In additional analyses we controlled for the possible confounding effects of socioeconomic position and previous symptoms of depression. Socioeconomic position was assessed through maternal questionnaire in pregnancy and included highest maternal educational attainment, we used four categories: university degree; A level or advanced level (which are after compulsory school qualifications); O level, or ordinary level (which are academic qualifications taken at the end of compulsory schooling. O levels are now defunct in the United Kingdom and have been replaced by GCSE examinations); and less than O level, which includes any other qualifications of a lower academic standard or no qualifications at all), and social class (professional or managerial or other occupations. We used the highest of maternal or paternal social class). We assessed symptoms of depression at age 13 years using the short mood and feelings questionnaire.35 Owing to the well established association of IQ with both self harm and school performance, we additionally controlled for child IQ when investigating educational and occupational outcomes. We assessed child IQ at age eight years using the Wechsler intelligence test for children, third edition.36
