Humans savour both fatty foods and carbohydrate-rich ones. But new experiments show that people place the highest value on foods that combine carbohydrates and fat — a finding that could help to explain the obesity epidemic.

Dana Small at the Yale University School of Medicine in New Haven, Connecticut, and her colleagues asked volunteers to place monetary bids on a variety of foods. Volunteers were willing to pay a premium for cake, chocolate and other fatty, sugary treats, but assigned lower values to equally liked and equally caloric foods, such as cheese, that are rich in only one category of nutrient.

Brain scans revealed that when volunteers pondered the merits of sweet, fatty snacks, their brains showed a surge of activity in cerebral areas involved with habit formation and reward. The authors suggest that human esteem for fat- and carbohydrate-rich foods, combined with the modern ubiquity of such foods, could contribute to overeating.