Our sample included rather a small number of children who had engaged in self harm, and this may have led to unstable or biased estimates of the size of the associations between bullying victimisation and self harm, particularly when controlling for confounders. Therefore our results need to be replicated in samples with a higher prevalence of such behaviours. Nevertheless, this study provides first evidence of an increased risk of self harming behaviours at the beginning of adolescence among bullied children from a population representative birth cohort unbiased by referral for treatment. The sample comprised twins, so the results may not generalise to singletons. However, the prevalence of bullying victimisation and mental health problems has been shown to be the same for twins and singletons.13 57 Moreover, using the twin design we were able to show that the association between bullying and self harm was maintained when environmental risks within the family were taken into account. Although we examined both mothers’ and children’s reports of bullying, we had access only to mothers’ reports about the children’s self harm, which may have
