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The excess of unipolar depression in females emerges in adolescence. However, studies of age effects on depression scale scores have produced divergent estimates of changes from childhood to adolescence.We explored possible reasons for this discrepancy in two large, longitudinal samples of twins and singletons aged 8–17.There were no differences between twins and singletons in their scores on the Short Mood and Feelings Questionnaire (SMFQ), a 13-item self-report depression scale. SMFQ scores for boys fell over this age-range, while those for girls fell from age 9 to age 11 and then increased from age 12 to age 17. The mean scores of girls under 12 and those 12 and over differed by only around one-fifth of a standard deviation. However, given the non-normal distribution of the scores, a cut point that selected the upper 6% of scores created the expected female:male ratio of 2:1.Implications for future research on adolescent depression are discussed.