The more overweight a child, the more likely he or she is to be absent from school, a new report suggests. Researchers studied 1,069 fourth- to sixth-grade students in nine schools in Philadelphia. They recorded height, weight, sex, race and days absent for each. The study appears in the August issue of Obesity.

The scientists classified each child in one of four weight categories by body mass index: underweight, normal, overweight and obese. On average, underweight children were absent 7.5 days, normal weight children 10.1 days, overweight children 10.9 days and the obese 12.2 days. Even after adjusting for race, ethnicity, age, sex and school attended, being overweight remained a significant predictor of absences.

Statistical analysis showed that weight, sex, age, school and race accounted for 11 percent of the variance in absences, meaning unknown factors are involved. The authors acknowledge that it is unclear whether the increased absences significantly affect overweight students’ performance.

Andrew B. Geier, the lead author and a doctoral candidate in psychology at the University of Pennsylvania, doubts that sickness among overweight children causes absences. “Even in fourth grade,” he said, “I believe that psychosocial factors, not physical ones, are keeping overweight kids from going to school.”