Progress in Understanding the Causes of Autism Spectrum Disorders and Autistic Traits: Twin Studies from 1977 to the Present Day

This chapter provides a comprehensive review of twin studies in the autism fi eld. While family studies have also made a substantial contribution to our understanding of autism, these have not been reviewed here for the practical reason of space and because several informative reviews of family studies of autism are available (e.g., Bailey, Palferman, Heavey, & Le Couteur, 1998 ; Sucksmith, Roth, & Hoekstra, 2011 ). It is also not within the scope of this chapter to include a systematic account of molecular genetic fi ndings in ASD; the reader is directed to the following review papers (Abrahams & Geschwind, 2008 ; Betancur, 2011 ; Freitag, Staal, Klauck, Duketis, & Waltes, 2010 ; Geschwind, 2011 ). In this chapter, we describe how the well-documented original twin studies of narrowly defi ned autism have been succeeded by twin studies of autism spectrum disorders (ASDs) and by a new wave of twin studies exploring the etiology of dimensional assessments of autistic traits in the general population. We discuss how this literature contributes to our understanding of the dimensional nature of autistic behaviors. Furthermore, we consider how twin research has added to our understanding of the overlap between autism and intellectual disability, language development, and psychiatric conditions, and how it has provided evidence for etiological heterogeneity in autistic symptoms. Finally, after considering some limitations and assumptions inherent in these twin studies, we provide suggestions for future research directions. Chapter 2 Progress in Understanding the Causes of Autism Spectrum Disorders and Autistic Traits: Twin Studies from 1977 to the Present Day