LORAIN, Ohio — It used to be called illegitimacy. Now it is the new normal. After steadily rising for five decades, the share of children born to unmarried women has crossed a threshold: More than half of births to U.S. women younger than 30 occur outside marriage.

Once largely limited to poor women and minorities, motherhood without marriage has settled deeply into middle America. The fastest growth in the past two decades has occurred among white women in their 20s who have some college education but no four-year degree, according to Child Trends, a research group that analyzed government data.

Among mothers of all ages, a majority — 59 percent in 2009 — are married when they have children. But the surge of births outside marriage among younger women — nearly two-thirds of U.S. children are born to mothers younger than 30 — is both a symbol of the transforming family and a hint of coming generational change.

One group still largely resists the trend: College graduates overwhelmingly marry before having kids. That is turning family structure into a new class divide, with the economic and social rewards of marriage reserved for people with the most education.

“Marriage has become a luxury good,” said Frank Furstenberg, a sociologist at the University of Pennsylvania.

The shift is affecting children’s lives. Researchers have consistently found that American children born outside marriage face elevated risks of falling into poverty, failing in school or suffering from emotional and behavioral problems.

The forces rearranging the family are as diverse as globalization and the pill. Liberal analysts argue that shrinking paychecks have thinned the ranks of marriageable men, while conservatives often say that the sexual revolution reduced the incentive to wed and that safety net programs discourage marriage.

Here in Lorain, a blue-collar town west of Cleveland where the decline of the married two-parent family has been especially steep, dozens of interviews with young parents suggest that both sides have a point.

Over the past generation, Lorain lost most of two steel mills, a shipyard and a Ford factory, diminishing the supply of jobs that allow blue-collar workers to raise families with a middle-class lifestyle. More women went to work, making marriage less of a financial necessity for them. Living together became routine, and single motherhood lost the stigma that once sent couples rushing to the altar. Women here often describe marriage as a sign of having arrived rather than a way to get there.

Meanwhile, children happen.

How much does it matter?

The recent rise in single motherhood has set off few alarms, unlike in past eras. When Daniel Patrick Moynihan, then a top Labor Department official and later a U.S. senator from New York, reported in 1965 that a quarter of black children were born outside marriage — and warned of a “tangle of pathology”— he set off a bitter debate.

By the mid-1990s, such figures looked quaint: A third of Americans were born outside marriage. Congress, largely blaming welfare, imposed tough restrictions. Now the figure is 41 percent — and 53 percent for children born to women younger than 30, according to Child Trends, which analyzed 2009 data from the National Center for Health Statistics.

Large racial differences remain: 73 percent of black children are born outside marriage, compared with 53 percent of Latinos and 29 percent of whites. And educational differences are growing. About 92 percent of college-educated women are married when they give birth, compared with 62 percent of women with some post-secondary schooling and 43 percent of women with a high school diploma or less, according to Child Trends.

Almost all of the rise in nonmarital births has occurred among couples living together. While in some countries such relationships endure at rates that resemble marriages, in the United States they are more than twice as likely to dissolve than marriages.

In Lorain as elsewhere, explanations for marital decline start with home economics: Men are worth less they used to be. Among men with some college but no degrees, earnings have fallen 8 percent in the past 30 years, according to the Bureau of Labor Statistics, while the earnings of their female counterparts have risen by 8 percent.

“Women used to rely on men, but we don’t need to anymore,” said Teresa Fragoso, 25, a single mother in Lorain. “We support ourselves. We support our kids.”

Rethinking priorities

Fifty years ago, researchers have found, as many as a third of U.S. marriages were precipitated by a pregnancy, with couples marrying to maintain respectability. In interviews with women here, the children were described as largely unplanned, a byproduct of relationships lacking commitment.

Some unwed mothers cite the failures of their parents’ marriages as reasons to wait. Brittany Kidd was 13 when her father ran off with one of her mother’s friends, plunging her mother into depression and leaving the family financially unstable.

“Our family life was pretty perfect: a nice house, two cars, a dog and a cat,” she said. “That stability just got knocked out like a window; it shattered.”

Kidd, 21, said she could not imagine marrying her son’s father, even though she loves him.

“I don’t want to have to rely on someone else,” she said. “I don’t want to wind up like my mom.”

Even as many Americans withdraw from marriage, researchers say, they expect more from it: emotional fulfillment as opposed merely to practical support.

“Family life is no longer about playing the social role of father or husband or wife. It’s more about individual satisfaction and self-development,” said Andrew Cherlin, a sociologist at Johns Hopkins University.

Certainly money helps explain why well-educated Americans still marry at high rates: They can offer each other more financial support, and hire others to do chores that prompt conflict. But some researchers argue that college-educated men have also been quicker than their blue-collar peers to accept roles that give women equal authority.

“They are more willing to play the partner role,” said Sara McLanahan, a Princeton sociologist.

Reviewing the academic literature, Susan Brown of Bowling Green State University in Ohio recently found that children born to married couples, on average, “experience better education, social, cognitive and behavioral outcomes.”