The structure of a family takes different forms around the world and even in the same society. The family's form changes as it adapts to changing social and economic influences. Until recently, the most common form in North America was the nuclear family，consisting of a married couple with their minor children. The nuclear family is an independent unit. It must be prepared to fend for itself. Individual family members strongly depend on one another. There is little help from outside the family in emergencies. Elderly relatives of a nuclear family are cared for only if it is possible for the family to do so. In North America，the elderly often do not live with the family ; they live in retirement communities and nursing homes.
There are many parallels between the nuclear family in industrial societies，such as North
America，and of families in societies such as that of the Inuits，who live in harsh environments. The nuclear family structure is well adapted to a life of mobility. In harsh conditions，mobility allows the family to hunt for food. For North Americans，the hunt for jobs and improved social status also requires mobility.
The nuclear family was not always the North American standard. In a more agrarian time，the small nuclear family was usually part of a larger extended family. This might have included grandparents，mother and father，brothers and sisters，uncles，aunts，and cousins. In North America today，there is a dramatic rise in the number of single-parent households. Twice as many households in the United States are headed by divorced，separated，or never-married individuals as are comprised of nuclear families. The structure of the family，not just in North America, but throughout the world，continues to change as it adapts to changing conditions.
nuclear /'nju:kh/ adj. 原子核的，中心的 harsh /ha:?/ n. 严酷的
emergency /I'm?:d??nsI/ n. 紧急状况 agranan / ?'gre?rl?n/ adj. 土地的，耕地的
在全世界，甚至在同一个社会中，家庭结构有着不同的形式。家庭的构成形式随着不断变化 的社会和经济影响而改变。直到最近，北美洲最普遍的家庭形式为核心家庭，由一对夫妇和他们 未婚的孩子构成。核心家庭是一个独立的单位。它必须能够照料自己。家庭成员紧密依赖着彼此。 在紧急情况下，外界提供给家庭的帮助微乎其微。核心家庭中，只有当条件允许的时候，才会照 料家中年长的亲属。在北美洲，老人很少和家人一起生活，他们‘般住在退休社区和养老机构。
在工业社会，例如北美洲，核心家庭之间有很多相似点。在居住在恶劣环境中的因纽特人的 社会中，家庭间也有很多相似点。核心家庭结构良好地适应于流动性的生活。在恶劣的条件下， 流动性能让一个家庭吃饱饭。对北美洲人来说，找工作和提高社会地位同样需要流动性。
核心家庭并不总是北美洲人的标准。在农业社会时期，小型的核心家庭经常是大家庭的一部 分。其中可能包括了祖父母、父母、兄弟姐妹、叔伯、阿姨以及表兄弟姐妹。在现在的北美洲， 单亲家庭的数目大幅度增加。由核心家庭构成的单亲家庭的数目是美国由于离婚、分居或者未婚 导致的单亲家庭的两倍。不仅在北美洲，在世界范围内，家庭结构都会随着不断变化的条件而改变。