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Deficient response inhibition as a cognitive endophenotype of ADHD.

OBJECTIVE
To investigate whether a deficient response inhibition is a cognitive endophenotype of attention-deficit/hyperactivity disorder (ADHD). The authors hypothesized that nonaffected siblings of ADHD probands would have a response inhibition between that of ADHD probands and normal controls, although they resembled the controls at a behavioral level.


METHOD
Participants were 25 ADHD probands with a family history of ADHD, their nonaffected siblings (n = 25), and 48 normal controls matched for age and IQ. All participants were between 6 and 17 years of age. The nonaffected siblings were compared with their ADHD siblings and with controls on measures reflecting different types of response inhibition.


RESULTS
The nonaffected siblings had results similar to those of the ADHD probands, who differed from the controls on all inhibition measures (p <.05).


CONCLUSIONS
Siblings of ADHD probands, while not behaviorally expressing the disorder, have ADHD-associated deficits in response inhibition. This suggests that subtyping based on measures of response inhibition can help identify genetic susceptibility to ADHD. Children with a genetic vulnerability to ADHD may have hidden cognitive deficits in the absence of manifest behavioral symptoms. Therefore, they should be monitored to detect possible learning problems.