We examined whether messages about junk food intake would motivate individuals to decrease their consumption of high calorie snack food. Viewing messages outlining the health benefits of limiting junk food intake and that a social norm is to limit junk food intake was associated with less high calorie snack food intake than viewing a control message. There was no evidence that the social norm message had a greater effect than the health message. Compared with the control message, participants in the experimental conditions ate less high calorie snack food (32% on average, which equated to 70 fewer calories in a single snack session). These findings are promising, given that healthy eating interventions typically only promote modest changes to dietary behavior . Although our measure of usual intake of junk food predicted the amount of high calorie snack food participants ate (usually high consumers ate more junk food), there was no evidence of an interaction between this measure and message type. Thus, the messages had similar effects on high calorie snack food intake across both high and low consumers of junk food. Total energy intake was also lower in the experimental conditions versus control condition, suggesting that the messages reduced junk food intake and there was little evidence of compensation or substitution with other food items.
Developing public health messages that encourage people to limit their consumption of high calorie foods is a priority, as the widespread availability and intake of these foods is thought to be a significant contributor to rises in adiposity [1, 3]. Although much research has tested the types of interventions that could encourage people to increase their intake of fruit and vegetables, messages specifically targeting junk foods have received less attention [8, 10]. The present study suggests two message types that may be effective. The use of social norm messages to encourage healthier eating is a novel concept but evidence has started to accumulate that they could be used to promote fruit and vegetable intake [20–22]. This is the first study to test the effectiveness of social norm message in reducing intake of junk food.
In a previous study, we observed that social norm messages about consumption of fruit and vegetables were associated with greater fruit and vegetable intake in individuals who were usually low consumers, but not high consumers. This is likely to be because high consumers were already adhering to the norm, whilst low consumers felt motivated to increase their intake to adhere to the presented norm . See Schultz et al. for a discussion of how norms influence behaviour as a function of whether individuals believe they are adhering to a presented norm . In the present study, we predicted that usual intake of junk food would moderate the effect of a social norm message on intake of high calorie snack food/junk food. There was no evidence for such moderation in the present study; lower junk food intake was associated with exposure to the experimental messages regardless of usual intake. One explanation for this finding may be that the low usual consumers of junk food did not perceive themselves to be adhering to the norm we presented. The norm message was that ‘others limit their junk food intake to 0 or 1 portion a day’ and very few participants reported normally consuming 0 portions a day. Thus, it could have been that all participants were motivated to adhere to the norm. Measuring participant perceptions of how their current behavior relates to a norm presented and testing the influence this has on junk food intake would allow for a more direct examination of this proposition in future studies.
Previously, we had also observed that social norm messages were more effective than health messages for promotion of fruit and vegetable intake . A similar pattern of results has been found when measuring intentions to consume fruit and vegetables . This was not the case in the present study as participants in both experimental conditions ate less high calorie snack food intake than the control condition. It is possible that health messages about fruit and vegetable intake are now commonplace [9, 11], so the health implications of eating fruit and vegetables are already well understood. Conversely, messages about the health implications of consuming high amounts of junk food may not be as commonly understood in this population, so individuals may be less well informed. This suggestion is speculative, so evidence will be needed to confirm it. The message types assessed here are likely to bring about changes to eating behavior through different mechanisms (social motives vs. health motivates), so it would be interesting to examine whether an additive effect would be observed on combining messages. Examination of individual differences in responsiveness to message types would also be valuable. Health messages might be particularly influential on people with high health concern but they may have little impact on people with low health concern [26, 27], whereas social norm messages may motivate both sets of people.
A strength of the present study was the inclusion of a control condition, in which participants were not exposed to food messages. This allowed us to make a direct comparison between intervention messages and a non-intervention message. In future studies it would be informative to test the effects of a message that instructs participants to reduce their junk food intake, without specific reference to health motives or social norms, as this would allow for a clearer examination of the importance of food-related message content. Eating behavior was measured shortly after exposure to messages, so although messages may have motivated participants to change their behavior in a healthy way, whether these changes would be maintained over longer periods requires examination. Social norm messages to promote other health behaviors have produced longer lasting effects, so longer term changes might occur . As we tested participants in a lab environment and instructed them to read poster messages, it will be important in the future to examine whether these findings can be generalized to applied ‘real world’ settings. It will also be important to examine the effect of different social norm reference groups . Here, young adult students participated and the social norm message was about what other students were eating. Thus, although this type of approach could be effective in university campus settings, further evidence will be needed to assess whether more encompassing norm messages would motivate healthy changes to dietary behavior and if effects would occur in other subsections of the population. That being said, some recent work has suggested messages about national eating norms can increase peoples intentions to eat healthily in the general population , so this deserves further attention. Examining whether social norms approaches could be used to promote healthy eating in young children also warrants attention, as Salvy and colleagues in particular have shown peers to have a substantial influence on snack food intake in a number of studies in this population [13, 30].
The results of the present study suggest that participants did not compensate for their reduction in junk food by eating more of the fruit and vegetables that were also on offer. It may be the case that if other more palatable non-‘junk foods’ were available, participants would have compensated by eating these foods. Further work will need to clarify this. In line with other research examining social norm effects, we used self-report measures of dietary behavior [20, 22]. Although we included a guided dietary recall and a measure of typical consumption, further work would benefit from measures less likely to be influenced by reporting bias.
The results from the present study show that messages about the health benefits and social norms surrounding ‘junk food’ intake are associated with reduced intake of high calorie snack food in young adults. The results also contribute to a new body of evidence that suggests social norm based messages can motivate people to eat more healthily.