As the shaded quadrangles of the nation’s elite campuses stir to life for the start of the academic year, they remain bastions of privilege. Amid promises to admit more poor students, top colleges educate roughly the same percentage of them as they did a generation ago. This is despite the fact that there are many high school seniors from low-income homes with top grades and scores: twice the percentage in the general population as at elite colleges.

A series of federal surveys of selective colleges found virtually no change from the 1990s to 2012 in enrollment of students who are less well off — less than 15 percent by some measures — even though there was a huge increase over that time in the number of such students going to college. Similar studies looking at a narrower range of top wealthy universities back those findings. With race-based affirmative action losing both judicial and public support, many have urged selective colleges to shift more focus to economic diversity.

This is partly because students are more likely to graduate and become leaders in their fields if they attend competitive colleges. Getting low-income students onto elite campuses is seen as a vital engine of social mobility.

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Yet as Anthony P. Carnevale, director of Georgetown University’s Center on Education and the Workforce, put it, “Higher education has become a powerful force for reinforcing advantage and passing it on through generations.”

It is true that low-income enrollment at some top colleges has been slowly climbing. And some studies suggest that colleges are well intentioned but simply ineffectual in addressing economic diversity. College leaders also point to studies showing that most low-income students with high grades and test scores do not apply to highly selective colleges.

But critics contend that on the whole, elite colleges are too worried about harming their finances and rankings to match their rhetoric about wanting economic diversity with action.

“It’s not clear to me that universities are hungry for that,” said Richard D. Kahlenberg, a senior fellow at the Century Foundation who studies college diversity. “What happens if low-income students start calling the bluff of selective universities, and do start applying in much larger numbers? Will the doors be open?”

There are elite colleges, both private and public, with three times as many recipients of Pell Grants — the main federal aid for low-income students — as some of their peers, which critics say shows that the others could be doing more. Prestigious schools like Vassar, Amherst, Harvard and the University of California system have managed to increase low-income enrollment.

“A lot of it is just about money, because each additional low-income student you enroll costs you a lot in financial aid,” said Michael N. Bastedo, director of the Center for the Study of Higher and Postsecondary Education at the University of Michigan. “No one is going to talk openly and say, ‘Oh, we’re not making low-income students a priority.’ But enrollment management is so sophisticated that they know pretty clearly how much each student would cost.”

Colleges generally spend 4 to 5 percent of their endowments per year on financial aid, prompting some administrators to cite this rough math: Sustaining one poor student who needs $45,000 a year in aid requires $1?million in endowment devoted to that purpose; 100 of them require $100?million. Only the wealthiest schools can do that, and build new laboratories, renovate dining halls, provide small classes and bid for top professors.

The rankings published by U.S. News and World Report, and others, also play a major role. The rankings reward spending on facilities and faculty, but most pay little or no attention to financial aid and diversity.

“College presidents are under constant pressure to meet budgets, improve graduation rates and move up in the rankings,” Carnevale said. “The easiest way to do it is to climb upstream economically — get students whose parents can pay more.”

A big part of that climb has been the rise of “merit aid,” price breaks offered to desirable students regardless of their parents’ wealth. A few dozen top schools give no merit aid, but they are the exception. Historically, American colleges gave far more need-based aid, but it is outweighed today by merit aid.

Since the late 1990s, top schools have made several high-profile moves to become more accessible to low- and middle-income families. The policy changes drew heavy coverage, but had limited effect, studies found, largely because poorer consumers were unaware of them.

Harvard, Princeton, the University of Virginia and the University of North Carolina at Chapel Hill eliminated early admission programs that were seen as favoring affluent students. Some colleges stopped including loans in financial aid packages, so that all aid came in the form of grants. Others lowered prices for all but affluent families, not requiring any contribution from parents below a certain income threshold, like $65,000.

But the colleges that ended early admissions reinstated them within a few years, after other elite schools declined to follow their lead, putting them at a disadvantage in drawing top students. Many of the benefits of the no-loan and no-parental-contribution policies went to middle-income families, and in any case, not all of the wealthiest schools, like Harvard, Princeton, Yale and Stanford, fully adopted both.

In 2006, at the 82 schools rated “most competitive” by Barron’s Profiles of American Colleges, 14 percent of American undergraduates came from the poorer half of the nation’s families, according to researchers at the University of Michigan and Georgetown University who analyzed data from federal surveys. That was unchanged from 1982.

