Archives of General Psychiatry November, 2011; Vol. 68; No. 11; pp. 1095-1102 Joachim Hallmayer, MD; Sue Cleveland, BS; Andrea Torres, MA; Jennifer Phillips, PhD; Brianne Cohen, BA; and 11 more authors FROM ABSTRACT Autism is considered the most heritable of neurodevelopmental disorders. Therefore these authors wanted to provide rigorous quantitative estimates of genetic heritability of autism v. the effects of the environment. These authors assessed 192 twin pairs pertaining to Autism Spectrum Disorder (ASD). The authors concluded that the susceptibility to ASD has moderate genetic heritability (37%) and a substantial shared twin environmental component (55%). KEY POINTS FROM THESE AUTHORS: 1) This study is the largest population-based twin study of autism that used contemporary standards for the diagnosis of autism. 2) Autism is a complex neurodevelopmental disorder that interferes with the normal course of social, communicative, and cognitive development. 3) Over the last 30 years there has been a substantial (10-fold) increase in the prevalence of autism. 4) The prevalence for autism spectrum disorders (ASDs) is about 1% [1 in 100 children]. 5) This study found a genetic heritability of 37% and an environmental component of 55% for ASD. 6) The results suggest that environmental factors common to twins explain about 55% of the liability to autism. 7) Although genetic factors also play an important role, they are of substantially lower magnitude than estimates from prior twin studies of autism. 8) This study provides evidence that environmental factors in autism have been seriously underestimated in previous studies and the influence of genetic factors on the susceptibility to develop autism, overestimated. 2 9) Because of the reported high heritability of autism, a major focus of research in autism has been on finding the underlying genetic causes, with less emphasis on potential environmental triggers or causes. 10) These authors hypothesize that at least the environmental factors impacting susceptibility to autism exert their effect during the critical period between prenatal and early postnatal (first year) of life.