Sample characteristics.

Self-reported height and weight is widely used to calculate Body Mass Index (BMI) and yet little is known about factors that affect accuracy. This study investigated the motivational characteristics–narcissism and social desirability—that influence the accuracy of self-reported weight and how they interact with accountability (telling participants that their weight will be verified). A two-way between-subjects design was used (accountable vs not accountable) with motivational moderators (narcissism and social desirability). Participants (N = 80; Mean Age = 34.63; 58.8% Female) were randomly allocated to accountable (told that their weight would be verified in a follow-up lab session) or not-accountable (no information given) conditions. In Session 1, participants self-reported motivational (social desirability and narcissism) and anthropometric (height and weight) measures online. In Session 2 (24 hours later), objective measures of height and weight were taken in the lab. There was a significant interaction between condition and maladaptive narcissism level. Being told that weight would be later verified improved accuracy of self-reported weight, but only for those low in maladaptive narcissism. Accountability improves the accuracy of self-report weight data, but not for individuals high in narcissism. Though based on a modest sample, these findings suggest that the under-estimation of self- report weight serves a self-protective function and maladaptive narcissism may be a useful covariate to include in research using self-report weight estimates.