Exercise, dieting and adherence to medicines are effective measures to reduce long-term consequences of diabetes; however, patients throughout the world fail to manage their condition. We propose the existence of motivated beliefs as an explanation for this paradox. We empirically test the economic model of motivated beliefs using data from 100 patients with diabetes. We operationalized beliefs by comparing real BMI to an individual’s BMI reference point where she is motivated to believe that she should start preventive effort. We measure an individual reference point to start prevention by using previously validated pictorial BMI-based body size guide. Most respondent’s report a reference BMI to initiate preventive effort larger than their real BMI; interestingly this reference BMI is uncorrelated with real BMI. The distortions between real and reference body image to start prevention are higher among males and among younger individuals. Those with a larger negative distance from the reference point are 0.64 points less likely to engage in self-management, which is 23% of the average behavior. These results open the possibility that personalized medicine should incorporate information about an individual’s beliefs to improve the efficacy of treatment. Our results have implications to explain the lack of self-management in other chronic conditions.