Portion sizes of commercially available foods have dramatically increased over recent decades [1
], and it has been consistently demonstrated that people eat more from larger than from smaller food portions [3
]. Large food portion sizes have therefore been identified as a possible contributor to the obesity epidemic [5
]. Recent insights show that portion sizes served in a given eating occasion not only affect immediate consumption, but also affect subsequent portion selection and consumption at later eating occasions [7
]. Particularly, it has been shown that when served a smaller portion, people select and consume a smaller portion of food in the future compared to when they are served a larger portion. Thus, changes to portion sizes in the environment have potential downstream consequences beyond a single eating occasion [7
]. However, relatively little is known about the mechanism responsible for this effect [9
Previous findings suggest that exposure to and consumption of smaller portion sizes may recalibrate perceptions of portion size, making smaller portions more ‘normal’. In one study, mere visual exposure to images of small (versus large) portions decreased subsequent perceptions of what constituted a ‘normal’ sized portion, and this resulted in participants selecting a smaller ideal portion of that food immediately afterwards, but this effect did not translate to actual food selection [10
]. Furthermore, in a series of studies, when participants were served a small (versus large) portion of a lunch meal they consumed significantly less one day later, and chose a smaller ideal portion of that same meal one week later [7
]. It was demonstrated that this effect partly occurs because being served a smaller (versus larger) portion size decreases peoples’ perceptions of what constitutes a ‘normal’ portion size [7
]. Comparable results on food intake were found in an experiment manipulating visual exposure to physically present portion sizes of snack foods. In this study, visual exposure to portion sizes affected perceptions of portion size normality 24 h later, although perceptions of portion size normality did not formally mediate the effect of visual exposure on subsequent consumption of the same snack food [8
]. In this previous work [7
], perceptions of portion size normality were measured by asking participants to indicate what they thought was a ‘normal’ portion size of food to eat in a given situation, but it remains unclear what these portion size normality judgments are based on.
Previous research has demonstrated that consumers possess divergent social and personal norms for portion size. A perceived ‘social norm’ represents what consumers believe other people
consider to be a normal and/or appropriate amount to eat. A perceived ‘personal norm’ represents the amount of food consumers consider to be a normal and/or appropriate amount for themselves
to eat [11
]. The construction of a ‘personal norm’ is a dynamic process that is influenced by the external environment [11
], and a served portion size may affect future consumption by affecting one’s ‘personal’ norm. There are two conceptually and motivationally distinct types of social norms that may be affected by exposure to different portion sizes. First, consumers may believe a served portion size is based on what other people consume (a ‘descriptive’ social norm) [13
]. This is consistent with evidence that social norms about food consumption are inferred from physical aspects of food environments. For example, previous experiments in laboratory and real-world settings have shown that consumers who were presented with a bowl of snacks surrounded by empty snack wrappers consumed more than those who were presented the same snack bowl but without empty wrappers [14
]. The wrappers may have communicated that others had previously consumed the snacks in the same situation, therefore exemplifying a descriptive social norm communicated by the eating environment. Second, an ‘injunctive’ social norm is what one perceives ought
to be done [13
]. Consumers are likely to assume that a served portion size was not chosen at random by the person serving it, but that there was some reasonable rationale behind providing that amount of food [16
]. They may therefore infer that a portion size served to them represents what others think is the appropriate amount for them to eat (an injunctive social norm), and this could affect later portion size selection and consumption.
Rather than via a perceived norm, an alternative explanation is that portion size communicates how much of that food one needs to consume in order to feel satisfied. This ‘expected satiety’ belief may be learned from the post-ingestive consequences of having consumed a given portion (the feeling of satisfaction and the avoidance of hunger after eating), or merely inferred from the amount presented (e.g., ‘this must be enough to keep me satisfied if someone has decided to serve this amount’). Higher expected satiety associated with a given food is associated with the selection of smaller portion sizes and reduced consumption of that food [8
], and may therefore play a role in the effect of portion size exposure on subsequent behavior.
