To Örebro University

Increasing evidence indicates a persistence of autism and autistic traits into adulthood, necessitating a lifespan approach to autism. Little is known about why autism persists into adulthood, and how genetic and environmental factors contribute over time. Genetically informative research has mostly focused on childhood and longitudinal designs following autistic individuals into adulthood are scarce. We aimed to determine the heritability of clinically diagnosed autism in adulthood and the etiological stability of autistic traits from childhood to adulthood. From 23,849 twin pairs in the Swedish Twin Register born between 1959 and 2010, we identified 485 individuals (1.01%, 31.5% female) diagnosed with autism. Using a liability threshold model, we estimated the relative contribution of genetic, shared environmental and non-shared environmental influences to autism liability in childhood (birth years 1996–2010) and adulthood (birth years 1959–1995). The heritability of autism was 96% (95% confidence interval, CI, 80–98%) in childhood and 89% (95% CI,67–94%) in adulthood. Non-shared environmental factors explained 4% (95% CI, 2–9%) of the variance in childhood and 11% (95% CI, 6–19%) in adulthood while shared environmental factors did not contribute. These results suggest that genetic factors play an important role for autism across the lifespan and that the shared environment is of minor importance. Additional analyses using data of 2925 twins from the longitudinal Twin Study of Child and Adolescent Development using Cholesky decomposition found stable as well as newly emerging genetic influences on autistic traits from age 8/9 to 19/20, as well as unique non-shared environmental influences at each age.