Abstract
People typically eat more from large portions of food than from small portions. An explanation that has often been given for this so-called portion size effect is that the portion size acts as a social norm and as such communicates how much is appropriate to eat. In this paper, we tested this explanation by examining whether manipulating the relevance of the portion size as a social norm changes the portion size effect, as assessed by prospective consumption decisions. We conducted one pilot experiment and one full experiment in which participants respectively indicated how much they would eat or serve themselves from a given amount of different foods. In the pilot (N = 63), we manipulated normative relevance by allegedly basing the portion size on the behavior of either students of the own university (in-group) or of another university (out-group). In the main experiment (N = 321), we told participants that either a minority or majority of people similar to them approved of the portion size. Results show that in both experiments, participants expected to serve themselves and to eat more from larger than from smaller portions. As expected, however, the portion size effect was less pronounced when the reference portions were allegedly based on the behavior of an out-group (pilot) or approved only by a minority (main experiment). These findings suggest that the portion size indeed provides normative information, because participants were less influenced by it if it communicated the behaviors or values of a less relevant social group. In addition, in the main experiment, the relation between portion size and the expected amount served was partially mediated by the amount that was considered appropriate, suggesting that concerns about eating an appropriate amount indeed play a role in the portion size effect. However, since the portion size effect was weakened but not eliminated by the normative relevance manipulations and since mediation was only partial, other mechanisms may also play a role.
Highlights
The size of the portion one is served has a strong and well recognized influence on the amount of food that people consume, such that more is consumed when the portion size increases
Portion Size Effect We calculated the portion size effect for each participant by first standardizing all consumption amounts, and subtracting the standardized consumption of the foods shown with a small portion size from the standardized consumption of the foods shown with a large portion size
The amount that was considered appropriate only partially mediated the relation between portion size and expected amount served
Summary
The size of the portion one is served has a strong and well recognized influence on the amount of food that people consume, such that more is consumed when the portion size increases (see for example Zlatevska et al, 2014; Hollands et al, 2015 for reviews). Two recurring explanations for the portion size effect are visual cues (Benton, 2015; English et al, 2015; Herman et al, 2015) and appropriateness (Rolls et al, 2002; Herman and Polivy, 2005, 2008; Wansink and van Ittersum, 2007; Steenhuis and Vermeer, 2009; Herman et al, 2015). We focus on the explanation that has been most dominant in the literature: appropriateness (Herman et al, 2015)
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