ABSTRACT
Abstract Autism spectrum disorder (ASD) is a neurodevelopmental condition of multifactorial origin. Aside from clinically relevant ASD phenotypes qualifying for a categorical diagnosis, data increasingly support the notion of broader/extended phenotypes and traits of ASD that are continuously distributed in the general population, with variation in ASD traits determined by factors overlapping with the clinical phenotypes. Earlier twin studies conrmed repeatedly the genetic causes for ASD, with only modest environmental inuences. Thus, ASD is considered one of the most heritable of psychiatric conditions with a
Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.1 Behavior genetics: Why conduct research on twins? . . . . . . . . . 16
2.1.1 Genetic inuence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.1.2 Shared and nonshared variation . . . . . . . . . . . . . . . . . . . 17
2.2 Twin studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2.1 Different twin designs . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2.2 Twin studies in ASD: What have we learned? . . . . . . . . 20 2.2.3 Twin studies of ASD as a continuous trait . . . . . . . . . . . 22
2.2.3.1 The Missouri Twin Study . . . . . . . . . . . . . . . . 22 2.2.3.2 The twin early development study . . . . . . . . . . 22 2.2.3.3 The Child and Adolescent Twin Study
in Sweden. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.2.4 Twin studies of ASD symptoms in toddlers . . . . . . . . . . 26 2.2.5 Discordant MZ twin pair design in ASD research . . . . . 26 2.2.6 Twin studies in ASD and comorbidity . . . . . . . . . . . . . . 28 2.2.7 Etiological overlap between different domains
of symptoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.2.8 Limitations of twin designs within ASD research . . . . . 30
2.3 Summary and directions for future research . . . . . . . . . . . . . . . 31 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
heritability estimates between 80% and 90%. However, recent twin studies have suggested a stronger environmental inuence on ASD etiologies. This chapter gives a short theoretical background on the basics of behavioral genetics and twin designs, summarizes the contribution of twin research to the knowledge of ASD etiologies, and discusses areas of interest for future twin research within the eld of ASD.
