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updated 9/21/2006 3:44:21 PM ET 2006-09-21T19:44:21

A baby born some time this October will be the 300-millionth American, but he or she shouldn't expect a house full of siblings.

The average number of people living in U.S. households has dropped almost one whole body each time the country adds 100 million citizens, according to the U.S. Census Bureau. When the 300-million milestone is reached, that number will be at a new low of 2.6 people per home—parents, offspring and extended squatters included.

A combination of cultural factors is behind the shrinking American household, experts say.

"It's the three Fs—family, freedom, and finance," said Gil Troy, professor of American history at McGill University in Montreal.

Goodbye Ozzie and Harriet
When the U.S. population reached 100 million in 1915, the average number of people sharing a home was 4.5. Larger, patchwork families were just more common back then, say historians

"In 1915, you might have had Granny and Gramps living [at home] as well as more little'uns scampering about," Troy told LiveScience in a recent email interview.

With the advent of better transportation and looser social restrictions in the years that followed, people were able to leave the nest, Troy explained. In 1967, when the U.S. counted 200 million citizens, households hovered at just over three residents each.

That trend continued to today, said Douglas Besharov, a professor at the University of Maryland's School of Public Affairs.

"I think the American family is in the throes of what you could call seismic change," Besharov wrote in the U.S. Department of State's 2001 electronic journal The American Family, "...caused by a combination of greater wealth, individuality and mobility."

The census data clearly supports the notion that traditional family structures are breaking down, he said.

"Many of these family changes are due to the explosion of freedom over the last century, freedom to roam beyond your home town, freedom to roam beyond the conventional ‘Ozzie and Harriet' family," agreed Troy, who explores some of these changing family roles in his upcoming book "Hillary Rodham Clinton: Polarizing First Lady" (University Press of Kansas, 2006).

Richer, older, freer
Americans also got a lot richer over the years, creating a domino effect of other factors that have changed the demographics, experts say.

"Feeding all this freedom are the plush finances of Americans—we can afford to indulge our whims, to move out of the house before getting married, to divorce if we're unhappy," Troy said.

Financial success also has had a hand in people getting married later in life, women delaying child rearing for their careers and helping people live longer—all elements that can contribute to smaller average households.

Ironically, though, as the number of people per home has dwindled, average house sizes have gotten larger, Troy said, because people can now afford their own private castles.

Can't stop the coupling
The sliding number should at least level off in the next few decades, historians say, as it's unlikely that people will stop hooking up in pairs.

"I don't see the institution of marriage disappearing, despite the hit it took over the 20th century," Troy said. "While we have seen more singles, and fewer conventional partnerships, there's still a heck of a lot of coupling going on." Individuals may have more partners in one lifetime than people did in the past, but people are still compelled to live in twos, he noted.

Then there's the pesky matter of kids, who will always need to be cared for, unlike the older generations that now have alternative housing options.

"Human children still have a long gestation process and an even longer maturing process (many would argue that it is getting even longer these days)," Troy said. "So whereas the Social Security, and old age home revolution really moved the older folks out of the house, kids still need a home."

Future family huddles?
A reversal in trends is possible even, Troy said, depending on what the future holds.

"We have seen a rediscovery of ‘monogamy,' a growing criticism of the impact divorces have on kids and on the society, and a continuing core commitment to family and family values," he said. "So all these could trigger a reverse—as could an economic shock…[or a] war, which sends people huddling together, out of desire or necessity."

"Anything can happen," Troy said. "The one constant, especially in America, is change."

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