Portion size effects vary: The size of food units is a bigger problem than the number

Appetite 140 (2019) 27–40

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Portion size effects vary: The size of food units is a bigger problem than the number

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Jolien Vandenbroelea,∗, Anneleen Van Kerckhovea, Natalina Zlatevskab a b

Ghent University, Faculty of Economics and Business Administration, Department of Marketing, Innovation, and Organisation, Tweekerkenstraat 2, 9000, Ghent, Belgium University of Technology Sydney, 15 Broadway, Ultimo, NSW, 2007, Australia

ARTICLE INFO

ABSTRACT

Keywords: Portion size Numerosity heuristic Perceived quantity Consumption

While it is well-known that larger food portions lead to increased consumption (i.e., the portion size effect), previous studies confound the effect of the size and the number of food units making up the larger portion. Moreover, empirical tests of the mechanism underlying the portions size effect are scarce. In response to these shortcomings, we present three experiments that test the impact of food unit-size and unit-number on consumption of increasingly large portion sizes, and assess whether perceptions of quantity (driven by unit size or number) mediate the portion size effect. Study 1 (n = 185), tracking actual consumption, shows that the portion size effect is determined more by unit-size than unit-number. Relative consumption ratios are higher when participants were served portions made up of enlarged food units compared to more food units. Since perceived quantity and consumption are thought to be negatively related, Study 2 (n = 193) reveals that consumers’ quantity perceptions of portions are lower for unit-size (vs. unit-number) increases. Study 3 (n = 189) considers both perceived quantity and consumption and demonstrates that perceived quantity indeed mediates the effect of food portion size on consumption. Finally, this study also shows that encouraging consumers to focus perceptually on size when portion size increases are in terms of unit-size, or focus on number when portion size increases are in terms of unit-number, supports them in increasing quantity perceptions and decreasing actual consumption. Hence, manipulating the perceptual focus of consumers helps to mitigate the portion size effect. The findings contribute to literature on the portion size effect and numerosity heuristic, and provide practical insights on food packaging so to tackle the obesity crisis.

1. Introduction

enlarged portion size (Herman, Polivy, Pliner, & Vartanian, 2015). An obvious way to manipulate food quantity is by increasing (or decreasing) the size of the food unit (i.e., unit-size changes), such as offering small, medium, or large versions of a sandwich (Rolls, Roe, Meengs, & Wall, 2004). But consumers might also consider portions that consist of one, two, or three sandwiches, all of the same size, such that the increase in portion size refers to the number of food units (i.e., unit-number changes). For example, for a long time one of the most popular chocolate bars called ‘Mars’ was offered not only as a regular sized single bar, but also in a king-size version. More recently, however, the enlarged single bar was replaced by a duo-pack. While the former illustrates a change in unit-size, the latter suggests that food enlargements can also be the result of changes in unit-number. Previous studies apply both methods (i.e., increases in unit-size or unit-number), and confirm that both can produce a portion size effect (Almiron-Roig, Solis-Trapala, Dodd, & Jebb, 2013; Chandon & Ordabayeva, 2009;

The contribution of increasing food portion sizes to the growing problem of obesity has encouraged a substantial amount of research into investigating its relationship with consumption (e.g., Diliberti, Bordi, Conklin, Roe, & Rolls, 2004; Jeffery et al., 2007; Rolls, Roe, Halverson, & Meengs, 2007; Stroebe, van Koningsbruggen, Papies, & Aarts, 2013; Young & Nestle, 2002). People's tendency to eat more when portion sizes increase has been defined as a “portion size effect,” and various studies of conditions that might mitigate or eliminate this effect show that it is fairly robust and resistant (French et al., 2014; Marchiori & Papies, 2014). In fact, a meta-analytic review revealed that consumption increases by 35% on average when portion sizes double (Zlatevska, Dubelaar, & Holden, 2014). Notwithstanding this extensive portion size literature stream, researchers tend to overlook what factor specifically produces the

Corresponding author. E-mail addresses: [email protected] (J. Vandenbroele), [email protected] (A. Van Kerckhove), [email protected] (N. Zlatevska). ∗

https://doi.org/10.1016/j.appet.2019.04.025 Received 20 September 2018; Received in revised form 4 April 2019; Accepted 28 April 2019 Available online 06 May 2019 0195-6663/ Crown Copyright © 2019 Published by Elsevier Ltd. All rights reserved.

