Comorbidity and child psychopathology: recommendations for the next decade.

This special section exemplifies and offers a number of important methodologic and conceptual advances that should provide investigators new tools for understanding comorbidity of child and adolescent psychopathology, including (a) the importance of making careful methodologic distinctions in how comorbidity is defined and operationalized, (b) specifying and justifying how data from different sources are combined, (c) teasing out the impact of potentially confounding risk factors that lead to symptom and syndrome overlaps, and (d) exploring the effects of time, timing, and order of disorder emergence on variable manifestations of comorbidity. These advances are much needed, but may still prove insufficient, given the daunting challenges in fully understanding comorbidity. Thus, future studies should be characterized by (a) more focused search for subgrouping factors and interactions related to the emergence of comorbidity, (b) careful exploration of setting- and/or informant-specific types of psychopathology, (c) development of studies that explore not just phenotypes and genotypes, but also environtypes and trajectory-types, (d) more discriminative use of information sources, including explicit efforts to reconcile (rather than combine) discrepant information, (e) clear descriptions and logical justification of when conjunctive, disjunctive, additive, and discriminative combinatorial approaches are used, (f) increased use of multidisciplinary research methods and teams, (g) increased application of multiple lines of evidence in comorbidity studies, (h) increased focus on understanding illness processes rather than just psychopathologic states, (i) development of creative new research designs, and (j) redrawing disorder boundaries when warranted.