ABSTRACT
Developmental disabilities include a broad spectrum of physical, cognitive, psychological, sensory, and speech and language impairments resulting from neurological damage or dysfunction, and are typically identified between birth and age 18 years. Mental retardation (MR) is a group of heterogeneous disorders with varying underlying etiologies. Epidemiologic studies of MR have generally used an intelligence quotient of 70 to define MR. This is because standardized intelligence testing represents a systematic and objective way to assess cognitive functioning across populations. Epidemiology has been useful in determining the prevalence of individual mutations and understanding the range of phenotypic expression in specific genetic mutations. Previous studies have examined recurrence risks for MR by severity, sex, and the presence of comorbidities. Many studies have shown that families with individuals with mild MR have a higher risk for recurrence than those with severe MR and that the presence of comorbidities decreases the chance for recurrence.