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Diliberti et al., 2004). However, it remains unclear whether food unit-size and unitnumber contribute equivalently to the effect of portion sizes on consumption. Therefore, we aim to determine the relative impact of food unit-number and unit-size in the portion size effect and argue that portion size enlargements caused by increases in unit-size lead to a greater portion size effect because such larger portions are perceived as lower quantities. On the opposite, we expect that increases in unitnumber will lead to a smaller portion size effect because these portions are perceived as higher quantities. Hence, we propose that unit-size and unit-number are differentially linked to quantity perceptions, and that quantity perceptions and consumption are negatively related.

to consumption. The first proposition can be constructed by considering the numerosity heuristic (Pelham, Sumarta, & Myaskovsky, 1994) referring to humans' overreliance on easily quantifiable or numerical information. For example, attribute differences appear larger on scales with many units (fine-grained scales) than on scales with fewer units (grossgrained scales) because consumers rely more on the number rather than the metric in which information is expressed (Pandelaere, Briers, & Lembregts, 2011). In a context of portion size perceptions, this implies that labeling a portion of candies as “16 gummy candies” (fine-grained) rather than “one serving of candies” (gross-grained) leads people to perceive portions as actually being larger (Lewis & Earl, 2018). Consumers’ greater sensitivity to numbers, due to the easier evaluability of discretized units (Lembregts & Van den Bergh, 2018), as reflected in the numerosity heuristic, might suggest that quantity perceptions are higher for increases in unit-number versus equivalent increases in unitsize. Besides, just like the relationship between portion size and consumption, the link between portion size and perceived quantity reflects an inelastic power function. That is, there is a positive relationship between actual and perceived quantity (Chandon & Ordabayeva, 2009, 2017; Ordabayeva & Chandon, 2013; Teghtsoonian, 1965), but this relationship is a declining function of increasing portions sizes (Chandon & Wansink, 2007; Krider, Raghubir, & Krishna, 2001; Wansink, Painter, & North, 2005). Taken together, these findings suggest that the declining positive function of increasing portion sizes and perceived quantity is more determined by unit-size, than unit-number. Stated differently, consumers will attribute relatively higher perceived quantity to portion increases stemming from changes in unit-number, while increases in unit-size will lead to relatively lower perceived quantity. This leads to the following hypothesis:

2. Consumption in relation to food unit-size and unit-number By studying how manipulations of unit-size and unit-number influence consumption, our research is akin to research into food partitioning effects and consumption, which indicates that dividing a portion into more, smaller pieces reduces consumption rates (Coelho do Vale, Pieters, & Zeelenberg, 2008; Holden & Zlatevska, 2015; Kerameas, Vartanian, Herman, & Polivy, 2015; Roose, Van Kerckhove, & Huyghe, 2017; Scott, Nowlis, Mandel, & Morales, 2008). That is, if there are two equivalent portion sizes, the one that consists of fewer, larger food units tends to encourage more consumption than an equivalent portion that consists of more, smaller food units. Such evidence implies that the portion size effect might be smaller (i.e., enlarging a portion spurs consumption to a lesser extent) when portions become enlarged by the addition of more units relative to increasing the size of the units. However, these studies of partitioning effects do not explore the effect of changing portion sizes but instead tend to leave the portion size fixed. With such a design, the unit-size and unit-number necessarily are confounded, so their relative effects are impossible to assess. Therefore, it is not clear whether reduced consumption of partitioned portions results from a decrease in the size of the food units or an increase in the number of food units. Partitioning research tends to offer explanations based on smaller unit-size, without addressing the possible contributing effect of unit-number. That is, studies usually describe the partitioned portion as “small” rather than “numerous” (e.g., van Kleef, Kavvouris, & van Trijp, 2014). To determine the relative influence of the unit-size versus unit-number in creating a portion size effect, it is necessary to compare the effect of increasing portion sizes in two different ways: increasing the size of the food unit (unit-size) or increasing the number of food units (unit-number). Note however, that the portion size effect is in general a declining function of increasing portion sizes. A meta-analysis affirms that consumption increases in line with portion sizes and that this portion size effect is a declining function of increasing portion size, such that as portion sizes get bigger, the rate at which consumption increases declines (Zlatevksa et al., 2014). Consumption should consequently decline more rapidly in response to increases in unit-number compared to unit-size. Therefore, the current study offers the following hypothesis:

H2. The declining positive function of increasing portion sizes and perceived quantity is determined more by food unit-size than by unitnumber. Secondly, some research offers support for the negative relationship of perceptions of quantity and consumption, such that Redden and Hoch (2009) find that people serve themselves larger portions (i.e., higher consumption) in situations in which perceived quantity tends to be lower. Wansink and van Ittersum (2003) also demonstrate that using elongated glasses leads to higher perceived quantity estimates and reduced consumption. Presumably for this reason, Ordabayeva and Chandon (2016) call for more attention to and research on the role of perceptual biases and how they might relate to consumption behavior. Accordingly, we investigate how perceived quantity mediates the portion size effect, taking food unit-number and unit-size into account. The portion size effect should be determined more by food unit-size than unit-number, and this relationship might be explained through the mediation of perceived quantity. Essentially, this means that we expect lower perceived quantity from portion increases in unit-size, so that consumption will be higher because quantity perceptions and consumption are negatively related. In contrast, for increases in unitnumber, we expect higher perceived quantity and lower consumption compared to increases in unit-size.

H1. The declining positive function of increasing portion sizes and consumption (i.e., the portion size effect) is determined more by food unit-number than unit-size.

H3. The effect of visual cues in a portion size (unit-number and unitsize) on consumption is mediated by perceived quantity.

