They prime us with more food, so guaranteeing higher consumption - and sales.
Research shows that portion size directly influences how much we eat. In one study, when participants were offered a portion of macaroni cheese 50 per cent bigger than a standard portion, their energy intake increased by 19 per cent; when the portion was increased by 100 per cent, intake increased by 30 per cent. In a US study, carried out by the applied economics and management department of Cornell University and the graduate school of library and information science at the University of Illinois, 40 students were offered snacks before a televised football game. Those offered large bowls ate 56 per cent more (142 calories per person) than students offered the same snacks out of smaller bowls.
The researchers put the same weight of peanuts, crisps and pretzels on two different tables, one set with two large bowls and one set with four smaller bowls. Students who were offered food from the big bowls took more and ate more than the others. The researchers, whose conclusions have been published in the Journal of the American Medical Association, believe that big serving bowls, just like big packets of crisps sold in supermarkets or supersized buckets of popcorn at the cinema, encourage overeating because they imply that eating more is acceptable. They suggest that "portion distortion" could be used to better effect by instead serving large bowls of fruit and vegetables.
In a third study, Nicole Diliberti and colleagues from the department of nutritional sciences at Pennsylvania State University investigated how varying the size of a meal affected the amount that was eaten. They served a standard (248g) and a large (377g) pasta dish to customers in a restaurant. The meals cost the same and customers didn't know which one they had been given. Those who were given the larger portion increased their energy intake by 43 per cent (172 calories). They also ate more of the accompaniments (tomato, roll, and butter). So the next time someone asks you if you want to supersize that meal, tell them that regular is just fine.
Dr Raj Persaud is Gresham professor for public understanding of psychiatry, and director of the Centre for Public Engagement, King's College London.
His latest book is The Motivated Mind (Bantam Press). Email: firstname.lastname@example.org