Autism is a pervasive developmental disorder (PDD) characterized by communication and language impairments, social deficits, and stereotyped or repetitive behaviors. These developmental abnormalities are apparent by early childhood, or 36 months of age. Autism is a complex psychiatric disorder with oligogenic inheritance (1). Twin studies and family studies show substantial evidence for genetic predisposition in the majority of idiopathic cases (2–9). The population prevalence of autism has been estimated at approximately 0.5–2 per 1000, but the rate among siblings of affected probands is estimated from multiple studies at 1–3%, which is profoundly higher (approximately 10–15 times greater) than the general population prevalence (6–9). The concordance rate for dizygotic (DZ) twins is similar to the rate for nontwin siblings, whereas the concordance rate for monozygotic (MZ) twins is approximately 60–90% (2). This suggests that the heritability for autism is greater than 90%, if you assume a multifactorial threshold model (10), which exceeds that of other common psychiatric disorders such as bipolar disorder, alcoholism, and schizophrenia. Other twin studies show that this predisposition extends to a broader autism phenotype of milder, related developmental disorders (2,3). This spectrum of nonpathological abnormalities in behavior is present in parents and siblings of autistic individuals (11). Most MZ co-twins who were nonautistic exhibited milder abnormalities similar to features of autism. Concordance for the broader phenotype of autism for DZ twins is considerably lower than the concordance for MZ twins: 10% and 92%, respectively (2). Concordance and relative risk continue to drop off dramatically for relatives outside the immediate family, such as cousins, although the concordance rates for relatives are still greater than the population prevalence (6,11).

Original languageEnglish
Title of host publicationAutism Spectrum Disorders
PublisherCRC Press
Number of pages20
ISBN (Electronic)9780824755164
ISBN (Print)9780824707156
StatePublished - 1 Jan 2003


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