Objective: To test whether the experience of being bullied has an environmentally mediated effect on internalizing symptoms in young children.
Design: A genetically informative, longitudinal 1994-1995 birth cohort.
Setting: A nationally representative sample from the United Kingdom.
Participants: We examined 1116 twin pairs who are participants in the Environmental Risk Longitudinal Twin Study. Main Exposure The experience of being bullied between the ages of 7 and 9 years.
Main outcome measures: Mothers' and teachers' reports of children's internalizing problems at 7 and 10 years of age.
Results: Monozygotic twins who had been bullied had more internalizing symptoms (mean, 0.23; SD, 1.00) compared with their co-twin who had not been bullied (mean, -0.13; SD, 0.86), indicating that being bullied has an environmentally mediated effect on children's internalizing problems (beta, 0.36 [95% confidence interval (CI), 0.18-0.54]). This effect remained significant after controlling for preexisting internalizing problems (beta, 0.26 [95% CI, 0.09-0.44]).
Conclusions: Being bullied at a young age is an environmentally mediated contributing factor to children's internalizing problems. Intervention programs aimed at reducing bullying behavior in schools and in the community have the potential to influence children's early symptoms of mental health problems.