Autism spectrum disorder has both genetic and environmental origins. Despite more than 50 years of research, however, conclusions about the exact origin and development of autism spectrum disorder remain controversial. Researchers designed a large multinational population-based cohort study to determine the contribution of genetic, maternal, and shared and nonshared environmental effects on the development of this condition.

The study population comprised all singleton births in Denmark, Finland, Sweden, and Western Australia between 1998 and 2007 and in Israel from 2000 to 2011. Of 2,001,631 children followed from birth to 16 years, 22,156 were diagnosed with autism spectrum disorder. The heritability of autism spectrum disorder was estimated to be approximately 80%, based on the population data from these five countries, indicating that the majority of risk is due to genetic factors. Nonshared environmental factors also consistently contributed to risk, but shared environmental factors had minimal effect. Maternal effects were nonexistent or minimal. There were modest differences in sources of risk between countries. Estimated effects of genetic, maternal, and environmental factors were similar for autism spectrum disorder and autistic disorder.

Among other limitations of this study, the authors point out that the effective sample size for individual countries was limited by the low prevalence of autism spectrum disorder.