The economic storms of recent years have raised concerns about growing inequality and questions about a core national faith, that even Americans of humble backgrounds have a good chance of getting ahead. Most of the discussion has focused on labor market forces like falling blue-collar wages and lavish Wall Street pay. But striking changes in family structure have also broadened income gaps and posed new barriers to upward mobility. College-educated Americans like the Faulkners are increasingly likely to marry one another, compounding their growing advantages in pay. Less-educated women like Ms. Schairer, who left college without finishing her degree, are growing less likely to marry at all, raising children on pinched paychecks that come in ones, not twos. Estimates vary widely, but scholars have said that changes in marriage patterns — as opposed to changes in individual earnings — may account for as much as 40 percent of the growth in certain measures of inequality. Long a nation of economic extremes, the United States is also becoming a society of family haves and family have-nots, with marriage and its rewards evermore confined to the fortunate classes. “It is the privileged Americans who are marrying, and marrying helps them stay privileged,” said Andrew Cherlin, a sociologist at Johns Hopkins University. About 41 percent of births in the United States occur outside marriage, up sharply from 17 percent three decades ago. But equally sharp are the educational divides, according to an analysis by Child Trends, a Washington research group. Less than 10 percent of the births to college-educated women occur outside marriage, while for women with high school degrees or less the figure is nearly 60 percent. Long concentrated among minorities, motherhood outside marriage now varies by class about as much as it does by race. It is growing fastest in the lower reaches of the white middle class — among women like Ms. Schairer who have some postsecondary schooling but no four-year degree. While many children of single mothers flourish (two of the last three presidents had mothers who were single during part of their childhood), a large body of research shows that they are more likely than similar children with married parents to experience childhood poverty, act up in class, become teenage parents and drop out of school. Sara McLanahan, a Princeton sociologist, warns that family structure increasingly consigns children to “diverging destinies.” Married couples are having children later than they used to, divorcing less and investing heavily in parenting time. By contrast, a growing share of single mothers have never married, and many have children with more than one man. “The people with more education tend to have stable family structures with committed, involved fathers,” Ms. McLanahan said. “The people with less education are more likely to have complex, unstable situations involving men who come and go.” She said, “I think this process is creating greater gaps in these children’s life chances.”