In the present research, we examined whether visual exposure to (Study 1) and being served (Study 2) smaller versus larger portion sizes would affect later portion size selection for a hypothetical meal (Study 1) and consumption of an actual meal (Study 2). We hypothesized that this effect would be explained by (1) the general perception of what constitutes a normal sized portion, as shown in previous research [7
], and (2) more specific perceptions of social (both descriptive and injunctive) and personal norms about what is a normal amount to eat in that situation. Particularly, it was expected that visual and actual exposure to smaller (versus larger) food portions would result in a lower intended and actual consumption of that food 24 h later and, accompanying perceptions of a normal-sized portion, perceptions of social (both descriptive and injunctive) and personal norms. We also tested an alternative explanation; that exposure and consumption of smaller portion sizes may affect later portion selection and consumption by affecting expected satiety.
Across two experiments, we tested whether perceptions of descriptive social norms (beliefs about what others do), injunctive social norms (beliefs about what should be done according to others), and personal norms (beliefs about what should be done according to oneself) underlie the effect of exposure to different food portion sizes on future portion size selection and consumption of the same food. The present findings indicate that participants who were actually served a smaller (versus larger) portion size of food served themselves and consumed less of that food the next day (Study 2), whereas mere visual exposure to a smaller (versus larger) portion size of food did not affect hypothetical portion size selection the next day (Study 1). Consistent with our hypotheses, Study 2 found that the relationship between the portion size exposure condition and later consumption was partially mediated by changes in perceptions of descriptive and injunctive social norms (but not personal norms) for portions of that food, although, contrary to predictions, no significant evidence was found for the role of general perceptions of portion size normality in this relationship.
The results of the present research are consistent with previous findings, demonstrating that consuming
smaller (versus larger) food portions decreases the amount of food that participants later freely serve themselves and consume [7
]. Together, this evidence supports the proposition that downsizing commercially available food products could have effects that extend beyond the consumption of the reduced food products, by affecting future portion size preference and consumption [7
]. An alternative proposition that has been explored in other research is that visual exposure to smaller food portions via digital media may ‘renormalize’ small portions. A feasibility study of a social media intervention that involved exposing students to images of peers’ snacks in small portion sizes resulted in participants reporting a smaller ideal snack portion size [23
]. However, the present findings that mere visual exposure to images of smaller (versus larger) portions was insufficient to significantly decrease future hypothetical consumption of that food reinforces the suggestion that food portions may need to be physically present in one’s environment in order to be able to adjust perceptions of a normal-sized portion and future consumption [8
]. Systematic exploration of the necessary conditions for altering perceived portion size normality and future consumption would be valuable to inform effective strategies to reduce overconsumption.
The present work did not replicate earlier findings that exposure to portion sizes affects consumers’ general perceptions of what is a ‘normal-sized’ portion. This was unexpected, as it has been repeatedly shown that exposure to a stimulus can alter people’s perceptions of size normality, demonstrated in relation to food portions [7
], as well as in other domains (e.g., perceptions of ‘normal’ body sizes) [24
]. A possible explanation for these non-significant findings could be that the more nuanced questions about portion size norms included in the present research (e.g., descriptive and injunctive social norms, personal norms) prompted participants to think more deeply about their beliefs about portion size, dampening the effect of prior exposure to portion sizes on their reported perceived ‘portion size normality’, in general terms. A different interpretation of the non-significant findings might be that the extreme small and large portion sizes included in our experiments were too different from what participants initially perceived as being ‘normal’ in size, as the portion sizes (both small and large) included in the initial exposure phase were selected based on their similar deviance from a normal portion.
