Officials at schools that utilize percentage plans say the plans increase diversity because rural and minority students automatically are admitted if they rank high enough in their graduating classes.
But according to the report, such plans only will have their desired effect if affirmative action and other supplemental recruitment, admissions and academic support programs remain in place.
States already using percentage plans include California, Florida and Texas.
UNC does not have a specified policy for the number of minority students it admits.
In California, graduates in the top 4 percent of their high school class are guaranteed admittance to at least one campus in the University of California system. In Texas, the top 10 percent automatically receive a promise of admission, while the top 20 percent in Florida are enrolled.
UC officials established the system's percentage program -- Eligibility in the Local Context -- two years ago.
UC-system spokesman Hanan Eisenman said ELC increases diversity at system campuses because it increases the universities' presence in high schools, which increases the number of applicants from around the state.
"There are some high schools in the state that have historically sent few graduates to the University of California system, and ELC increases that number," he said.
The UC system wanted to reach out to high school students across the state and to encourage them to attend public universities rather than to go out of state, he said. "We did (ELC) to reward students in all parts of California. ELC sends a message to work hard and succeed and (the UC system) will recognize you."