A twin study of the relationships among inattention, hyperactivity/impulsivity and sluggish cognitive tempo problems

We investigated the etiological relationships between the three ADHD dimensions of Inattentive Problems (INP), Hyperactivity-Impulsivity Problems (HIP) and Sluggish Cognitive Tempo (SCT) as measured by the CBCL 6-18 questionnaire. Multivariate models were applied to 398 twin pairs (374 boys and 422 girls) aged 8 to 17 years (M = 13.06, SD = 2.59) belonging to the population-based Italian Twin Registry. The INP, HIP and SCT problem scores were moderately-to-substantially (range 0.29-0.47) intercorrelated. The best fitting model showed that these 3 dimensions are correlated both at the genetic (correlations' range: 0.65-0.83) and the environmental (correlations: 0.29 and 0.44) levels, but they are also distinct. While SCT showed moderate heritability and large non-shared environmental influences, variance for both INP and HIP was substantially explained by genetic influences. We also found evidence of negative sibling interaction for INP, implying that a given behavior in one twin leads to an opposite behavior in the co-twin. Our results support at the etiological level the findings of previous psychometric and longitudinal studies of ADHD, which yielded evidence of the 3 distinct-albeit correlated-problem dimensions of inattentiveness, hyperactivity-impulsivity, and sluggish cognitive tempo.