Better at what cost? The contrasting effect of norm feedback on exercise behaviours and body image.

Descriptive norm feedback informing people they have performed below average on an exercise task can motivate improvements in their subsequent performance and exertion. However, no research has examined whether such feedback has concurrent negative effects. We addressed this in a pre-registered lab experiment. Participants (N = 120) completed a brief no-equipment exercise workout twice. Prior to and during Trial 2, participants either received no feedback (control condition) or feedback that their Trial 1 performance was 30% below (high norm condition) or 30% above (low norm condition) the average of fellow students. Mixed model ANOVAs indicated that participants in the high norm condition demonstrated significantly greater improvements in both their objective performance (vs. low norm: t(78) = 7.77, p < .001; vs control: t(78) = 9.36, p < .001) and exertion (vs low norm: t(78) = 2.41, p = .046; vs control: t(78) = 4.20, p < .001). However, participants in the high norm condition also demonstrated significantly greater reductions in their body satisfaction (vs low norm: t(78) = -4.43, p < .001; vs control: t(78) = -3.63, p = .001) and increases in their negative mood (vs. low norm: t(78) = 5.80, p < .001; vs control: t(78) = 3.38, p = .003). Moderation analyses indicated that the effects of the norm feedback on performance (F(2, 114) = 4.55, p = .13) and body satisfaction (F(2, 114) = 7.19, p = .001) were moderated by social identification such that its effects were stronger among those who strongly identified with the norm reference group. Descriptive norm feedback may improve exercise outputs but carries a psychological cost. Its use should be sensitive to potential negative impacts on body image and mood, with further research needed to examine ways to offset these unintended effects.