Excess consumption of energy is a key driver of increasing rates of obesity, which in itself contributes to a range of non-communicable and communicable diseases.
One way to tackle excess consumption of energy is to use health warning labels (HWLs) that include aversive images illustrating the negative consequences of consuming high-calorie foods. Previous studies have found that image-and-text HWLs increase dietary self-control in relation to snacks and reduce hypothetical selection of high-calorie snacks.
However, there is currently a lack of evidence of the underlying mechanisms by which HWLs impact the selection and consumption of high-calorie snacks. We were interested in whether the effects of labelling are driven mainly by a simple unpleasant association, or whether it is necessary for the label to portray a causal relationship between the product (e.g. chocolate bar) and the unpleasant outcome (e.g. heart disease). To investigate this, we compared the effects of HWLs (portraying an outcome that was causally related to over-consumption of high-calorie snacks) and irrelevant aversive labels (IALs; portraying an equally unpleasant image that was unrelated to over-consumption) on implicit (approach behaviour) and explicit (liking, wanting, selection) motivation.
In Study 1 (lab) and Study 2 (online), participants were randomised to three groups. In the HWL group, they were presented with four chocolate bars with labels that contained unpleasant images and warning texts that were relevant to excess calorie consumption. In the IAL group, chocolate bars had labels that contained unpleasant images and warning texts that were irrelevant to excess calorie consumption. In the no label control group, participants were presented with chocolate bars with no labels. To measure the speed of response in the implicit motivation task, participants were instructed to either approach or avoid the chocolate bars on the computer screen. For the measure of explicit motivation, participants were asked to rate on a scale how much they wanted and liked each chocolate bar. They were also asked to select a snack from a choice of unhealthy snacks (chocolate bars) or a healthier snack (fruit or snacks less than 100 calories).
Findings across the two studies show that both HWLs and IALs reduce implicit and explicit motivation towards high-calorie snacks but, with some exceptions relating to explicit motivation measures, the labels need to be present on the chocolate bar to produce an effect. This suggests that the mechanism of action on implicit behaviour may be a simple association-based devaluation of the snack.
It was also found that HWLs were more effective at reducing high-calorie snack selection and subjective wanting than IALs, suggesting that aversive warning labels may influence explicit choice-related behaviours by portraying a causal relationship between the product and the outcome.
These findings provide an insight into mechanisms that underlie the effect of warning labels, which could help improve the design and delivery of interventions using HWLs.