3. Perceived quantity in relation to food unit-size and unitnumber

Importantly, in this study we focus on how visual cues (unit-number and unit-size) affect perception and consumption. However, notwithstanding the fact that the current research focuses on perceived quantity, we do not discard the existence of additional contributors to the portion size effect. Herman et al. (2015) and Zuraikat, Smethers, and Rolls (2019) identify several mechanisms that might be underlying the portion size effect. For example, larger portions induce larger bites, which, in turn, decrease exposure time and sensory-specific satiety,

Why might a change in unit-number lead to a diminished portion size effect relative to a change in unit-size? In this research, we propose a mechanism based on quantity perceptions to endorse the idea that more units result in lower consumption. This proposition builds on two lines of reasoning. First, unit-size and unit-number are differentially linked to quantity perceptions and, second, these perceptions are linked 28

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yielding higher consumption. The dominant explanation for the portion size effect, however, suggests that portion sizes can act as normative eating cues as such that consuming more from larger portions is perceived as appropriate. Hence, when portions are perceived to be larger, consumers feel licensed to consume more (Reily & Vartanian, 2016). This observation runs counter the predicted negative relationship between perceived quantity and consumption. It is important to note though that Reily and Vartanian (2016) suggest that consumers consider it appropriate to consume more from larger portions in case they fail to recognize the portion as large. In contrast, studies advancing a negative relationship between perceived portion size and consumption suggest that the motivation to consume less from larger portions stems from a conscious recognition of the portion as large. Hence, although our proposition and the findings of Reily and Vartanian (2016) are seemingly in opposition, they both make sense when awareness is factored into the equation. From the above reasoning it becomes clear that we do not assume that consumers are necessarily unaware that they are served larger portion sizes, which would imply that the relationship between perception and consumption would be unconscious. On the contrary, recent findings that show that people are mostly aware that they are eating less or more than intended (Keenan, Childs, Rogers, Hetherington, & Brunstorm, 2018).

information on the procedure and visuals of our stimuli can be found in Appendix A. First, our analysis verifies the positive relationship between portion size and actual consumption by a regression with served portion size as predictor of the actual consumption. We also assess whether the quadratic effect of the served portion size is a significant predictor in this regression to demonstrate the curvilinear relationship of the portion size effect. Next, to assess our main hypothesis (H1), it is impossible to compare on absolute consumption levels as participants were offered different total portion sizes. Instead, we use the consumption ratio of the actual consumption of the participant on the served portion size as key dependent variable, which shows how much participants proportionally ate. In this way, we disentangle in a regression analysis the influence of unit-number and unit-size as independent predictors on the consumption ratio. 4.2. Results and discussion The results confirmed the portion size effect, in the positive relationship between the served portion size and consumption by the participant (β = 1.374, t = 6.83, p < .001). Moreover, the quadratic effect was significant, indicating a curvilinear relationship between portion size and consumption (β = -.845, t = −4.20, p < .001). Consumption increased as portion size increased, but the rate of change is attenuated for relatively larger portion sizes. In addition, an inspection of a visual presentation of the data also seems to confirm our expectations. Fig. 1 illustrates that the logarithmic trendline relating portion size increases originating from an increased unit-number to absolute consumption flattens more rapidly than the trendline linking portion size increases that stem from increased unitsize to absolute consumption. Even though this graph nicely illustrates the anticipated effect, it should be interpreted with caution as trendlines are based on a subset of the data (i.e., the first row and first column in Table 1, cf. infra). Table 1 reports the consumption ratio (i.e., actual consumption on served portion size) averaged across participants in each cell, ranging from zero to one, where zero stands for no consumption and one for complete consumption. Total served portion sizes are indicated in italic, and equivalent total portion sizes are shaded in the same color. For example, participants could have received a served portion size of 64g, consisting of 4 pieces of 16g, of which they ate on average 61% (i.e., 39g). Comparing the effect of food unit-size and unit-number indicated that mean consumption ratios are higher for increases in unit-size (changes across rows) than increases in unit-number (changes across columns) for virtually all portion sizes. Participants consumed on average a higher proportion of the portion sizes made up of larger unitsizes, compared with equivalent portion sizes made up of increasing unit-numbers. To illustrate, for any substantial portion size (32g or higher), the mean consumption ratio was greater for portions manipulated in terms of unit-size rather than unit-number. For example, when offered a 32g portion of brownies in the form of 4 × 8g brownies, participants on average consumed 65% of the portion. But for a single 32g brownie, their consumption amounted to 80% on average. The regression analysis shows that unit-number (β = -.40, t = 5.99, p < .001) acted as a negative predictor of consumption ratio, and unitsize was only marginally significant (β = -.12, t = −1.71, p = .089). That is, increased unit-number led to a lower consumption ratio compared to increased unit-size, which is in line with our expectations of H1. The declining positive function of increasing portion sizes and consumption was thus more determined by the number of food units, than by the size. This study demonstrated that portion size effects vary, depending on whether portion size changes involve the food unit-size or unit-number. Therefore, the portion size effect is stronger for increases in food unitsize than unit-number. As a first implication, this study shows that

