Almost nobody can stop eating at just one normal serving if there’s extra food on their plate, Penn State researchers have shown, and this tendency coupled with the spread of megaportions may be contributing to the American obesity epidemic. In the first systematic, controlled study of the response to portion size in adults, the researchers found that the bigger the portion, the more the participants ate. On average, they ate 30 percent more from a five-cup portion of macaroni and cheese than from one half its size ? without reporting feeling any fuller after eating. Dr. Barbara Rolls, who holds the Guthrie Chair of Nutrition in Penn State’s College of Health and Human Development, led the study. She says, “Men and women, normal-weight and overweight individuals, restrained and unrestrained eaters, all responded to larger portion size by eating more.”