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.”