factors contribute to the endophenotype, then the means should be ordered as follows: controls ← unaffected siblings ← probands. This approach has been successful in other fields of neuropsychiatric research suc as schizophrenia.38,39 However, initial failures to substantiate measurements of sustained attention, cognitive flexibility, encoding, impulsivity,40 memory,40,41 visual information processing,42 and executive functions41 as potential ADHD endophenotypes may have contributed to a relative lag in the study of this disorder.43 Fortunately, recent research is more promising, with stronger evidence from means comparisons analysis for specific cognitive measurements such as reaction time (RT) data, response inhibition (typically indexed by commission errors), and sustained attention (usually indexed by omission errors44–48), motor measurements,49 and activity level measurements.50 However, means comparison analyses have failed to consistently validate some measurements, such as delay aversion and motivation.51 Although several promising endophenotype candidates have not produced convincing results in molecular genetic studies, several significant associations have been reported: Actigraph data (a mechanical assessment of activity level) is associated with the 7-repeat allele of the DRD4 receptor gene52; measurements of response inhibition with the 10/10 repeat genotype on DAT153; RT data and the absence of the ADHD-risk allele on the DRD4 gene54–56; and homozygosity for the 10-repeat allele