4. Study 1: relative importance of food unit-size and unit-number for consumption Demonstrating that a bigger portion results in increased consumption levels confirms the portion size effect. This finding is secondary to the primary research objective for Study 1, namely, to establish that consumption heightens more in response to increased unit-sizes rather than a higher number of food units (H1). 4.1. Method A sample of 186 participants were recruited from the respondent pool of a large European university to participate in a laboratory experiment (73 men, 112 women; one person failed to provide socio-demographic information; MAge = 22.9 years, SD = 6.1, range = 19–63 years) in exchange for monetary compensation. The invitation to the lab session indicated that the experiment ostensibly comprised several unrelated tasks. Ethical approval of this study was obtained by the Ethical Commission of the Faculty of Economics and Business Administration of Ghent University. To obtain this approval, the study design had to conform the ethical guidelines, including specific guidelines on experiments that involve human subjects. Participants filled in an informed consent before starting the survey. They were informed that the study involved food, but that there was no obligation to consume it. The data for this study were collected in October 2016. The participants watched a video, during which they were offered chocolate brownies as a snack. They were not informed of the intention of the research study or that their consumption of the brownies would be monitored. Participants could only eat the brownies during the movie, afterwards they were asked to give the plate back to the instructor, so that the consumption time remained equal between conditions (i.e., 12 min). The 3 × 4 between-subjects design reflected the different portion sizes offered to the participants, manipulated in terms of the size of the brownies offered (unit-size: 8g, 16g, or 32g) and number of brownies (unit number: 1, 2, 4, or 8 pieces). Participants were randomly assigned to only one of the possible combinations, such as for example 16g (unit-size) x 4 pieces (unit-number), which establishes a served portion size of 64g. Hence, the total portion size served differed between conditions and ranged from 8g to 256g. After returning the plate, the instructor (a) noted whether or not the plate was completely empty (43% of participants, see Table A2 in Appendix A for an overview per cell) and, where relevant, (b) measured the weight of the remaining brownie(s) to obtain actual consumption. Additional 29

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Fig. 1. Trendlines linking increasing portion size to absolute consumption, depending on unit-size and unit-number. Note. Fig. 1 illustrates the logarithmic trendline relating portion size increases originating from an increased unit-number and unit-size to absolute consumption. The data for the logarithmic trendline is based on the first row and column of Table 1.

portion size of 75g of chocolate, consisting of three pieces (unitnumber) of 25g (unit-size). Participants viewed four photographs, one at a time, selected at random from the set of 16 photographs and had to estimate the total weight (i.e., perceived quantity) on a slider that ranged from 0 to 10 ounces (i.e., common measuring unit in U.S., equivalent to 284g). Participants rated only four out of 16 photographs to avoid survey fatigue and obstruct biases by demand. See Appendix B for additional information on the procedure and visual stimuli. First, our analysis verifies a) the positive and b) quadratic relationship between portion size (defined as the actual total quantity shown in the photographs) and perceived quantity (as estimated by the participants). Therefore, we regress a) the portion size and b) the quadratic effect of the portion size as predictors on perceived quantity. To analyze our main hypothesis H2, we create the perceived quantity ratio (i.e., perceived quantity by the participant divided by the actual quantity shown) to account for the varying total portion sizes across photographs. A perceived quantity ratio lower than 1 indicates an underestimation of the portion size, while perceived quantity ratios higher than 1 indicate an overestimation of the portion size. Next, a hierarchically structured model regresses both the unit-number and the unit-size as first-level predictors on the perceived quantity ratio as dependent variable. The participant serves as a second-level variable in this model, because each participant was exposed to four photographs.

Table 1 Mean consumption ratio (i.e., actual consumption on served portion size), depending on unit-size and unit-number.

increasing unit-number mitigates the portion size effect at least to some extent. Larger portion sizes lead to increased consumption—but less so when the increase in portion size comes from an increase in the number of food units offered, confirming H1. 5. Study 2: relative importance of food unit-size and unit-number for perceived quantity Study 2 further explores what causes the differential outcomes of unit-size and unit-number increases on consumption. The portion size effect might be greater for unit-size portions because of a difference in perceived quantity, so Study 2 tests the link between portion size and perceived quantity and how unit-number and unit-size each relate to quantity perceptions (H2).

5.2. Results and discussion Similar to Study 1's finding that portion size and consumption are positively related, the results of Study 2 exhibit a positive relationship between the portion size and perceived quantity (β = .936, t = 8.68, p < .001). In line with research showing that people's perceptions of quantity decrease as portion sizes increase in volume (Chandon & Ordabayeva, 2009, 2017; Ordabayeva & Chandon, 2013; Teghtsoonian, 1965), we find a significant quadratic effect that depicts a curvilinear relationship between the two variables (β = -.332, t = −3.08, p = .002). Moreover, from Fig. 2, which presents the trendlines linking increases in unit-size and unit-number to perceived quantity, we can infer that portions that are made up of more units yield higher perceived quantity compared to portions made up of larger units. Again, this graph should be interpreted with caution, as trendlines are based on a small subset of the data. Table 2 shows the mean of the perceived quantity across conditions. Total portion sizes are indicated in italic, and equivalent total portion sizes are shaded in the same color. For example, participants that saw a photograph of a total portion size of 112.5g, consisting of 3 pieces of

