Another possibility is that factors within the family environment (such as poverty, parental psychopathology, domestic violence) could account for the association we observed between bullying and self harm. To rule out the influence of these shared environmental risk factors, we compared twins within the same family to determine whether bullied twins were more likely to self harm than their non-bullied age and sex matched co-twin. Using mothers’ reports, 162 twin pairs were discordant for bullying, and the bullied twins were significantly more likely to self harm than were their non-bullied co-twins (13 (8.0%) v 3 (1.9%); ratio=4.3, 95% confidence interval 1.3 to 14.0). Turning to children’s self reports, 144 were discordant for bullying, and here we saw a similar trend for the bullied twins to self harm compared with their co-twins (12 (8.3%) v 7 (4.9%); ratio=1.7, 0.71 to 4.1).
