Prof Meredith Young found women consumed fewer calories dining with men than when dining with women.
Researchers at Montreal's McGill University secretly observed 460 college students eating in the campus cafeterias.
They found that when a woman was with a man, she ate about 100 calories less than when she was with a woman.
The more men present in larger eating groups, the fewer calories a woman had on her tray. Women ate roughly 100 fewer calories for each man at the table. But there was no such effect on men.
Women who only ate with other women tended to slightly increase their calorie consumption.