Assessment of child ADHD symptoms was based on items derived from the DICA (Reich, 2000) and the C-SSAGA. It has been suggested that measures of symptom count should be used rather than categorical diagnoses (Levy et al., 1997), and much research has been conducted using this strategy. For purposes of these analyses, DSM-IV ADHD symptom endorsement (see Table 1) was based solely on maternal report and defined as a sum total of items endorsed and required onset of each symptom prior to age 7. Separate summary scores were created for Inattentive (INATT) and Hyperactive/Impulsive (HYP/IMP) dimensions. Twins’ self-report of ADHD data was not obtained.
