Molecular genetic studies of ADHD: 1991 to 2004

AbstractAttention deficit hyperactivity disorder (ADHD) is highly heritable but is likely a complex disorder involving multiple genes of moderate effect (Smalley [1997: Am J Hum Genet 60:1276–1282]). Over 100 studies have examined the genetics of ADHD by linkage or association, though no article has presented a comprehensive overview of all published reports. We reviewed all ADHD studies, including 3 genome‐wide linkage studies, and association studies of 94 polymorphisms in 33 candidate genes. To simplify comparisons across heterogeneous articles, demographics and comorbidity were ignored; analyses of subtype and haplotypes were excluded; and only the most positive finding for each polymorphism in a study was reported. Thirty‐six percent of all findings were positive (P ≤ 0.05), 17% were trends (0.05 < P ≤ 0.15), and 47% were negative (P > 0.15). Studies utilizing dimensional measures of ADHD tended to result in higher rates of positive findings than those using categorical diagnoses (χ2 = 5.6, P = 0.018), and case‐control studies tended to result in higher rates of positive findings than family‐based studies (χ2 = 18.8, P < 0.001). However, for either dichotomy, no signi