It has become fashionable for our most selective colleges to worry about becoming as representative of American diversity as suburban country clubs.

College admissions experts conferring at the University of Southern California this year were so alarmed that they suggested our most prestigious campuses add space for another 100 students in each class and fill those slots with low-income kids.

Why are our choosiest colleges so dominated by affluent white or Asian students? The explanations are many: not enough financial aid, inadequate preparation in inner-city high schools, poor students’ discomfort mixing with rich kids.

But a new study by researchers from the University of Michigan and the University of Arizona suggests something different. Great high schools and families like those in the Washington area may be at fault, at least in part. In the last 32 years, low-income students have significantly raised the grades and test scores that affect college admissions, but have made little headway because students from affluent families have improved even more.

No region of the country is quite as focused on preparing for college as ours is. On average, the Washington area has the wealthiest and most educated parents in the country, and the most demanding high schools. Our yearning for admissions to the best- known colleges has produced a new generation of college students better than any before.

Yet, Michael N. Bastedo of Michigan and Ozan Jaquette of Arizona say, this has frustrated the dreams of hard-working kids in D.C. and Baltimore and Chicago and Philadelphia.

“We find that although low-income students have shown strong gains in the indicators that lead to admission to highly selective schools . . . higher income students have simultaneously made even stronger gains on these same indicators,” they say in their paper “Running in Place: Low-Income Students and the Dynamics of Higher Education Stratification” in the September issue of the journal Educational Evaluation and Policy Analysis.

“Thus, enrollment in selective colleges has become a horse race in which wealthier students always remain at the head of the pack.”

Bastedo told me in an e-mail: “Many of the hard-working low-income students who are turned away from highly selective colleges today would have been happily admitted in 1975.”

The researchers suggest admissions officers take another look at measures used to decide who is most worthy of a selective college slot. They say giving the nod to applicants with higher SAT scores, for instance, is “not well-supported by evidence linking these differences to future achievement.”

SAT scores correlate with family income, the researchers say. The more your parents make, the higher your score is likely to be. But “the predictive validity of SAT scores lies primarily in its ability to serve as a proxy for high school quality rather than predicting a student’s individual achievement,” they say. “Colleges that have made SAT scores optional for admissions report that these students achieve at the same levels at their SAT-reporting counterparts.”

There appears to be no definitive research on how admissions officers treat differences in SAT scores and extracurricular activities when they decide whom to admit, Bastedo and Jaquette say. I have attended meetings of alumni interviewers for one selective college where the SAT controlled the discussion. If a student had excellent grades and stellar after-school achievements but not a top SAT score, almost no one wanted to recommend that their alma mater admit that young person.

Some research suggests that getting into a selective college is not important. Student character traits, not the college’s reputation, define their success in life. They can find as much of what they need at the State University of New York at Buffalo as they can at Yale.

But Bastedo and Jaquette note that the more selective a college is, the more likely its students will earn degrees. That may have nothing to do with any value added by those colleges. Their graduation rates are higher because they attract more academically capable students.

Maybe, but because little is known about the mechanics of selective college choices, it can’t hurt for those schools to take another look at what they are doing. They ought to make sure they are not creating useless barriers to less privileged applicants without realizing it.