Familiality and heritability of subtypes of attention deficit hyperactivity disorder in a population sample of adolescent female twins

Objective: Attention deficit hyperactivity disorder (ADHD) is a highly heri table but clinically heterogeneous syndrome. The study examined the familia lity and heritability of ADHD subtypes as defined by DSM-IV and by latent-c lass analysis in a population sample of adolescent female twins. Method: To determine which elements of ADHD cluster together, latent-class analysis was applied to data obtained from parents on the 18 DSM-IV ADHD sy mptoms in 4,036 female twins age 13-23 years in a population sample identif ied from the registry of all births in Missouri for the years 1968-1996. Re lative risk and odds ratios were used to assess within-subtype and between- subtype familiality and heritability of both DSM-IV and latent-class ADHD s ubtypes. Results: Latent-class analysis was most compatible with the existence of th ree mild and three severe classes of ADHD symptoms in the general populatio n. The three severe classes showed moderate overlap with DSM-IV ADHD subtyp es. The primarily inattentive and combined subtypes of DSM-IV ADHD co-clust ered within families. The primarily hyperactive/impulsive DSM-IV subtype an d the individual latent-class analysis subtypes did not co-cluster. Subtype s defined by both approaches were highly heritable. Conclusions: Unlike DSM-IV subtypes of ADHD, latent-class ADHD subtypes app ear to be independently transmitted in families. These classes may be more appropriate targets for molecular genetic studies of ADHD.