Relationship Between School-Based Bullying Behaviours and Happiness, and the Mediating Role of Body Mass Index at School-entry: Evidence from the Born in Bradford Primary School Years Study

<ns3:p>Background Despite extensive research on school-based bullying and its effects on children’s well-being, the role of body mass index (BMI) as a moderating factor in these dynamics is not fully known. Methods This study aims to assess the relationships between experiencing bullying and being mean to others, and happiness in school-aged children, assessed through self-reported surveys at ages 7–10 years, while evaluating whether BMI at school-entry (measured at ages 4–5 years) influenced these associations. Data were drawn from the Born in Bradford Primary School Years study, a large, ethnically diverse longitudinal cohort study in the UK. Logistic regression models were used to analyse the relationships between school-based bullying involvement/ being mean to others and happiness, and whether these were moderated by BMI at school-entry, to explore potential long-term effects of weight status on social and emotional well-being. Results 51.4% of children reported being bullied (54.7% girls vs. 48.3 boys). Those who were bullied by others had lower odds of feeling happy all the time (OR = 0.60, 95% CI: 0.42–0.86, p = 0.006). This association was observed among those who were mean to others regarding feeling happy all the time (OR = 0.34, 95% CI: 0.24–0.49, p &lt; 0.001) or sometimes happy (OR = 0.20, 95% CI: 0.13–0.30, p &lt; 0.001). Although children with obesity had a slightly higher prevalence of being bullied (52.6%) compared to their healthy-weight peers (48.6%), BMI at school-entry did not modify the relationship between school-based bullying or mean behaviour and happiness. Conclusions The findings of our study highlight the negative impact of school-based bullying on children’s happiness, regardless of BMI at school-entry. Implementing school-based interventions and strengthening social support systems may help mitigate these effects and enhance children’s emotional well-being. Future research in different longitudinal designs is warranted to confirm the long-term interplay between school-based bullying, BMI at school-entry, and psychological outcomes.</ns3:p>