5.1. Method The sample included 193 U.S.-based residents, recruited via Amazon Mechanical Turk (93 men, 100 women; MAge = 37.2 years, SD = 12.0, range 20–69 years) in exchange for a monetary compensation. Ethical approval of this study was obtained in a similar way as in Study 1 and the data were collected in June 2016. Chocolate that is easily broken down into discrete units of a predefined size was used. A total of 16 photographs with chocolate chunks were created, with various quantity combinations of four different unit-sizes (12.5g, 25g, 37.5g, 50g) and four different unit-numbers (1, 2, 3, 4). The total portion size shown on the photograph thus ranged from 12.5g to 200g. For example, participants could be shown in one photograph a total 30

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Fig. 2. Trendlines linking increasing portion size to absolute perceived quantity, depending on unit-size and unit-number. Note. Fig. 2 illustrates the logarithmic trendline relating portion size increases originating from an increased unit-number and unit-size to absolute perceived quantity. The data for the logarithmic trendline is based on the first row and column of Table 2.

perceived quantity ratios than unit-number (or, increases in unitnumber yield higher quantity estimates than changes in unit-size). These results confirm H2.

Table 2 Mean perceived quantity ratio (i.e., perceived quantity on portion size), depending on unit-size and unit-number.2

6. Study 3: effects of perceptual focus and food unit-number and unit-size on perceived quantity and consumption Study 2 demonstrated that the relative decline in perceived quantity with increasing portion sizes is more a function of the size than the number of food units that constitute a portion. However, the viability of this finding to serve as an explanation for the portion size effect depends on the extent to which perceived quantity mediates the portion size effect (H3). Modifying the portion size in terms of unit-number versus unit-size has distinct effects on consumption (S1) and perceived quantity (S2), so testing whether perceived quantity mediates the portion size effect is the first objective of Study 3. Second, this study also aims to provide evidence for the mediating role of perceptions by adopting a moderation of process approach (Spencer, Zanna, & Fong, 2005). Specifically, interventions to reduce perceptual insensitivity (i.e., increase perceived quantity) should mitigate the portion size effect. Sensitizing consumers to unit-size information might increase perceptions of quantity and reduce consumption if the portion consists of larger sized units (i.e., increased unit-size). Inducing a perceptual focus on the number instead might be expected to increase perceived quantity and reduce consumption when the portion is made up of more, smaller units (i.e., increased unitnumber). In effect, matching consumers’ perceptual focus (unit-size vs. unit-number) with the presentation format of a portion size (i.e., larger vs. more units) could be more effective for reducing the portion size effect. Formally,

37.5g, indicated a perceived quantity ratio of .89 on average (i.e., 100.12g). The left upper quadrant of Table 2 indicates that for smaller portions, consumers infer relatively higher quantities (ratios > 1). With increasing portion sizes, consumers tend to rather underestimate the portion size, as indicated by the lower mean ratios in the lower right quadrant. For both increases in unit-size and unit-number, the mean perceived quantity ratio decreases as portion size increases. However, it appears that the decline in the mean perceived quantity ratio is smaller for increases in unit-number versus unit-size. That is, consumers attribute higher perceived quantity ratios to portion size shifts that result from changes in unit-number (changes across columns) rather than unit-size (changes across rows). More formally, the hierarchically structured model suggests that both unit-number (B = -.078, SE = .018, t = −4.43, p < .001) and unit-size (B = -.25, SE = .018, t = −13.84, p < .001) are significant negative predictors of the perceived quantity ratio. To test the hypothesis that the unit-number (β = -.105) and unit-size (β = -.339) standardized regression coefficients would be statistically and significantly different, their corresponding 95% confidence intervals (CI) were estimated with bias-corrected bootstrapping (1,000 resamples). The 95% CI of unit-number is [-.15, -.06] and for unit-size [-.39, -.29]. If the confidence intervals overlap by less than 50%, the beta weights would be considered significantly different (p < .05; Cumming, 2009); the CIs actually exhibit no overlap. The negative standardized regression coefficient for unit-size is significantly larger than that for unitnumber, confirming that unit-size has a stronger negative impact on

H4a. Invoking a size focus increases perceived quantity when portions comprise larger units, while invoking a number focus increases perceived quantity when portions comprise more units. H4b. Invoking a size focus reduces the portion size effect (i.e., lower consumption) when portions comprise larger units, while invoking a number focus reduces the portion size effect when portions comprise more units. Fig. 3 provides a visual depiction of the predictions for Study 3. Specifically, perceived quantity should mediate the portion size effect (H3), and perceptual focus should moderate the relationship between the portion size presentation (larger vs. more units) and perceived quantity (H4). When the focus (size or number) matches the portion size presentation (larger unit-size or greater unit-number), perceived 31

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Fig. 3. Moderated mediation model of portion size presentation and perceptual focus on perceived quantity and consumption.

quantity should be higher, which then should result in lower consumption.

is the mediator. A follow-up analysis tests for the expected interaction effect of portion size presentation and perceptual focus (i.e., moderator) on perceived quantity and consumption (H4). Therefore, we apply twoway analyses of variance (ANOVAs), with portion size presentation (larger vs. more units) and perceptual focus (size vs. number) as independent variables, and perceived quantity and consumption, respectively as dependent variables.

