BackgroundResearch shows that social values often function as truisms—ie, these values are convictions that are widely held and strongly endorsed, but rarely questioned. Thus individuals tend to behave in accordance with the value only when they can do so fairly easily. However, value-consistent behaviour can be increased by building cognitive support for the value, which can be achieved by thinking about reasons supporting or opposing the value. We examined whether health values function as truisms and explored the effect of building cognitive support for health values on fruit and vegetable consumption and exercise. MethodsIn experiment 1, 150 participants (aged ≥18 years) were recruited via local advertisements, posters, and flyers, and tested in community and workplace settings. This sample was comparable to the general population in terms of age, sex, ethnicity, education level, smoking status, and BMI, and there were no significant differences between experimental and control groups in terms of these characteristics. Participants rated the importance of a range of values (including four health values), on a scale from −1 (opposed to my values) to 7 (extremely important). Participants then wrote down reasons why health values were important or unimportant to them (experimental group, n=75), or wrote reasons why they liked or disliked particular television shows (control group, n=75). Finally, all participants completed a second questionnaire assessing value importance, including the four health values. In experiment 2, 43 students (aged ≥18 years) either analysed reasons for health values (experimental group, n=22) or completed anagram and word search tasks that made the value of health salient (control group, n=21). 1 week later, they reported their diet and exercise behaviours during the previous week (number of days that they had consumed fruit and vegetables; average number of servings eaten; number of times they had participated in vigorous physical activity; and number of times they had exercised for ≥20 min). We analysed data with ANOVA and post-hoc t tests. FindingsExperiment 1 showed that health values ratings changed more after the analysis of reasons than in the control group (p<0·02; mean [SD] in experimental group=0·62 [0·41], mean [SD] in control group=0·44 [0·45]), indicating that health values were functioning as truisms. This effect occurred across a range of different social groupings, and irrespective of whether individuals lead healthy or unhealthy lifestyles. In experiment 2, responses to the health and diet items were standardised and combined to form diet and exercise indices. Participants who completed the reasons analysis subsequently reported more exercise (p=0·08, mean [SD]=0·56 [2·08]) and less food consumption (p=0·06, mean [SD]=–0·44 [1·33]) than did those in the control group (mean [SD] exercise=–0·78 [2·51]; mean [SD] food=0·48 [1·68]). This research shows that health values function as truisms. InterpretationThis research shows that health values function as truisms. As with other values, people do not think to question the importance of health. That this effect was true for a range of different social groupings suggests that health reasons analysis could be a helpful strategy for most people. Consistent with this hypothesis, experiment 2 showed that thinking about reasons for health values resulted in participants reporting higher amounts of exercise 1 week later. That they also reported lower amounts of food consumption could be an indication of increased dieting. Further research with physiological and observational measures, and more detailed dietary questionnaires, should help to confirm both this interpretation and the usefulness of this technique in motivation of health behaviour change. FundingThe research was funded by an UK Economic and Social Research Council (ESRC) grant “Lifestyle change: values and volition” awarded to KT, GRM, GH, and ML, and by an ESRC grant “Explicit and implicit bases of social values: implications for behaviour change” awarded to GRM and GH.