Prevention of the Portion Size Effect

An increase in the portion size leads to an increase in energy intake, a phenomenon which is also referred to as the portion size effect. Previous research has shown that this effect occurs for different kinds of people, in different settings, and for different foods. Furthermore, the portion size effect persists over longer periods of time and has therefore been connected to the worldwide increase in overweight and obesity. Portion and pack sizes have increased in recent years and it is thus important to find ways to prevent the occurrence of the portion size effect. In Chapter 2 I review what is known about the portion size effect to date and discuss possible causes of the effect. I expand on the view that consumers are uncertain about how much they should eat and therefore rely on external reference points, such as the portion size, to determine their consumption amount. The way the portion size is incorporated in the consumption quantity decision can vary, which leads to four different explanations for the portion size effect: cleaning the plate, unit bias, anchoring, and portion sizes as social norms. To prevent the portion size effect, people can be encouraged either to rely less on external reference points or to use other external reference points in the consumption decision. In Chapter 3, we tested whether the portion size acts as a social norm and as such communicates how much is appropriate to eat. In two online experiments, we manipulated the normative relevance of the portion size, either by providing information that the portion size communicated the behaviours of an in-group (own university students) or out-group (students from a different university), or that it was approved by a minority or majority of a relevant social group. Results showed that participants expected to eat and serve more from larger than from smaller portions, but that this portion size effect was less pronounced when the normative relevance of the portion size was decreased. Furthermore, in Experiment 3.2, the relation between portion size and the expected amount served was partly mediated by the amount that was considered appropriate, providing further evidence that the portion size is indeed an indicator of the appropriate amount to eat. At the same time, mediation was only partial, and although the normative relevance manipulation weakened the portion size effect, it did not prevent it. Hence, social concerns about eating appropriately certainly play a role in the portion size effect, but there seem to be other causes as well. In Chapter 4, we focussed on preventing the portion size effect by motivating people not to use external reference points, such as the portion size, in their consumption quantity decision. Previous research has shown that providing diet-concerned individuals with a reminder of their dieting goal can help them control their consumption, and we investigated whether such a reminder would also be effective in reducing the pack size effect. In Experiment 4.1, expected consumption of a number of snack foods was measured, and a dieting magazine served as the diet prime. In Experiment 4.2, actual consumption of M&M’s was measured, and dieting commercials served as diet primes. The results of both 138 experiments indeed showed that restrained eaters lowered their (expected) consumption from large snack packages when exposed to a diet prime and as a result the pack size effect disappeared. In Chapter 5, we tested whether instead of motivating people not to use an external reference point in the consumption decision, it might be effective to provide people with an alternative, better reference point in the form of a serving size recommendation. In three experiments, we measured (expected) consumption of unhealthy snack foods which were provided in either a small or large package which did or did not contain the serving size recommendation. Furthermore, we tested both a pictorial serving size recommendation, which displayed food amounts visually and a non-pictorial recommendation, which communicated the recommended amount in grams only. Only the pictorial recommendation reduced (expected) consumption from large packs and hence weakened the pack size effect. Although the serving size recommendation reduced the pack size effect, it did not seem to fully remove it, suggesting that it might not have provided sufficient motivation for everyone to no longer incorporate the portion size in the consumption quantity decision. In summary, it is difficult, but not impossible to weaken the portion and pack size effect. These findings also imply that the tendency to eat more when more food is provided, is to a certain extent under conscious control. This means, that with sufficient help, people will be able to rely less on environmental cues, such as the portion size, when making consumption decisions.