An analysis of cohabitation, marriage and divorce data from 13 countries, including the U.S., shows that living together has become so mainstream that growing numbers of Americans view it as an alternative to marriage.

The National Marriage Project study of a sampling of Western European and Scandinavian nations, Australia, Canada and New Zealand found that cohabitation elsewhere is far more common and indeed viewed as an option to matrimony. The study found that anywhere from 15 percent to 30 percent of all couples identified themselves as living together, compared with about 10 percent right now in the U.S.

"We're still the most marrying of all these countries, but the data are clearly headed in the one common direction. It's headed in the direction of cohabitation as an alternative," says David Popenoe, the report's author and co-director of the National Marriage Project at Rutgers University, which studies marriage and child well-being.

Because the most recent data analyzed from some countries are at least 2 years old, and because increasing numbers of celebrities are living together, Popenoe says his projections consider slight increases over time.

"Today, celebrities from Hollywood and elsewhere are looked up to," he says. "They have become role models."

A previous study by the same group showed that since 1970, the number of Americans living together has increased from about 500,000 opposite-sex couples to more than 5 million.

Using databases of Census-like information in the countries studied, the analysis found the marriage rate is down in all countries except Norway and Sweden, which have traditionally low marriage rates. In the U.S. from 1995 to 2005, the marriage rate declined almost 20 percent.

Joselin Linder, 33, of Brooklyn lives with a boyfriend and has lived with two others in the past. Now she's co-author of the new book "The Good Girl's Guide to Living in Sin" and says many women her age and younger view living with a romantic partner as a convenience. She says it's not about avoiding marriage.

"It's what's happening in the world of dating, and it's not necessarily a path anywhere," she says.

The new report cites Census data showing that about 40 percent of all opposite-sex, unmarried couples live with their own child under 18.

"We often think of cohabitation as a phenomenon of young adulthood before people start having kids, but as marriage is being delayed to later and later ages, more children are born before marriage, and many of the couples are cohabiting before the birth," says R. Kelly Raley, associate professor of sociology at University of Texas-Austin, who did not participate in the study.

Raley isn't convinced that cohabitation is being viewed as a marriage alternative, citing a 2001 study of her own. The evidence, she found, didn't suggest people cohabit to start a family, which she says is what would be expected if cohabitation were considered a marriage alternative.