What have we learned from recent twin studies about the etiology of neurodevelopmental disorders?

The relative influence of genes and environment on the liability to neurodevelopmental disorders (NDDs) can be investigated using a twin design. This review highlights the results of the most recent twin studies of NDDs. Recent twin studies have confirmed that NDDs show moderate-to-high heritability, and that from an etiological viewpoint both autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) are best regarded as the extremes on a continuous liability distribution. Both ASD and ADHD show high heritability in childhood and a substantial drop in heritability in adulthood, which is likely explained by the use of different assessment strategies in childhood versus adulthood, or by a complex mechanism of gene-by-environment interaction. NDDs show substantial comorbidity among each other, and with other mental health problems, which is partly because of a shared genetic etiology between different disorders. The findings of twin studies implicate substantial heritability of NDDs, and warrant large-scale molecular genetic studies for such traits.