Objective The aim of this study was to determine whether specific psychological factors influence intervention effects for children with severe obesity in a clinical setting.

Design Secondary analyses of data about attention deficit/hyperactivity disorder (ADHD) characteristics, body satisfaction, social and emotional functioning, and the primary outcome, change in body mass index (BMI), were available for 41 out of 72 children and their families randomised to family-based behavioural treatment over 6 months or waiting list control. Regression analyses, with an interaction term for treatment condition, were performed to explore baseline factors and moderators of outcome.

Results Parents reporting their child’s emotional well-being as high and high maternal education significantly predicted less weight loss for the total sample, with no effect of ethnicity, age, sex or baseline BMI. Children’s social functioning was a significant moderator of treatment effect; children with high social function showed a decrease in BMI after 6 months of therapy (R²=0.08–0.13), whereas an increase in BMI was observed in children with high social function who waited for treatment. For children with poor social function, no treatment effect was observed—subjects lost weight in both conditions. No significant moderation effect was found for body (dis)satisfaction, emotional status, comorbid depression or ADHD, adjusting for baseline BMI, age, sex and ethnicity.

Conclusions These preliminary findings suggest directions for development of tailored obesity programmes. Professionals engaged in treatment of childhood obesity should consider a child’s emotional and social functioning when considering group obesity intervention, as well as the risks of no intervention.