6.1. Method A sample of 189 European undergraduate students (100 men, 89 women; MAge = 21.5 years, SD = 4.9, range 20–65 years) participated in the experiment, in exchange for a course credit. Ethical approval and informed consent for this study was obtained in a similar way as in the former studies, and the data were collected in February 2016. On arrival at the lab, participants were assigned to isolated cubicles, to reduce any influence of other participants. The study design was a 2 (perceptual focus: size vs. number) × 2 (portion size presentation: larger vs. more units) between-subjects design. Participants were randomly assigned to one of the four conditions. The invitation to the lab session indicated that the experiment ostensibly comprised several unrelated tasks. The first task encouraged a perceptual focus on either size or number, such that participants had to estimate quantities depicted in pictures of three different categories: garbage, motor oil, and water. For each category, participants saw four illustrations. The first showed a single quantity, and then the three remaining images revealed greater quantity (in volume) as either a larger unit (size focus) or more units (number focus). Participants had to estimate the quantities of the larger depictions. The checks on the accuracy of their responses revealed that all participants’ answers were accurate, regardless of unit-number or unit-size condition—as expected for this very easy task. See appendix C for more information on the visuals and procedure of the study design. Participants next watched two movie fragments of 5 min each, and they were told they would be questioned about the videos afterwards. Participants also were provided with a plated portion of 100g brownies, comprising either two 50g pieces (larger units) or six 16.6g pieces (more units), to snack on while they watched the movie fragments. After watching the movie fragments, participants were instructed to return the plate to the supervisor before completing the rest of the questions, so that the consumption time remained equal between conditions (i.e., 10 min). By weighing the remaining brownies, it was possible to calculate the amounts consumed. Strictly speaking, there were no plate cleaners in this study. Nevertheless, we note that 35% of the participants left hardly anything on their plate. That is, they ate more than 90% of the total portion size served (see Appendix C). Perceived quantity was measured after watching the movies, participants had to estimate the total weight of the plated brownies. A first analysis will determine whether perceptions mediate the effects of portion size on consumption (H3), by using a moderated mediation bootstrapping analysis (Preacher & Hayes, 2008). Total portion size was held constant in this study, so absolute consumption provides the dependent variable, and the measure of perceived quantity

6.2. Results and discussion The moderated mediation analysis shows that the effect of the portion size presentation (larger vs. more units) on consumption is mediated by perceived quantity, both when participants focused on unit-size (indirect effect estimate = 2.17, SE = 1.65, 95% CI = .05, 6.92) or unit-number (indirect effect estimate = −2.69, SE = 1.71, 95% CI = −7.20, -.24). These findings are in line with H3. In the ANOVA with perceived quantity as the dependent variable, the results indicate a non-significant main effect of portion size presentation (F(1,185) = .14, p = .711) and a non-significant main effect of perceptual focus (F(1,185) = 2.26, p = .14). However, the interaction of these two variables is significant (F(1,185) = 12.242, p = .001). As Fig. 4 shows, participants induced to focus on size offer higher perceived quantity estimates when their portion contains a few, larger brownies rather than more, smaller brownies. Participants induced to focus on the number of units show a reverse pattern of results: They indicate significantly higher perceived quantities when the food portion comprises more, smaller versus few, larger brownies. These results confirm H4a. A second ANOVA, using consumption as the dependent variable, reveals non-significant main effects of both portion size presentation (F (1,186) = .19, p = .664) and perceptual focus (F(1,186) = 2.43, p = .121), but a significant interaction of these two variables (F (1,186) = 9.53, p = .002). According to Fig. 4, the observed pattern is the inverse of what is observed for perceived quantity. When participants focus on unit-size, the presentation of portions comprising few, larger brownies result in lower consumption relative to portions comprising more, smaller brownies. When participants instead focus on the unit-number, consumption is lower for portions comprising more, smaller versus few, larger brownies. These findings support H4b. The findings of Study 3 thus confirm that perceived quantity mediates the effect of portion sizes on consumption and highlights how changing consumers’ perceptual focus can alter the magnitude of the portion size effect. 7. General discussion Increases in portion size result in an increase in consumption (Zlatevska et al., 2014). Prior research, however, has ignored the two 32

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Fig. 4. Interaction effect of portion size presentation and perceptual focus on 1) perceived quantity and 2) consumption.

