Introduction: Reaction time (RT) variability is one of the strongest findings to emerge in cognitive-experimental research with those diagnosed with attention deficit/hyperactivity disorder (ADHD). -- We set out to confirm the association between ADHD & slow & variable RTs & investigate the degree to which RT performance improves under fast event rate & incentives. Using a group familial correlation approach we tested the hypothesis that there are shared familial effects on RT performance & ADHD. Methods: 144 ADHD combined-type probands, 125 siblings of the ADHD probands & 60 Cs, 6-18y, performed a 4-choice RT task with baseline & fast-incentive conditions.. Results: 1/ ADHD was associated with slow & variable RTs, -- & greater improvement in speed & RT variability from baseline to the fast-incentive condition.. 2/ RT performance showed shared familial influences with ADHD: assuming no shared environmental contribution 60-70% of the phenotypic correlation was estimated to be due to shared familial influences. Conclusions: A) The data are inconsistent with models that consider RT variability as reflecting a stable cognitive deficit in ADHD, but instead emphasise the extent to which energetic (see Russell et al. 2006) or motivational factors can have a greater effect on RT performance in ADHD.B) The findings support the role of RT variability as an endophenotype, mediating the link between genes & ADHD.