Groups that work with poorer students and administrators at colleges with high disadvantaged enrollment say that one main factor is simply making the effort to get low-income students to apply. Last year, researchers at Stanford and Harvard reported that the vast majority of high-achieving, low-income students do not apply to any selective colleges.

“Kids who’ve never heard of most elite institutions, who don’t know anyone who’s gone to one, who assume they can’t afford one, aren’t going to apply unless you go out and recruit them,” said Anthony W. Marx, president of Amherst from 2003 to 2011.

Most of the top private colleges rely on nonprofit groups like Questbridge and the Posse Foundation to help them find promising disadvantaged students. However, some recruit heavily that way while others only take one or two students per year. And some elite institutions, like Washington University, do not work with such groups.

For colleges that have the resources, another factor is how willing they are to spend the money to hunt for those applicants, and the much larger amount needed to help lower-income students go to a $60,000-a-year college. In addition, poor students face bigger challenges to remain enrolled and colleges often spend money on support services for them.

Among the 50 most selective private colleges, as measured by test scores and the percentage of applicants admitted, about four-fifths have need-blind admissions, meaning that they pledge to judge applicants without considering their ability to pay. Most of the colleges that do consider financial need to varying degrees, including Colby College and Washington and Lee, have lower Pell enrollments than most of their peers.

At a forum last fall, Mark S. Wrighton, the chancellor of Washington University, said of need-blind admission, according to the student newspaper, “It’s not our highest priority” — a comment that Mr. Berg said was taken out of context.