A growing number of applicants to US universities are refusing to disclose their race, further complicating the debate over affirmative action.
A report in the Journal of Blacks in Higher Education says that high numbers of students decline to identify their race at 11 of the nation's 29 highest-ranked universities.
Up to one in five students does not indicate their race on application forms to Harvard University, and at many other top schools the proportion is about one in ten.
Admissions officials said it was probable that white students thought they were at a disadvantage at private universities such as Harvard, which still have aggressive affirmative-action programmes aimed at increasing minority enrolment. They said those fears were unfounded.
Officials speculated that some conservative black students might also not disclose their race, preferring to be judged exclusively on their merits.
Admissions officers said that other applicants might be biracial, and do not neatly fit the racial categories from which they are asked to choose.
The US Supreme Court ruled this summer that race could continue to be used by state universities in admissions selections, as long as it was not the only or overriding factor. In a close vote, the court also said that affirmative action should eventually stop.
While private universities are not technically affected, the decision thrust the issue of race into the forefront of public discourse - and applicants' minds.
Not only does this reluctance make it harder for universities to ensure racially diverse enrolment, but it also complicates a requirement that they report their racial breakdown to the government to receive federal funding.
For now, however, no universities appear to be in jeopardy of losing this support. They simply report "no racial classification" for increasing numbers of their students.