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Depressive symptoms and disorder are experienced by a significant proportion of young people and have long-lasting deleterious effects. The aims of the current investigation were to examine the aetiology of depressive symptoms using a twin design. In particular to examine the effects of sex, age, maternal depression and anxiety symptoms and to examine the aetiology of high depression scores.Questionnaires were sent to the families of a population-based sample of twins aged between 8 and 17 years. Parents and children over the age of 11 were asked to complete the Mood and Feelings Questionnaire and Hospital Anxiety and Depression Scale (mothers only). Responses were obtained from 1463 families and data were analysed using genetic model fitting and DeFries and Fulker regression analysis.Depressive symptoms, particularly when self-rated, were significantly genetically influenced. There was evidence of significant heterogeneity according to age, with shared environmental factors more important and genetic factors less important for children aged 8 to 10 than for adolescents aged 11 to 17 years. Some but not all of the shared environmental influences on parent-rated depressive symptoms were accounted for by maternal symptoms of depression and anxiety. There was a significant effect of gender for self-rated depressive symptoms. For boys, genetic factors were of greater importance and common environmental influences of less importance than for girls. Shared environmental effects had a substantial influence on high self-rated depression scores. Adolescents who scored highly on self-rated depression questionnaires experienced significantly more shared life events and their mothers had significantly higher internalising symptoms than adolescents who scored within the normal range.The results of this study add to the evidence that the aetiology of depressive symptoms differs by age, with genetic factors becoming more important from childhood to adolescence. Some but not all of the shared environmental effect observed for mother-rated depression scores is due to maternal depression and anxiety symptoms. For self-rated depressive symptoms, the importance of genetic and environmental factors may also differ by sex, with genetic influences more important for boys. The aetiology of high depression symptom scores, when self-rated, appears to differ from scores within the normal range in that shared environmental factors appear to be more important. Further research is needed to identify these shared environmental factors using longitudinal models that test genetic and environmental mediation.