The cultural truth acknowledged in that last sentence is one of the reasons why the Census Bureau's new data on family structures in America are so interesting. For the past half-century, this country has tended to think of the state of the nuclear family as a leading moral indicator. Now, for the first time, fewer than a quarter of the households in this country are made up of nuclear families - 23.5 percent to be exact, down from 25.6 percent in 1990 and 45 percent in 1960. Also, for the first time, the number of people living alone is greater than the number of nuclear families. Meanwhile, the number of unmarried couples nearly doubled in the past decade to 5.5 million, still vastly fewer than the 55 million married couples.

It is certainly possible to read these figures as unwelcome deviations from a fundamental norm, as conservative groups like the Family Research Council do. But the dominance of the nuclear family in the 1950's and 1960's was just as much an expression of prevailing economic and sociological forces as its lack of dominance is now. The broad social shifts underlying these data - the prevalence of women working outside the home, the decline of agricultural households, the ever-growing mobility of Americans, the postponing of marriage and childbearing, the extension of life spans - do not necessarily point toward a more fragile social structure, even though some changes, like the increase in the number of single women with children, are troubling. What they point to is the flexibility and the increasing complexity of American family arrangements.

The ideological emphasis on the nuclear family in recent decades has caused us to forget that the nuclear family itself, especially in its suburban, Ozzie and Harriet form, is a reduction of what family has meant in most places at most times in human history. The American nuclear family has been notable, to borrow George Murdock's words, for not combining into larger aggregates, like extended families, but for sticking to itself.

You only have to have grown up in a nuclear family to understand the good it can do. But even children who grew up in nuclear families have contributed to their decline as they have gone on in many cases to form non-nuclear families of their own. What the new census data underscore is an increasing awareness that the nuclear family is not the only kind of family or even the only healthy kind of family. In modern America no type of family can really be recognized to the exclusion of all others.