Firstly, we tested the relation between mothers’ and children’s reports of frequent bullying and mothers’ reports of their self harm by using modified Poisson regression to estimate relative risks and robust 95% confidence intervals,34 both unadjusted and then adjusted for the potentially confounding effects of physical maltreatment by adults, internalising and externalising problems at age 5, and IQ at age 5. We also calculated relative risks separately for girls and boys and evaluated the equality of the bullying coefficients across the sexes with an approach called “seemingly unrelated regression” using the “suest” command in Stata (v11.2). Secondly, we did a discordant twin analysis to rule out family-wide influences on the association between bullying victimisation and self harm.35 Here, we tested the hypothesis that bullied twins would be more likely to self harm than their non-bullied co-twins over and above shared familial environmental risks. Thirdly, we did a series of logistic regressions to test which risk factors could help to identify which bullied children would engage in self harm. We evaluated the following risks: family adversities (socioeconomic deprivation, family history of
