Increasing serving size increases amount consumed: Catch-22.

The effect of serving size on consumption is well-established (see Chandon and Wansink, 2011 for a review). The larger the serve, the greater the amount consumed. However, little attention has been given to quantifying the serving size effect. We know that size influences volume consumed, but by how much? The present research used a meta analysis of 67 studies, and a combined N of 2792 respondents, to determine the relative effect of serving size on consumption volume (d=.47). More importantly, we extended our analysis to determine the absolute size of the effect: we found that a doubling of serving size increases consumption by 22%. Finally, we show that the serving size effect is larger among adults than children. 1 Working Paper, December 2011. This paper was first presented at Bond University, October 2011. A short abstract was submitted to the Society for Consumer Psychology’s First International Conference (Florence, 2012), and a full paper is being prepared for submission to a journal. Increasing Serving Size Increases Amount Consumed: Catch -22 The effect of serving size on consumption is well-established (see Chandon and Wansink, 2011 for a review). The larger the serve, the greater the amount consumed. However, little attention has been given to quantifying the serving size effect. We know that size influences volume consumed, but by how much? The present research used a meta analysis to determine the effect of serving size on consumption volume. We also develop an absolute measure of the effect in line with psychophysical theory, in particular Weber’s Law (1834). A meta-analysis using 67 separate studies with a combined N of 2792 respondents, revealed a substantial and significant effect of serving size on consumption (d=.469, CI95=[.376, .562]), see Table 1). A moderator analysis shows the serving size effect to be reliable across a number of different domains (see Table 2). Although the effect varies under a range of conditions, the only significant difference observed was that the effect is significantly smaller for children relative to adults (dadult=.55 vs dchildren=.24, see Table 2). While the serving size effect would be characterized as ‘moderate’ by Cohen (1988), speaking of the ‘size’ of the effect sizes is, as he acknowledges problematic. For instance, in the current dataset, the size of an effect depends to a great extent on the size of the difference between the ‘large’ and ‘small’ serves. To avoid the problem of relative effect size measures and in order to develop an absolute measure of the serving size effect, we developed ratio measures that captured the relative change in serving sizes (the size of the difference between the large serve and the small serve divided by the small serve size) and the relative change in consumption (between large and small serve). We then regressed the change in amount consumed on the change in amount served. Each individual study included in the regression was weighted according to the meta-analytic weights which are a function of sample sizes and standard errors. Our results show that the change in serving size significantly predicts the change in consumption (B=.22, t=26.43, p < .001; see Figure 1). Importantly, the regression coefficient tells us that if the serving size doubles (i.e., the large serve is 100% larger than the small serve), the consumption is 22% higher than in the small serve condition.