Etiology of Autism: Genetic Influences
Infantile autism was first described by Kanner in 1943.1 Based on the observation that symptoms often began shortly after birth and always by two or three years of age, Kanner believed that autism was caused by an unknown, inborn defect. Because this syndrome was so severe and peculiar, it seemed unlikely to be an understandable outcome of a child's life experiences. In the years since Kanner first described this disorder, research findings have supported his initial interpretations. It is now generally accepted that autism has a biologic cause, and considerable research has been carried out with the aim of uncovering its nature. Both hereditary factors and the prenatal and perinatal environment have been considered. This paper will review the role of genetic factors in the cause of autism; the role of the prenatal and perinatal environment is considered in the paper by Nelson.51 Three types of genetic associations have been described: (1) the familial aggregation of autism, per se—autism is more common in the sibs of affected children; (2) the familial aggregation of other disorders in the family members of autistic children—a variety of disorders that are mild, but probably conceptually related, have recently been described in relatives; and (3) autism appears in association with a few particular disorders of known genetic etiology. FAMILIAL AGGREGATION OF AUTISM Population and Family Studies In most case series and a few population-based studies, the prevalence of autism in the siblings of autistic children has been estimated to be about 2% to 3%.2,3 While this number is small, it is 50 to 100 times greater than the expected rate of autism of 4 to 5 per 10 000 in the population.4,5