And at a narrower, more elite group of 28 private colleges and universities, including all eight Ivy League members, researchers at Vassar and Williams Colleges found that from 2001 to 2009, a period of major increases in financial aid at those schools, enrollment of students from the bottom 40 percent of family incomes increased from just 10 percent to 11 percent.

Even with the best intentions, tapping the pool of high-performing, low-income students can be hard. Studies point to many reasons poorer students with good credentials do not apply to competitive colleges, like lack of encouragement at home and at school, thinking (correctly or not) that they cannot afford it or believing they would be out of place, academically or socially.

What distinguishes those who apply to elite schools is not family income or their parents’ level of education, according to a groundbreaking study published last year, but location. Exposure to just a few high-achieving peers or attending a high school with just a few teachers or recent alumni who went to highly selective colleges makes a huge difference in where low-income students apply.

So does face-to-face recruiting.

“You can make big statements about being accessible, and have need-blind admissions and really low net prices for low-income kids, but still enroll very few of those low-income kids, by doing minimal outreach,” said Catharine Bond Hill, president of Vassar College. “There has to be a commitment to go out and find them.”

But admissions officers can visit only a small fraction of the nation’s 26,000 high schools, so they rarely see those stellar-but-isolated candidates who need to be encouraged to apply. The top schools that have managed to raise low-income enrollment say that an important factor has been collaborating with some of the nonprofit groups, like QuestBridge and the Posse Foundation, that are devoted to identifying hidden prospects, working with them in high school and connecting them to top colleges.

The programs can be expensive for colleges, and many balk at the cost, refusing to join or taking only a handful of students each year.

“Last year, we had only 680 slots for 15,000 students nominated by schools and community groups,” said Deborah Bial, president of the Posse Foundation, which has 51 partner colleges but is trying to recruit more. “We could easily triple in size, tomorrow.”

Troy Simon, 21, said that without significant help from two nonprofit groups, he would not be where he is now, preparing for his junior year at Bard College. It is a long way from his chaotic childhood in a poor section of New Orleans, where he was held back in school — so far, in fact, that his story has been hailed by Michelle Obama as an example of what can be overcome.

“I was illiterate,” he said. “I clowned around in class, I didn’t understand what was going on, and no one asked too many questions about my failing grades at home.”

Separated from his family during Hurricane Katrina, which destroyed their home, he took refuge with cousins in an abandoned building. He was placed for a time in a school in Houston, where he said that at age 12, he finally began to learn to read, but after returning to New Orleans, he was arrested on suspicion of theft.

But in high school — he attended three of them — a group called College Track took him in, providing tutoring, test preparation, counseling and advice on applying to college. Then he won a Posse Foundation scholarship that paid for college and provided support services there.

“The kids that I hung out with when I was a kid that were smart, some of them are dead now, some are in prison, some of them have several kids that they can’t support,” he said. “I consider myself blessed.”

Ten to 15 years ago, when some elite colleges got more serious about economic diversity, there was a view that increasing financial aid could turn the dial, but “I think we were a little naïve,” said Morton O. Schapiro, president of Northwestern University, a former president of Williams College and, like Hill at Vassar, an economist specializing in the economics of higher education.

Cost remains a barrier, but so does perception, he said, adding, “It’s a psychology and sociology thing, as well as a pricing thing.”

College presidents like to say that consumers should not be scared off by high sticker prices, around $60,000 a year for top schools. But those are the numbers that draw national attention, concealing a much more tangled picture, in which true costs are hard to discern, hard to compare and wildly variable.

Public colleges remain less expensive, but in an era of declining state support, their prices have risen faster than those of private colleges, and they vary widely from state to state. Private colleges have sharply increased financial aid since the turn of the century, so that the average net price that families really pay has barely changed over the last decade, adjusted for inflation — and for low-income students, it has actually dropped.

But even top private colleges with similar sticker prices differ enormously in net prices, related to how wealthy they are, so a family can find that an elite education is either dauntingly expensive or surprisingly affordable. In 2011-12, net prices paid by families with incomes under $48,000 averaged less than $4,000 at Harvard, which has the nation’s largest endowment, for example, and more than $27,000 at New York University, according to data compiled by the Department of Education.

None of that complexity is apparent to most consumers.

“If you come from a family and a neighborhood where no one has gone to a fancy college, you have no way of knowing that’s even a possibility,” said Anthony W. Marx, president of the New York Public Library, and a former president of Amherst. “And if you go on their website, the first thing you’re going to look for is the sticker price. End of conversation.”