Given the lack of availability of measured anthropometric data for the whole of Spain, this paper combines data from the 2006 Catalan Health and Health Examination Surveys to compute the size of weight and height self-reporting biases. The underlying determinants of these biases are then analyzed, placing special emphasis on the role played by social norms. Our findings show that social norms regarding ''ideal'' weight (proxied by the average weight of a reference group based on gender and age) tend to affect the selfreporting weight (relative) bias. This finding suggests that the more satisfied individuals feel with their own body image the less prone they are to under-report their weight, although this effect is contingent upon the definition of social norms and the correction of endogeneity. However, we found no evidence of a similar impact caused by the social norms governing height. The relationship found between the measured and self-reported anthropometric data was applied to the Spanish National Health Survey (NHS) so as to correct the self-reported information contained in it. After correcting for self-reporting errors, both the BMI and the prevalence of obesity were found to be significantly underestimated, with instances of misreporting being more prevalent among women.