Several research groups have started to employ twin studies to identify endophenotypes for ADHD. Initially, samples were too small to draw robust conclusions from parameter estimates gained from biometric genetic analyses,37 but recent analyses on larger-scale twin studies have been promising. For example, the heritability of RT data was estimated at 50% to 80%,4,62 with lower estimates increasing to around 70% when corrected for measured test-retest unreliability.63 This estimate is close to the average heritability of ADHD (~60% to 70%).1,3 Other measurements such as commission errors show somewhat lower estimates in the range of 18% to 48%, although these increase to 68% when also corrected for test-retest unreliability.15 Nevertheless, it is important to realize that biometrical genetic studies, including SEM analyses of twin data, transcend simple estimation of heritability. Such studies can address additional issues in the search for endophenotypes, in particular the identification and selection of appropriate endophenotypes from the many potential candidates.
