Background. Prevalence of childhood anxiety disorders at specific ages and genetic etiological influences on anxiety disorders in young children have been little studied. The present study reports prevalence estimates in a community sample of 6-year-old twins, and patterns of genetic and environmental influences on these early-onset anxiety disorders.

Method. Using a two-phase design 4662 twin-pairs were sampled and 854 pairs were assessed in the second phase by maternal-informant diagnostic interview using DSM-IV criteria.

Results. The most common conditions were separation anxiety disorder (SAD) [2·8%, 95% confidence interval (CI) 2·1–3·8, for current disorder] and specific phobia (10·8%, 95% CI 8·4–13·6, for current disorder). Behavioral genetic modeling was feasible for these two conditions, applied to two phenotypes: symptom syndrome (regardless of impairment) and the narrower one of diagnostic status (symptom syndrome with associated impairment). The heritability estimate for SAD diagnostic status was high, 73%, with remaining variance attributed to non-shared environment. The heritability estimates for specific phobia were also high, 80% for the symptom syndrome and 60% for diagnostic status, with remaining variance attributed in both cases to non-shared environment.

Conclusions. Compared with previous epidemiological surveys of children and adolescents in wide age-bands, the current estimates suggest that rates of anxiety disorders assessed in young childhood are generally at least as high and perhaps higher compared with those found in older children. The heritability estimates suggest that the genetic effects on these early-onset anxiety disorders are substantial and more significant than environmental effects, whether shared or non-shared.