Updated

If it seems that the definition of what constitutes an average American household has broadened in the past decade, it has. According to data from the 2000 census to be released Thursday, seismic shifts in the nation's demographics have dramatically altered the face of the American family.

From empty-nesting baby-boomers to the decision by many Generation Xers to delay marriage and child-rearing, from overall increased life expectancies to the social acceptance of single-motherhood, unmarried co-habitation and same-sex unions, a variety of social trends has resulted in a picture of the American household in which a mother, father and 2-3 offspring is no longer the norm.

One statistic already getting wide media attention is that the number of unmarried couples living under one roof jumped by 71 percent since 1990, while the number of individuals living alone increased 21 percent. Comparatively, the number of married-couple households rose just 7 percent.

Overall, there were 54.5 million married-couple families in 2000, or about 52 percent of the country's 105.5 million households, the census reported. In 1990, there were 50.7 million married-couple homes, 55 percent of all households then.

By comparison, homes containing unmarried partners number 5.5 million, or about 5 percent of all households, up from 3 percent a decade ago. The data did not detail, however, how many of those unmarried partnerships were same-sex couples. That information, as well other more specific relationship data, will be released as early as next month.

Meanwhile, the number of women raising children without fathers at home surged 25 percent between 1990 and 2000.

"Social norms have changed. The neighbors don't whisper behind your back as often," said Dorian Solot, co-founder of the Alternatives to Marriage Project.

The rise in people living alone could be explained by the lengthening life expectancy of Americans. There was an 38 percent increase in the number of people age 85 and older, but since women still tend to live longer than men, many members of this group could be widows.

The impact of the mammoth baby-boomer generation reaching middle age was reflected in a drop in the number of married couples raising children under 18. In 2000, married couples raising children accounted for 24 percent of all households, down from 26 percent in 1990. Many boomers, who ranged in age from 36 to 54 in 2000, finished raising their kids during the 1990s and are now "empty nesters." The drop was also facilitated by today's generation of twenty- and thirty-somethings delaying marriage and family.

The increases in unmarried partnerships and the number of women raising children without husbands were not unexpected, considering relatively high divorce rates and the fact that more young professionals are delaying marriage until later in life, said Martin O'Connell, chief of the Census Bureau's fertility and family statistics branch.

Thomas Coleman, executive director of the American Association of Single People, said groups like his have become more vocal in advocating for non-married persons the last 10 years. Too much attention is placed in Congress on getting married couples better tax breaks and employee benefits at the expense of singles, Coleman said.

Also being released Tuesday were the first detailed national breakdowns by age, and a more detailed look at the country's Asian and Pacific Islander populations. State-level data will start being released later this week.

The Associated Press contributed to this report