Teenagers judge their bodies against the classmates around them. Students who are among the youngest in their class, and so smaller than many peers, are more likely to describe a normal-weight body as “too thin,” and they eat and exercise in ways that match that belief.
The study is by Christopher S. Carpenter of Vanderbilt University and Brandyn F. Churchill of American University. It was published online in Health Economics on June 30, 2025, and appears in the journal’s September 2025 issue. An earlier version circulated as NBER Working Paper 32629 in June 2024.
Using birthdays as a natural experiment
Within any classroom, students born just after the school entry cutoff can be nearly a year older than those born just before it. In adolescence, that year means real differences in size. If students form their body image by comparing themselves with classmates, the younger ones should feel small and the older ones large.
Parents sometimes hold children back a year, so relative age is not purely random. To get around that, the researchers used an instrumental variables approach based on the gap between each student’s birth month and the official school entry cutoff month in their country.
The data came from the Health Behaviour in School-Aged Children study, a cross-national survey of European students. The working paper used survey waves from 2002 to 2018 across 33 European countries, with analyses of body image covering more than 460,000 students. Researchers compared how students described their bodies with the clinical BMI category implied by their reported height and weight.
In the working paper, about 20% of adolescents described themselves as lighter than their BMI implied, 60% described themselves accurately and 20% as heavier. Girls were more likely to have a harsh view of their bodies than boys.
Normal-weight students who feel too thin
Relatively younger students were more likely to describe their bodies as lighter than the BMI category their height and weight implied, compared with same-age students who were relatively older within their own classrooms.
The effect was driven by relatively younger students classified as normal weight for their age describing themselves as “too thin.” In other words, the misperception came from kids with no weight problem who looked around and felt small.
Eating and exercising to match
The beliefs showed up in behavior. Relatively younger students were more likely to say they wanted to gain weight, ate more calorie-dense foods and reported lower levels of physical activity.
In the working paper, the authors put a rough number on the diet gap. Students one standard deviation older in relative age than their classmates ate about 1,381 fewer calories over a year, based on reported servings of sweets and sugary drinks. The paper also found that relatively younger students were, if anything, more likely to be overweight for their age, even as they reported a desire to gain weight.
The authors note that the physical activity and diet measures are self-reported and fairly coarse. They argue, though, that there is no reason misreporting should track a student’s birth month relative to a school cutoff.
Their conclusion is that relatively younger students base their weight-related expectations and behaviors on their older peers. The working paper adds a broader warning: if young people compare themselves with heavier reference groups, social comparison could feed rising rates of overweight rather than curb them.
Study Details:
- Title: Social Comparisons and Adolescent Body Misperception: Evidence From School Entry Cutoffs
- Authors: Christopher S. Carpenter, Brandyn F. Churchill
- Journal: Health Economics
- Publication Date: June 30, 2025
- DOI: 10.1002/hec.4980