Child mental health outcomes were measured using both the parent- and teacher-completed Strengths and Difficulties Questionnaire (SDQ) at the age of 11 years [22]. These were administered by post and followed by reminder questionnaires if there was no response. The SDQ is a widely used dimensional measure of childhood mental health and has been validated in a large, nationally representative, community sample [23]. It includes four sub-scales relating to emotional problems, conduct problems, hyperactivity/inattention, and peer relationships and higher scores (scale of 0–10) indicate greater levels of severity. These are summed to provide a total problems score (0–40). Our analyses focus on the total problems score as well as the two behavioural problem sub-scales (conduct problems and hyperactivity/inattention).Academic outcomes were assessed using standardised, age-adjusted total scores from results on the Key Stage 2 (KS2) examinations taken in the final year at primary school (at ages 10–11 years). These scores provide an objective real world measure of academic performance and were obtained by data linkage with the National Pupil Database. In England, the national curriculum at KS2 relates to the school
