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What it does bring home to us is that we can no longer pretend that marriage is the central organizing principle of society.

A lower percentage of American adults are married than at any point in history, with just more than half — 51 percent — saying "I do," according to a report released by the Pew Research Center Wednesday.

Using Census data, the center documented continuation of a trend that includes more couples living together and couples waiting until they're older to get married, both of which may change the makeup of the American family.

In 1960, the report says, nearly three-fourths of all adults were married. The number of new marriages in America fell 5 percent from 2009 to 2010, "a sharp one-year drop that may or may not be related to the sour economy."

It also noted that the same trend is occurring in most "advanced post-industrial societies, and these long-term declines appear to be largely unrelated to the business cycle. The declines have persisted through good economic times and bad."

The most dramatic drop in marriage rates has been among young adults: In 1960, 59 percent of those between 18 and 29 were married; today, it's 20 percent. In that half-century, the median age of first marriage has risen by six years.

"It is not yet known whether today's young adults are abandoning marriage or merely delaying it," the Pew report says. "Even at a time when barely half of the adult population is married, a much higher share — 72 percent — have been married at least once. However, this 'ever married' share is down from 85 percent in 1960."

The report doesn't speculate on why marriage has declined. It notes that the declines are smaller among college-educated adults than among the less educated.

The Washington Post noted that in the nation's capital, "which has experienced an influx of young adults over the last decade," one in four adults is married and more than half have never been married. But it points out that they might later; median age of marriage for women is 26 and for men is 29, "an all-time high."

"In the 1950s, if you weren't married, people thought you were mentally ill," marriage and family expert Andrew J. Cherlin, a sociologist at Johns Hopkins University, told the Post. "Marriage was mandatory. Now it's culturally optional."

The findings don't mean that marriage is dead, as family life expert Stephanie Coontz of Evergreen State College in Olympia, Wash., told NPR's Jennifer Ludden. Many of the young unmarrieds will wed at some point.

"But what it does bring home to us is that we can no longer pretend that marriage is the central organizing principle of society. We have to take account of the many, many social networks and relationships that people cycle through, marriage being just one of them."

While social scientists are watching the marriage trends to see what they will mean, some organizations say losing marriage as a cornerstone for family life would be detrimental. According to data from FamilyFacts.org, the benefits of marriage are wide-ranging. The group says married couples have greater sexual satisfaction, married women are healthier and married people are more likely to volunteer. It also notes marriage comes with a higher likelihood of affluence, less depression and substance abuse and lower mortality risk.

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