Monozygotic twins discordant for attention-deficit/hyperactivity disorder: ascertainment and clinical characteristics.

ABSTRACT Objective Nongenetic factors and phenomenology of attention-deficit/hyperactivity disorder (ADHD) were examined in monozygotic (MZ) twin pairs discordant for ADHD. Method Recruitment included telephone screening ( n = 297 pairs), behavioral ratings obtained from parents and teachers ( n = 59 pairs), and, finally, in-person assessment ( n = 25 pairs; structured classroom observation, diagnostic interview, psychoeducational evaluation, birth record review, establishment of monozygosity, and anatomic brain imaging). Affected twins were further contrasted with previously studied affected singletons. Results Of the 25 MZ twin pairs qualifying for in-person evaluation, only 10 proved discordant for ADHD. Affected twins were mostly comparable with affected singletons on clinical measures, although fathers’ self-ratings of childhood ADHD status were significantly lower in twins than in singletons. Conclusions Discordance for ADHD in MZ twins appears to be ascribable to greater environmental discordance and decreased familiality. Despite these differences, affected twins were phenotypically comparable with affected singletons. Thus MZ twins discordant for ADHD, while rare, can inform research on the etiology and pathophysiology of this disorder.