Linkage studies were the earliest method for scanning the genome to narrow the search for genes involved in complex outcomes. As the findings reviewed above indicate, few regions reach conventional thresholds for suggestive or significant linkage, and there is little consistency among the regions identified across samples, with the exception of the region on chromosome 2 identified in COGA with suggestive evidence of replication in IASPSAD. The limited success of linkage studies for conduct disorder likely reflects multiple factors. Most notably, linkage methods are well suited for finding genes of large effect size and have thus had the greatest success in identifying genes for monogenic disorders rather than genes for complex disorders, which typically involve many genes of more modest effect (Bush and Haines, 2010). In view of the modest effect sizes expected for genes implicated in conduct disorder, these early linkage analyses were likely underpowered.
