The study findings demonstrate that preschool depression was a significant and robust predictor of meeting full DSM-5 criteria for major depressive disorder in later childhood and early adolescence (i.e., at ages 6–13). The predictive power of preschool depression for school-age depression remained strong and undiminished even when other key environmental and familial risk factors were included in the model. Preschool conduct disorder also remained significant, although its effect was diminished when nonsupportive parenting was accounted for. This finding extends the available data by demonstrating that preschool depression not only shows homotypic continuity up to 2 years later but also converts to meet all formal DSM-5 criteria for major depressive disorder during school age and early adolescence (a mean of 6 years later). This finding of major depressive disorder as a longitudinal outcome of preschool-onset depression further supports the significance and robustness of the preschool depression construct.
Preschool depression was also a risk factor for school-age anxiety disorders (odds ratio=3.48) and ADHD (odds ratio=3.69), underscoring its heterotypic in addition to homotypic outcomes. Heterotypic, and well as homotypic, outcomes of prepubertal depression have been reported in several other longitudinal samples (16, 17). Weissman et al. (17) have suggested further that children with prepubertal depression and a family history of depression have the highest risk of a recurrence of depression during adulthood. Consistent with the literature on risk for depression in older children, preschool conduct disorder also emerged as a significant predictor of later major depressive disorder. The finding that disruptive disorders are a risk factor for childhood depression has been established in several studies of school-age children (37). In a previous analysis of this sample, disruptive disorders during the preschool period also emerged as a risk factor for depression at the 2-year follow-up (6). However, the predictive power of this relationship was diminished in the present study when the effects of nonsupportive parenting were considered in the analysis. Formal testing for mediation showed that the effect of preschool conduct disorder on school-age depression was partially mediated by nonsupportive parenting. This suggests that nonsupport in the context of conduct disorder is an important factor in the mechanism by which the risk for later depression is transmitted. Targeted study of this risk trajectory is indicated.
After accounting for other known significant risk factors, we found that preschool depression remained a highly significant predictor of later meeting full criteria for major depressive disorder. These findings suggest that the preschool diagnosis is a stronger predictor of later major depression than maternal history of depression or traumatic life events. This finding contradicts common clinical belief and practice, in which these latter risk factors are viewed as highly significant risk markers, while symptom characteristics in young children and a preschool depression diagnosis are generally considered more secondarily (or not at all) in the domain of risk for later depression. While these findings do not address the question of whether preschool depression represents minor depression or a risk state, they should dispel any doubt that children with preschool-onset depression constitute a group that is clearly and uniquely at high risk for later depression and therefore should be targeted for early intervention.
This study also provides data to inform the relationship between preschool anxiety disorders and later childhood depression. In contrast to findings in adults and older children, preschool anxiety disorders did not emerge as a precursor of school-age and early adolescent depression. However, preschool depression emerged as a risk factor for school-age anxiety disorders (as well as school-age ADHD). The inconsistency of this finding relative to the established risk trajectory in older children and adults could be related to a number of developmental factors. One possibility is the more transient nature of early-onset anxiety disorders, which in some cases may represent a developmental extreme (e.g., separation anxiety disorder) rather than an enduring clinical disorder.
The most salient limitations of the study include the relatively small sample of children with preschool depression and the fact that the majority of study participants have not yet been followed through puberty. Thus, further studies that carefully track the sample through the pubertal period are needed to identify which preschoolers displaying depressive symptoms will not be at risk for a recurrent course. Nevertheless, the finding that preschool depression is a robust predictor of future major depressive disorder makes a strong case for using validated age-adjusted criteria to identify this early depressive phenotype before school age. The study findings are also of importance when considering that early intervention for depression in the preschool period, a point of high relative neuroplasticity, may provide a window of therapeutic opportunity to alter the chronic and relapsing course known in depressive disorders.