The effect of unit number and inter-unit distance on perceived food portion size

Previous studies have demonstrated that perceived portion size of food influences consumption, so modifying perceived portion size may affect food intake. The present study evaluated the effects of unit number and inter-unit distance on perceived food portion size through eye-tracking and behavioral experiments. After observing one reference portion size picture, 34 participants in Experiment 1 and 29 participants in Experiment 2 evaluated 60 chocolate pictures with 10 different total portion sizes, with a unit number of 9 or 16 and an inter-unit distance of 100%, 120%, or 130%. The results demonstrated that unit number and inter-unit distance independently influenced the perception of overall portion size, while only the unit number could influence the perception of unit portion size. Making estimations about unit portion size requires more cognitive resources than that of overall portion size. These findings imply that we can adjust people's perceived portion sizes by varying the inter-unit distance or unit number of foods, thus nudging individuals to make more rational portion size decisions.