Instead of serving as a springboard to social mobility as it did for the first decades after World War II, college education today is reinforcing class stratification, with a huge majority of the 24 percent of Americans aged 25 to 29 currently holding a bachelor’s degree coming from families with earnings above the median income.

Seventy-four percent of those now attending colleges that are classified as “most competitive,” a group that includes schools like Harvard, Emory, Stanford and Notre Dame, come from families with earnings in the top income quartile, while only three percent come from families in the bottom quartile.

Anthony Carnevale, director of the Georgetown University Center on Education and the Workforce and co-author of “How Increasing College Access Is Increasing Inequality, and What to Do about It,” puts it succinctly: “The education system is an increasingly powerful mechanism for the intergenerational reproduction of privilege.”

These anti-democratic trends are driven in part by a supposedly meritocratic selection process with high school students from the upper strata of the middle class performing better on SAT and ACT tests than those from poor and working class families.

Contrary to those who say that this is the meritocracy at work, differences in scores on standardized tests do not fully explain class disparity in educational outcomes. When high-scoring students from low-income families are compared to similarly high-scoring students from upper-income families, 80 percent of the those in the top quarter of the income distribution go on to get college degrees, compared to just 44 percent of those in the bottom quarter.

Post-secondary education is not, in fact, functioning to dissolve long-standing class hierarchies. There are various ways of examining these trends, which I’ve outlined below. However you look at it, the cultural and political implications of the deepening of the income achievement gap are profound.

Beginning in the early 1980s, according to the Census, the college “premium” – the difference in annual earnings of a high school graduate and a college graduate – rose from 50 percent to approximately 80 percent. In 2007, workers with a high school degree made an average of $31,286 compared to $57,181, 82.8 percent more, for those with a bachelor’s degree. A college degree does not guarantee affluence, but it puts the recipient in a far better position to achieve or maintain upper-middle-class status than those without degrees.

Higher education itself has polarized: Competitive four-year colleges, as defined by Barron’s, have seen enrollments rise from 41 percent of all post-secondary students to 46 percent from 1994 to 2006; 2-year community colleges at the bottom have seen their share of enrollment grow from 46 to 49 percent. In the middle ground, the percent enrolled at the less competitive four-year colleges has been cut in half, from 13 to 6 percent, according to the Carnevale study mentioned above.

Source: National Education Longitudinal Study: Base Year through Fourth Follow-Up, 1988–2000 (Washington, D.C.: U.S. Dept. of Education, National Center for Education Statistics, 2000); The Education Longitudinal Study of 2002: Base Year through Second Follow-Up, 2002–2006 (Washington, D.C.: U.S. Department of Education, National Center for Education Statistics, 2006).

Student bodies in competitive colleges and in community colleges reflect two very different economic worlds. At the 1,044 competitive colleges, 76 percent of the freshman came from families in the upper half of the income distribution. In the nation’s 1,000-plus community colleges, almost 80 percent of the students came from low-income families.

“The education system is an increasingly powerful mechanism for the intergenerational reproduction of privilege.” — Anthony Carnevale

Low income students are heavily dependent on scholarship aid to go to college, and especially dependent on grants as opposed to loans. Need-based scholarships are one way to increase low-income enrollment, but over the past three decades, the value of Pell Grants – the basic form of federal scholarship aid to poor students – has steadily declined as tuition costs have grown at a much faster rate than inflation. In 1979-80, the maximum Pell Grant covered 99 percent of the cost of a community college, 77 percent at a public four-year college and 36 percent at a private four-year college. By 2010-11, these percentages had dropped to 62, 36 and 15 percent respectively, according to Education Week.

At the same time, colleges, both public and private, have shifted their own spending priorities, modestly increasing the investment in students from families in the lowest income quintile, while sharply boosting their investment in education of students from the top income quintile. The Education Trust has produced the charts showing the shift over a 12-year period, from 1995 to 2007.

SAT scores, in turn, correlate directly to students’ family income. The following charts show this linkage:

Source: College Board

There is a substantial body of evidence that the system is failing to reward students with high test scores who come from low-income families. In a 2005 report, the College Board found that among those scoring highest in math tests in 1992, just under three-quarters of students from families in the highest quartile went on to get bachelor’s degrees by the year 2000. Among those from families in the bottom quartile, less than half that number, 29 percent, went on to get degrees.

As the value of a college degree has nearly doubled, in terms of future earnings, the percentage of low income college students actually graduating by age 24 has grown by only 2.1 points, from 6.2 percent in 1970 to 8.3 percent in 2009. Among students from families in the highest income quartile, the graduation rate by age 24 has surged by 42.2 percentage points, doubling from 40.2 percent to 82.4 percent over the last four decades.

Source: Postsecondary Education Opportunity

“The income-achievement gap is now more than twice as large as the black-white achievement gap. Fifty years ago, in contrast, the black-white gap was twice as large as the income gap,” according to Sean Reardon, a professor of education and sociology at Stanford, writing in “The Widening Academic Achievement Gap between the Rich and the Poor: New Evidence and Possible Explanations:”

At the same time that family income has become more predictive of children’s academic achievement, so have educational attainment and cognitive skills become more predictive of adults’ earnings. The combination of these trends creates a feedback mechanism that may decrease intergenerational mobility. As the children of the rich do better in school, and those who do better in school are more likely to become rich, we risk producing an even more unequal and economically polarized society.

The “income achievement gap” – differences in standard test scores and grade point averages – between children from families in the top 10 percent of the income distribution and those from families in the bottom ten percent has been growing. Reardon has found that the income achievement gap between children from the highest and lowest income deciles is roughly 30 to 40 percent larger among children born in 2001 than among those born in 1976.

“The children of the rich increasingly do better in school, relative to the children of the poor — that is, they score higher on standardized tests and they graduate from college at much higher rates. This has always been true, but is much more true now than 40 years ago,” Reardon told The Times in an email. “This means that social mobility has gotten rarer – the ‘American Dream’ is increasingly difficult to attain.”

Not only does a college degree significantly boost income, it also helps protect against downward mobility. The Pew Charitable Trusts found in a 2011 study that those with just a high school degree or less are 13 percent more likely to experience significant downward mobility – seeing their income fall substantially below what their parents made – than those with college degrees.

The class-reinforcing trends of higher education pose an acute dilemma for the American political system. “The built-in tension between postsecondary selectivity and upward mobility is particularly acute in the United States. Americans rely on education as an economic arbiter more than do other modern nations,” Carnevale wrote in “How Increasing College Access Is Increasing Inequality, and What to Do About It.” “Americans always have preferred education over the welfare state as a means for balancing the equality implicit in citizenship and the inequality implicit in markets.”

Politically, the lack of access to a four-year college education is a crucial problem for one of the key battleground constituencies of 2012: whites without college degrees. Several issues that can be mined by enterprising politicians cluster around this debilitating lack of access — in fact they help cause it — including the enormous debt loads carried by students and recent graduates, as well as the emergence of for-profit colleges saddling low-income students with loans for programs they cannot complete. The data show that a disproportionately large percentage of young adults from working-class families who, according to their test scores and grade point averages, are equipped to earn a B.A., are either not going to college, or failing to finish — relegating them to a life of stagnant or declining wages. There is a reservoir of resentment over this fate waiting to be tapped by either party.