The study will include comprehensive phenotyping of 65 patients meeting criteria for SMD, including assessment of comorbid psychopathology, language disorder, neurocognition, and EEG. Potential endophenotypes and diagnostic boundaries will be assessed in relation to our large existing database of children and adolescents with internalizing and externalizing disorders, as well as non-clinical controls. Eligible participants with SMD will proceed to a pilot study of sequential pharmacotherapy with an initial 4-week titration of open label lisdexamfetamine (LDX) followed by 8 weeks adjunctive therapy with randomized fluoxetine or placebo. Statistical analyses will address diagnostic boundaries of SMD compared with other disorders and emphasize initial determinations of the potential efficacy and tolerability of stimulant and SSRI treatments for SMD. There will be an added emphasis on effect size estimates and determination of optimal outcome measures in anticipation of future large-scale studies.