Our objective is to describe the contribution of rare diseases to our understanding of the epidemiology of neurodevelopmental disabilities (NDDs) by comparing and contrasting the epidemiologic features of NDDs classified according to key characteristics of developmental delay or deviance in such areas as behavior or cognition (the phenotypic approach; autism spectrum disorders and intellectual disability as examples) versus classification based on the identification of an etiologic diagnosis (the etiologic approach; 22q11.2 deletion syndrome and fragile X syndrome as examples). We suggest specific applications in which consideration of rare etiology-based NDDs might further our understanding of NDD epidemiology overall; what is needed to integrate the two classification approaches; and identify practical challenges in achieving that integration. Understanding commonalities and differences in the epidemiologic features of the phenotypically and etiologically defined NDD classifications provides a useful framework for furthering our understanding of the prevalence, distribution, and causes of NDDs, as well as delivering appropriate diagnostic resources, appropriate treatments, accurate prognostic information, and estimates of recurrence risk for these disorders.
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