Peer Problems and Hyperactivity–Impulsivity Among Norwegian and American Children

Peer problems are linked to ADHD-symptoms and the serotonin system is thought to be involved in ADHD-related behavior. Hence, from a gene-x-environment perspective, we examined whether the serotonin transporter 5-HTTLPR moderated the effect of peer problems on the development of later hyperactivity-impulsivity symptoms. In study 1, involving 642 Norwegian children, results indicated that for s-allele carriers only, caregiver-reported peer problems at age 4 predicted more parent-reported hyperactivity-impulsivity symptoms over the next two years. In Study 2, similar results emerged involving 482 American children. Discussion focuses on differential sensitivity to the adverse effects of poor peer relations.​​