The term Autism Spectrum Disorder (ASD) has come into use to reflect findings that autistic features are manifest in degrees and in various combinations, rather than simply being present versus absent. Genetic research has revealed that ASD—like many other complex human traits—is affected by numerous genetic factors, each of which may have very small effects.
To obtain more precise estimates of genetic and environmental contributions to autistic traits, a Dutch team analyzed scores on the CBCL/1½-5 DSM-oriented Pervasive Developmental Problems scale (now called the Autism Spectrum Problems scale) obtained from mother and father ratings of 38,798 3-year-old twins (de Zeeuw et al., 2017). The data were obtained from the Netherlands Twin Register, which includes data for about 40% of all multiple births in the Netherlands from 1989 through 2010. It was found that 78% of variation in autistic traits for boys and 83% for girls was accounted for by genetic effects. Nevertheless, 29% of the monozygotic (identical) twin pairs differed as to whether they were in the clinical versus normal range for CBCL scores. According to de Zeeuw et al., this indicated that—among identical twins at risk for ASD—“Either adverse environmental risk factors might have triggered ASD or protective environmental risk factors might have prevented a child from developing ASD” (p. 899).
Reference: de Zeeuw, E.L. et al. (2017). The etiology of autistic traits in preschoolers: a population-based twin study. Journal of Child Psychology and Psychiatry, 58, 893-901.