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MENTAL RETARDATION, now referred to widely as intellectual disability, is currently defined as having an IQ below 70 accompanied by marked deficits in adaptive functioning. Both terms have been broadly used to label a variety of cognitive impairments linked to prenatal or early postnatal brain abnormalities. For decades, subsets of individuals with rare intellectual disability syndromes, such as Rett syndrome or fragile X syndrome, have been characterized by their genetic etiologies. We are now beginning to elucidate the complex genetics of more prevalent neurodevelopmental disorders without distinct physical features that distinguish them, including so-called idiopathic or nonsyndromic forms of autism spectrum disorder (ASD). The combination of insights resulting from the intensive study of rare genetic syndromes coupled with successes in unraveling the genetics underlying idiopathic ASD has transformed our understanding of normal and pathological development of the human brain.

Common to all of these disorders are mental impairments that persist throughout life, hampering development and learning. Generally speaking, even if all mental functions seem to be affected, conditions with distinct etiologies and natural histories can be differentiated because some cognitive domains tend to be more impaired than others. And indeed, these differences are reified in diagnostic schemes that draw distinctions between developmental abnormalities that affect primarily general cognition, social cognition, or perception. These differential cognitive and behavioral vulnerabilities may provide useful clues about the origin and developmental time course of specific mental functions in normal development.

In this chapter, we focus principally on neurodevelopmental disorders that include abnormalities in social functioning, including ASD, fragile X syndrome, Williams syndrome, Rett syndrome, and Angelman and Prader-Willi syndromes. These conditions all impair highly sophisticated brain functions including social awareness and communication. ASD is a prime focus for several reasons: the high prevalence in the population; the overlap in genetic risks with other common neuropsychiatric conditions, including schizophrenia; and the absence of a defining neuropathology. They are also exemplars of the etiological and phenotypic heterogeneity common to many psychiatric syndromes. In this respect, ASD is a paradigmatic neuropsychiatric syndrome.

Autism Spectrum Disorder Phenotypes Share Characteristic Behavioral Features

Profound social disability has probably always been with us, but the characterization of autism as a medical syndrome was first described in the literature in 1943 by Leo Kanner and in 1944 by Hans Asperger. Today, clinicians and researchers think of autism as a spectrum of disorders with two defining but highly variable diagnostic features: impaired social communication and stereotyped behaviors with highly restricted interests.

Until recently, the term “Asperger syndrome” was used to describe individuals who met these two diagnostic criteria, but in whom language acquisition was not delayed and IQ was in the normal range. In the most recent edition of the standard psychiatric diagnostic manual, Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), Asperger syndrome along with a distinct disorder known as pervasive developmental ...

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