Consistent with expectations, current findings suggest that perceptions of descriptive and injunctive social norms jointly underlie the effect of being served (but not visually exposed to) a smaller compared to a larger food portion on consumption of that food 24 h later. To our knowledge, this is the first study empirically showing that served portion sizes can signal normative information about both what others would eat (descriptive social norm) and what others believe is the appropriate amount to eat (injunctive social norm), indirectly affecting future consumption. It should be noted that next to the combined
indirect effect of descriptive and injunctive norms, there was a specific
indirect effect of descriptive social norms, suggesting that this factor is relatively more important than perceptions of injunctive social norms. However, when including covariates in the model, only the total
indirect effect of both descriptive social norms and injunctive social norms remained significant and these norms were highly correlated. Therefore, results should be interpreted with a focus on the overall pattern of the joint indirect effect, which indicates that portion size norms are anchored in social groups [3
]. Our work supports the notion that social norms can affect eating behavior (e.g., see reviews of Higgs [26
], Robinson, Thomas, Aveyard, and Higgs [27
], and Stok, de Vet, de Ridder, and de Wit [28
]). Furthermore, our research is one of the first studies that provides empirical evidence for the proposition that social norms are embedded in physical elements of food environments, guiding eating behavior accordingly [29
Contrary to expectations, the indirect effect of portion size condition on consumption via personal norms was not supported, suggesting that exposure to portion sizes does not adjust the amount of food an individual considers to be a normal amount for themselves
to eat. We also did not find evidence for the alternative explanation that expected satiety underlies the effect of portion size exposure on later consumption. The results of the current study may therefore indicate that portion size norms are derived from the specific eating situation [30
], rather than reflecting an individualized norm (e.g., this is normal for me to eat or this is enough for me to feel satisfied), as only significant effects for social consumption norms were observed. Specifically, when participants were served a food portion size which was inconsistent with their personal norm in the present research, they may have inferred that this portion size signals what is seen by others in that context as a ‘normal’ or ‘appropriate’ amount to eat. As a result, they conform to these social norms, which might explain the indirect effects demonstrated in this and other studies [7
]. In other words, serving smaller (larger) portions leads to smaller (higher) food intake, as people think others in that eating context believe that this is the normal/appropriate amount for them to eat, and this effect occurs even if the portions were initially deemed abnormal and irrespective of their own personal standards. This line of reasoning is consistent with Herman and Polivy [31
], who argue that portion sizes determined by another person represent a judgment about what one ought to eat in a specific eating situation. Future research should examine whether the observed effects extend to more realistic settings (e.g., at home) in which social norms may be less salient [26
]. One could reason that, in a home setting, participants may be more strongly guided by a personal norm (which was not significantly affected by the portion size manipulation in the present research) than a social norm.
In the present research, a large number of participants incorrectly recalled the portion size to which they had previously been exposed. Participants who were exposed to (Study 1) or served (Study 2) larger (versus smaller) portion sizes had a poorer recall of the exposure portion size, which is in line with previous research indicating a general underestimation of especially large portion sizes [32
]. The effects of exposure to portion sizes on norm perceptions and expected satiety were potentially stronger (and in the predicted direction) among those participants who were able to correctly identify the portion size to which they were exposed to. This finding may suggest a potential role for making people aware of the portion size as a factor to moderate the relation between portion size exposure and later portion size evaluations, although recent research has failed to demonstrate that training attentive or mindful eating successfully reduces intake [33
Study 1 was conducted via an online survey platform, and therefore may be susceptible to bias due to lack of participant attention. This could potentially explain the observation that fewer participants in Study 1 (correct identification: 44.6%) were able to correctly identify the portion size to which they were exposed than in Study 2 (correct identification: 69.7%). The exclusion of male participants from Study 2 means that it is unclear whether our findings are generalizable to men, and future research should replicate this study including males. Another limitation of this research is that the social and personal norm measures were not validated, although these instruments were developed based on the general norm measure used in Robinson & Kersbergen [7
]. Hunger was measured retrospectively in Study 1 and this is a limitation of the current research, although this method has been used previously [10
]. Lasagna and spaghetti were selected as stimuli in the present experiments, as they are commonly consumed and are widely available in mainstream supermarkets in both the UK and the Netherlands. However, we did not assess participants’ familiarity with the foods in either study, or their frequency of consumption in Study 1, and we therefore cannot conclude that the foods were equally as well-known across the studies. It is possible that these factors moderate the effect of portion size exposure on future portion size preferences and this could explain the different pattern of results between the studies, but this remains a question for future research. The current research does not permit conclusions about whether exposure to smaller versus larger portion sizes would affect participants’ urge to compensate for a small portion size by eating additional food, but this would be a valuable direction for future research. Although there were no significant effects of visual exposure to portion sizes on normative evaluations of an ‘incongruent’ food in Study 1, future research should test these ‘transfer’ effects in a laboratory-based setting using actual food. Finally, it is unclear whether the effects that portion size had on social norms in the present studies were context specific (i.e., specific to portion size evaluations when eating in laboratory) or whether they would transfer and influence perceived social norms in other contexts (i.e., outside of the laboratory setting).