Why some children with externalising problems develop internalising symptoms: testing two pathways in a genetically sensitive cohort study

Background Children with externalising problems are at risk of developing internalising problems as they grow older. The pathways underlying this developmental association remain to be elucidated. We tested two processes that could explain why some children with externalising problems develop internalising symptoms in preadolescence: a mediation model whereby the association between early externalising and later new internalising symptoms is explained by negative experiences; and a genetic model, whereby genes influence both problems. Methods We used data from the Environmental Risk (E‐Risk) Study, a 1994–1995 birth cohort of 2,232 twins born in England and Wales. We assessed externalising and internalising problems using combined mothers’ and teachers’ ratings at age 5 and 12. We measured bullying victimisation, maternal dissatisfaction and academic difficulties between age 7 and 10 and used linear regression analyses to test the effects of these negative experiences on the association between early externalising and later internalising problems. We employed a Cholesky decomposition to examine the genetic influences on the association. Results Children with externalising problems at age 5 showed increased rates of new internalising problems at age 12 (r = .24, p