The company instituted the policy last year, after two black men were arrested while sitting at a Starbucks store in Philadelphia, waiting for a friend, and not buying anything.
The incident created an uproar.
The study, done by the University of Texas at Dallas and Boston College, found the new, open-bathroom policy has caused an almost seven-percent drop in store attendance, compared to shops nearby.
Researchers say people may be finding the bathrooms are dirty, and the tables are crowded, so they turn around and leave without buying anything.
Starbucks issued a statement disputing the research, saying customers are visiting its stores in record numbers.