(Pelham et al., 1994). In these studies, discrete information is not only more evaluable (evaluability referring to the ease with which a value can be mapped onto an evaluation, Lembregts & Van den Bergh, 2018), it is also presented in a more tangible format (visual presentation). Consequently, it is unclear whether it is the evaluability or the tangibility of information that is driving the numerosity heuristic. The current context is essentially different: Both continuous size (i.e., unit-size) and discrete numerosity (i.e., unit-number) information are visually presented, which makes that physical size is even more tangible than numerosity. Size can be understood on a perceptual level only, whereas numerosity still requires counting, which is an abstract, cognitive process (Brannon, Abbott, & Lutz, 2004). As tangibility and evaluability are clearly dissociated in our research context, and we still observe an overreliance on numerosity information, we provide evidence for an evaluability account of the numerosity heuristic. Third, this study extends understanding of the portion size effect by demonstrating the mediation by perceived quantity. Similar studies have demonstrated the influence of priming manipulations (Werle, Wansink, & Payne, 2011) and labeling (Chandon & Wansink, 2007; Wansink & Chandon, 2006) on quantity perceptions and food consumption. However, the novelty of this study design is the demonstration that also visual cues in the portion size (i.e., unit-number and unit-size) affect perceived quantity and consumption. By supporting the notion that perceived quantity and consumption relate negatively, we could also infer that actual consumption is lower for portions that reflect an increase in unit-number compared with unit-size. This is a striking finding as few studies have uncovered successful ways to attenuate the impact of the portion size effect on actual consumption. As a limitation of our studies, we do not know whether quantity perceptions are actually spontaneous activated as a means to monitor consumption in daily situations and how they relate to consumer judgement. Perhaps, visual cues, such as unit-number and unit-size, serve as guidelines for judgements of the appropriateness of the serving. In this way, higher unit-number can trigger higher quantity perceptions, so that consumers judge the portion size as less appropriate (i.e., too large) and lower their consumption accordingly. Moreover, unitnumber might also affect consumers' expectations on the consumption outcome: Research demonstrates that multiple smaller food units increase expected satiety (Oldham-Cooper, Wilkinson, Hardman, Rogers, & Brunstorm, 2017). Another prediction asserts that people do not actually make portion size judgments, and overconsumption represents an unconscious process, in line with the unit-bias proposed by Geier, Rozin, and Doros (2006). In this view, people tend to finish what is on

relevant ways in which portion sizes can be manipulated, namely, by varying food unit-size or food unit-number. Given the increase in the number of snack-sized and bite-sized, partitioned food items and multipack offerings on supermarket shelves (e.g., Cadbury Dairymilk Chocolate can be purchased in a single 200g and 50g chocolate block or a 180g multipack consisting of 15 individually wrapped snack sized blocks of 12g each and a 55g roll consisting of 7 chocolate buttons weighing approximately 7.9g each), this question is both of practical and theoretical importance. The current research demonstrates that not all portion size effects on consumption are equal and that the effect is driven more by unit-size than by unit-number. In studying this question, we advance insights into three areas. First, the results resonate with emerging research into partitioning effects that offers evidence of decreased consumption of food partitioned into more, smaller sized units. Research investigating partitioning effects tend to confound the size and the number of units (Coelho do Vale et al., 2008; Holden & Zlatevska, 2015; Kerameas et al., 2015; Scott et al., 2008). This research offers the first attempt to disentangle the independent influences of unit-number and unit-size on the portion size effect, and in doing so reveals that creating many, smaller food units might help people resist the temptation evoked by larger portion sizes. Additional issues that might bound our findings need to be considered in this regard though. For example, our studies did not explore the role of personality traits, and some research suggests that the offer of many, smaller units can increase consumption for some people, such as those who are diet-conscious (Coelho do Vale et al., 2008; Holden & Zlatevska, 2015; Scott et al., 2008). Moreover, the effectiveness of many, smaller units might depend on the extent to which the food units are perceived as reasonable units for guiding consumption. For example, a small bag of chips may seem useful and reasonable as a unit (Geier, Wansink, & Rozin, 2012), but single chips probably are not perceived as meaningful units by consumers. Our studies applied reasonable units, such as brownie or chocolate pieces, however, our effects might break down for smaller units. Secondly, we provide additional insights into the numerosity heuristic. Extant research has primarily considered how consumers trade-off continuous and discrete information. For example, when solving addition problems, people seem to incorporate the number of elements of which the addition problem consists in their value estimates (i.e., discrete information), rather than solely relying on the value of the constituting elements (i.e., continuous information). Or, when estimating the monetary value of arrays of American coins, the number rather than the nature of the coins influences value estimates 33

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their plates—at least up to some acceptable portion size. Beyond this level though, Geier et al. (2006) suggest that this unit-bias might break down. Moreover, as larger portions became more commonplace, people's perceptions of what constitutes an appropriate portion size, especially for energy-dense foods, also may have become distorted (Cornil & Chandon, 2014; Wansink & Ittersum, 2007). Zuraikat et al. (2019) point in their recent review article to the many different factors contributing and moderating the portion size effect, and their complex interplay and synergy. The challenge is on how to implement these ideas in the real world to tackle health issues and obesity. The portion size effect is powerful, but its influence can be mitigated if increased portion sizes result from an increase in unit-number (cf. duo-pack Mars) rather than unit-size (cf. king-size Mars). It also might be reduced if people can gauge portion sizes more accurately, in that people appear inclined to eat less of what they perceive to be bigger portions. For example, package designs might be revised, to inform people about enlarged portion sizes. For food usually presented as one unit (e.g., cake), it may be useful to add the volume increase associated with enlarged portion sizes. A larger cake might feature the caption “+25% volume” or “twice as big as normal”; French fries could be ordered in a standard, +25% volume, or

+50% volume portion size, rather than being offered with less informative labels as small, medium, and large. In this way, consumers’ attention will be more likely drawn to the increased size of a bigger portion, triggering a unit-size perceptual focus so to increase quantity perceptions and lower consumption. This research, along with studies that continue to explore the intricacies of portion size effects, suggests the need for field research that identifies effective implementations for tackling obesity. Declarations of interest None. Acknowledgments The authors thank Stephen Holden, Maggie Geuens, and Bram Van den Bergh for their extensive comments and feedback. A grant was awarded to the first author by the Special Research Fund of Ghent University (BOFDOC2016001601) and the National Bank of Belgium (BOFDOC20160016C1).

