The study, conducted by social scientists at the Pew Research Center in coordination with Time magazine, also shows that Americans' attitudes toward family issues differ by race, age and social class. It's based on interviews with 2,691 adults reached on their cell or land-line phones during the first three weeks of October. Pew researchers also analyzed demographic and economic data from the U.S. Census Bureau.
Among the biggest changes in Americans' attitude toward marriage was the number of those who now believe the institution is becoming obsolete. Thirty-nine percent of respondents said they believe that's the case, compared with 28 percent who said that when Time magazine asked the same question in 1978.
Those most likely to think marriage is becoming obsolete are people who are part of the trend: 62 percent of unmarried couples who live together and have children. Forty-two percent of self-described conservatives also believe marriage is becoming less popular and say they're troubled by it.
"If four in 10 are saying it's becoming obsolete, they're registering an awareness of a very important social change," Pew researcher Paul Taylor told USA Today. "It doesn't necessarily mean marriage is about to disappear or has disappeared."
Overall, 67 percent of respondents said they're still optimistic about the future of marriage and family in America, more than the country's education system (50 percent optimistic), its economy (46 percent) or its morals and ethics (41 percent).
In 1960, 68 percent of American twenty-somethings were married. By 2008, 26 percent were. Among adults of all ages, 72 percent were married in 1960, compared with just over half -- 52 percent -- in 2008, according to the Pew research.
So while marriage is becoming less popular, attitudes toward it also differ by class, age and race, the Pew survey found.
For instance, the gap between marriage rates for college graduates versus those with only a high school diploma or no degree is widening. In 1960, the gap was 4 percentage points, with 76 percent of college graduates marrying versus 72 percent of non-graduates. By 2008, the gap had jumped to 16 points, with 64 percent of college graduates getting married compared with 48 percent of those with a high school degree or less. Those less educated still said they would like to marry, but do so in smaller numbers because of financial concerns, the research found.
Americans are also more likely to believe people can come together to form a family without being married, said Andrew Cherlin, a professor of sociology and public policy at Johns Hopkins University.
Some 86 percent of respondents said they consider a single parent and child to be a family, and 80 percent said unmarried couples living together with children are families. Nearly two-thirds (63 percent) said they think a gay or lesbian couple raising a child constitutes a family. But even though a majority (88 percent) think a childless married couple is a family, a similar majority thinks a childless unmarried couple is not.
The Pew survey has a total margin of error of plus or minus 2.6 percentage points, larger for subgroups.