You Can Be Too Thin (But Not Too Tall): Social Desirability Bias in Self-Reports of Weight and Height 
In the United States, as in many other countries, self-reported values of weight and height are sometimes used to estimate obesity rates. Despite the need to obtain accurate measures of body mass index (BMI) and obesity rates in order to guide public health policy, it is widely recognized that self-reported values of weight and height are often inaccurate. A leading explanation for what causes these errors contends that self-reported values are influenced by prevailing social norms for weight and height by gender. If so, this would represent an instance of "social desirability bias," a broader phenomenon in which survey-takers bias their answers in the direction of socially desirable responses, such as reporting that one voted in a recent election when in fact one did not. .. To date, however, evidence of social desirability bias in self-reported values of weight and height is not conclusive, in part due to the absence of a precise framework describing how social norms might influence self-reporting behavior. To fill the gap, this paper proposes a theoretical model in which an individual's self-reported body weight is a function of the person's true weight and an exogenously determined social norm; essentially, the respondent faces a tradeoff between reporting a truthful value and a socially desirable one. The model generates testable predictions that can be compared with predictions based on alternative explanations for self-reporting errors. These various predictions are tested using a sample from the National Health and Nutrition Examination Survey that records both self-reported values of body weight and height and examined values for the same individuals. The empirical analysis is also used to make inferences about social norms for BMI.
Key Findings
- Individuals whose true body weight exceeds a given weight threshold or norm tend to understate their weight, while those below the norm tend to overstate their weight. The degree of overstatement or understatement of weight increases the farther is true weight from the norm in a given direction. These empirical tendencies agree strongly with the theoretical model in which self-reported weight is influenced by social desirability bias.
- An alternative explanation for misreporting of weight holds that many individuals are not aware of their current weight because do not weigh themselves frequently and their weight has changed over time. We find that this combination of factors may help to account for large underreporting margins among those with high levels of examined body weight. However, robust evidence of social desirability bias is present over most of the weight distribution.
- We infer a social norm for BMI of 22 for women. This social norm is slightly above the middle of the normal BMI range for women, 18.5–24.9, and corresponds to a slender physique. For men, the inferred social norm for BMI, at 26, falls into the overweight range of 25-29.9. This suggests that the BMI social norm for men conforms to a more muscular ideal.
Exhibits
Implications
The data suggest that the self-reporting errors in weight and height measurements, which are used to calculate BMI, are on average mild to moderate rather than severe, a finding which suggests that individuals care about giving an accurate report in addition to wanting to report socially desirable values. To the extent that the psychic costs of dishonesty counteract social desirability bias, surveys might seek to incorporate some mechanism to provide external validation of self-reported values in order to incentivize truthful self-reporting behavior.
Abstract
Previous studies of survey data for the United States and other countries find that on average women tend to understate their body weight, while on average both men and women overstate their height. Social norms have been posited as a potential explanation for misreporting of weight and height, but researchers disagree on the validity of that explanation. This paper is the first to present a theoretical model of self-reporting behavior for weight and height that explicitly incorporates social desirability bias. The model generates testable implications that can be contrasted with predictions based on alternative explanations for self-reporting errors. Using data from the National Health and Nutrition Examination Survey (NHANES) from 1990–2010, we find that self-reporting patterns for both weight and body mass index (BMI) offer robust evidence of social desirability bias, such that reports are biased (from both sides) towards social norms. The BMI norm inferred for women lies squarely within the range considered "healthy" by public health officials, while the BMI norm inferred for men lies just above this healthy range. Lack of awareness of one's current body weight may explain the presence of large (negative) self-reporting errors among those with very high values of examined weight, but the evidence of social desirability bias is robust to this alternative explanation over most of the weight distribution. Social desirability bias in self-reporting of height is observed primarily among those of below-average height and no clear height norms are discernible. The framework also helps to explain previous findings that the degree of self-reporting bias in weight depends on the survey mode.