Background: 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.
Method: We explored possible reasons for this discrepancy in two large, longitudinal samples of twins and singletons aged 8–17.
Results: 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.
Conclusions: Implications for future research on adolescent depression are discussed.