate from extended kin.
However, it's important to note that the American cultural ideal of the two-parent household with one or more children has seen diversification with the rise of single parents, same-sex parents, and cohabitating parents. Despite changes in the structure, the fundamental aspects of emotional closeness and support within a family unit continue to be valued highly in society.