Cross-National Study on the Convergent, Discriminant, and Concurrent Validity of the "Body Size Perception" Item in the Health Behaviour in School-aged Children Survey.

Background: Despite its significant usefulness in adolescent health studies, the single-item "body size perception" question, developed within the Health Behaviour in School-aged Children (HBSC) survey, has yet to undergo multidimensional validation. Objectives: To assess the convergent, divergent and concurrent validity of the HBSC body size perception question among adolescents. Methods: The single-item HBSC body size perception question is as follows: "Do you think your body is…?," with answers ranging from "much too thin" to "much too fat." Fifteen-year-old participants included in the analysis were 72,086 from 45 HBSC countries in 2017/18 (concurrent validity), and 595, 127, and 615 in 2021/22 in French-speaking Belgium, Ireland, and Poland, respectively. The convergent, divergent, and concurrent validity was assessed with body dissatisfaction, social desirability, and selfesteem, respectively. The concurrent validity was also examined with body mass index (BMI) from the 2017/18 HBSC data. All analyses were sex-stratified. Results: Cohen's Kappa values were 0.67 [confidence interval (CI): 95%: 0.62, 0.72] and 0.64 (0.59, 0.69) for boys and girls, respectively, in all 3 countries together. Body size perception was associated with social desirability, selfesteem, and BMI, with a stronger association in girls than that in boys. For instance, girls with higher social desirability were less likely to perceive themselves as "too thin" [Relative Risk Ratio (RRR) = 0.78 (0.69, 0.89)] rather than as the "right size." Boys with higher selfesteem were less likely to perceive themselves as "too fat" [0.93 (0.90, 0.97)] rather than the "right size." Girls with underweight were less likely to perceive themselves as "too fat" [0.38 (0.34, 043)] rather than "right size" and girls with overweight/obesity were more likely to perceive themselves as such [8.19 (7.49, 8.95)]. Conclusions: The single-item HBSC body size perception question demonstrated good convergent, divergent, and concurrent validity. It reflects adolescents' own perception of body size, possibly influenced by societal norms and ideals.