Epidemiology: Who Is Affected?

Intellectual disability is the most common developmental disorder and the most handicapping of the disorders beginning in childhood. It ranks as first among chronic conditions that limit full participation in society. Epidemiologic approaches provide a basis for understanding the distribution and dynamics of health, disease, and disorder for persons with intellectual disability; epidemiology is the foundation of public health practice. Because it relies largely on statistical methods, accurate data and clear definitions are essential. The interpretation of epidemiologic information requires background knowledge of demography, social sciences, environmental science, and the clinical sciences. Although epidemiologic studies are essential in establishing prevalence, and in describing the demography of a disorder, the role of epidemiology is far more extensive than this. Epidemiology can teach us about the nature and scope of intellectual disability and associated general medical, behavioral, emotional, and psychiatric problems. In so doing, epidemiologic approaches may be combined with neurobiologic and psychosocial measures. Moreover, epidemiologic studies can disclose individual developmental trajectories and the influences that shape those trajectories. Some of these influences promote risk; others provide protection and promote resiliency in the individual. Finally, experimental approaches in epidemiology allow the study of causative processes, factors that influence the course of the disorder, and service needs. It is these more extensive uses of epidemiology that are called for in future research. Chapter 3 outlines the classification of intellectual disability. This chapter will discuss the use of definitions of intellectual disability in establishing its prevalence, factors affecting prevalence, variability in rates in the various states, demographic features including the impact of increasing life expectancy, associated physical, behavioral, and emotional impairments, and new research directions. Accurate estimates of the number of intellectually disabled individuals are required for planning purposes and to gain better knowledge of the impact of interventions. Studies of the prevalence and incidence of intellectual disability date back to at least 1811, when Napoleon ordered a census of “cretins” to be made in one of the Swiss cantons (Kanner, 1964). Although little information is available about how this census was used, many surveys have been carried out since that time.