APPENDIX A Study 1 Additional procedure information 1. No a-priori power calculation was calculated to determine the number of participants in each condition. All participants in the respondent pool were invited, so the sample size was determined by feasibility constraints. 2. The general intake was on average 34.68 g (SD = 32.89) and ranged from 0 to 162g. We aimed to have our served portion sizes as close as possible to the amount that was intended for each condition, however, sometimes they were slightly off. Therefore, we measured also the actual amount served, and this measure was used in our analyses. 3. Additional variables that are relevant in the context of a consumption study were measured. Though, none of these measures led to the exclusion of participants, mainly because no extreme values were detected. a. BMI (based on self-reported length and weight): M(SD) = 22.03(2.91), range 16.59–32.69 b. Time (in hours) between last consumption and entering the lab: M(SD) = 2.66(2.77), 1-20 c. Dietary consciousness, 4 items from (1) not at all to (11) very much: M(SD) = 6.49(2.28), consisting of the following items: i. How diet-conscious would you say that you are? ii. How watchful are you of your weight? iii. How much thought do you give to eating healthily? iv. To what extent have you been trying to limit your dietary intake over the last week? d. Restrained Eating Behavior subscale of the Dutch Eating Behavior Questionnaire (Van Strien, Frijters, Bergers, & Defares, 1986), 10 items from (1) never to (5) very often: M(SD) = 2.51(.88).

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Table A.1

Stimuli Study 13

Table A.2

Proportion of participants that consumed the full portion in each cell4

APPENDIX B Study 2 Additional procedure information 1. No a-priori power calculation had been conducted to determine the required sample size. Nonetheless, we recorded multiple trials per respondent (i.e., within-subjects), which is a valuable strategy to increase the power of studies (Meyvis & Van Osselaer, 2018). 2. Participants read the following instructions: “In this task, you will see four pictures of chocolate chunks. We would like to gain insight in how people evaluate the size of these portions of chocolate. On each of the next pages we will display a picture of one or more chunks of chocolate. We will ask questions on the perceived magnitude of each entire portion (all depicted chunks together). We are interested in your instantaneous perception of the portion size. We would like to urge you not to deliberate much about your estimations, but rather to answer based on your initial thoughts and feelings.” 3. Participants spent on average 3 min on the complete task. 4. Besides measuring the estimated weight of the portion sizes, hungriness was measured from 0 (not at all hungry) to 100 (very hungry) on a slider: M(SD) = 43.18(26.95). This measure led not to the exclusion of participants, mainly because no extreme values were detected.

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Table B.1

Stimuli Study 25

APPENDIX C Study 3 Additional procedure information 1. No a-priori power calculation was calculated to determine the number of participants in each condition. As this study was part of a course credit task for students, the sample consisted of a convenience sample of all course students. 2. The general intake was on average 52.60 g (SD = 31.90) and ranged from 0 to 123.60g. We aimed to have our served portion sizes as close as possible to the amount that was intended for each condition, however, sometimes they were slightly off. 3. When participants where induced to focus on unit-number, 13% of the participants which were served more units ate 90% of more of their portion (cf. “plate-cleaners”), while this was 42% of the participants in the condition wherein they were served larger units. For the participants with a focus on unit-size, 20% of the participants that were served more units ate 90% or more of their portion, while only 2% of the participants did so in the condition with larger units. 4. Some other variables (not mentioned in the text) were measured, see below. None of these measures led to exclusion of participants, mainly because no extreme values were detected. a. Restrained Eating Behavior subscale of the Dutch Eating Behavior Questionnaire (Van Strien et al., 1986), 10 items from (1) almost never to (7) almost always, M(SD) = 3.31(1.28). b. BMI (based on self-reported length and weight): M(SD) = 21.64(2.62), range 16.56–37.74 Manipulation perceptual focus: SIZE On the next three pages you will see four different quantities of three different products, namely garbage bags, motor oil, and water. For the reference quantity, the volume is indicated below. Please provide estimates of the other depicted quantities. Use round numbers only (no decimals necessary).

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Manipulation perceptual focus: UNIT On the next three pages you will see four different quantities of three different products, namely garbage bags, motor oil, and water. For the reference quantity, the volume is indicated below. Please provide estimates of the other depicted quantities. Use round numbers only (no decimals necessary).

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Manipulation portion size presentation: LARGER units.

Manipulation portion size presentation: MORE units.

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