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 American women under 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 last 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 Washington 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 children in the United States are born to mothers under 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, who overwhelmingly marry before having children. That is turning family structure into a new class divide, with the economic and social rewards of marriage increasingly 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 children born outside marriage face elevated risks of falling into poverty, failing in school or suffering 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 let blue-collar workers raise middle-class families. 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.

Amber Strader, 27, was in an on-and-off relationship with a clerk at Sears a few years ago when she found herself pregnant. A former nursing student who now tends bar, Ms. Strader said her boyfriend was so dependent that she had to buy his cigarettes. Marrying him never entered her mind. “It was like living with another kid,” she said.

When a second child, with a new boyfriend, followed three years later — her birth control failed, she said — her boyfriend, a part-time house painter, was reluctant to wed.

Ms. Strader likes the idea of marriage; she keeps her parents’ wedding photo on her kitchen wall and says her boyfriend is a good father. But for now marriage is beyond her reach.

“I’d like to do it, but I just don’t see it happening right now,” she said. “Most of my friends say it’s just a piece of paper, and it doesn’t work out anyway.”

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 United States 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 under 30, according to Child Trends, which analyzed 2009 data from the National Center for Health Statistics.

Still, the issue received little attention until the publication last month of “Coming Apart,” a book by Charles Murray, a longtime critic of non-marital births.

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 a summary of research, Pamela Smock and Fiona Rose Greenland, both of the University of Michigan, reported that two-thirds of couples living together split up by the time their child turned 10.

In Lorain as elsewhere, explanations for marital decline start with home economics: men are worth less than 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.”

Fifty years ago, researchers have found, as many as a third of American marriages were precipitated by a pregnancy, with couples marrying to maintain respectability. Ms. Strader’s mother was among them.

Today, neither of Ms. Strader’s pregnancies left her thinking she should marry to avoid stigma. Like other women interviewed here, she described her children as largely unplanned, a byproduct of uncommitted relationships.

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.”

Ms. Kidd, 21, said she could not imagine marrying her son’s father, even though she loves him. “I don’t want to wind up like my mom,” she said.

Others noted that if they married, their official household income would rise, which could cost them government benefits like food stamps and child care. W. Bradford Wilcox, a sociologist at the University of Virginia, said other government policies, like no-fault divorce, signaled that “marriage is not as fundamental to society” as it once was.

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.

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 educated men have also been quicker than their blue-collar peers to give women equal authority. “They are more willing to play the partner role,” said Sara McLanahan, a Princeton sociologist.

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

Lisa Mercado, an unmarried mother in Lorain, would not be surprised by that. Between nursing classes and an all-night job at a gas station, she rarely sees her 6-year-old daughter, who is left with a rotating cast of relatives. The girl’s father has other children and rarely lends a hand.

“I want to do things with her, but I end up falling asleep,” Ms. Mercado said.

In fact, the tacit judgment of the New York Times-style liberal is in many ways more pernicious than the overt moralizing of conservatives on the downfall of family and marriage. It is easy to dismiss the Santorum faction for its cartoonishly old-fashioned view of extramarital sex, and this group is at least forthright about its view, whereas the subtle psychologizing put-down of the New York Times-style liberal, the slight hint of self-congratulation that they are not a single mother in Lorain, Ohio, bringing their son to the bar where they work, is more poisonous for its pretense of fairness and open-mindedness.