Abstract
Researchers continue to pursue a better understanding of the symptoms, comorbidities, and causes of autism spectrum disorders. In this chapter, we review the twin studies of autism spectrum disorders (ASDs) and autistic traits that have contributed to this endeavor. These twin studies have reported on the heritability of ASDs and autistic traits in different populations and using different measurement and age groups. These studies reveal that the etiology of clinical autism and autistic traits assessed in the general population is more similar than different, which contributes to the question of where the boundary lies between ASD and typical development. These studies have also stimulated debates and new hypotheses regarding what causes ASDs; their comorbidity with intellectual disability, language delay, and psychiatric disorders such as ADHD; and why ASDs show substantial symptom heterogeneity. Lastly, methodological assumptions of the twin design are given consideration in relation to autism research. We conclude with suggesting a range of future research directions for studying ASDs and related phenotypes.