Introduction to special section: mental retardation and mental illness.

Historically, the perceived relation between mental illness and mental retardation has undergone substantial changes. During the past 2 centuries, clinical observations and systematic research addressing these 2 constructs have developed along separate lines. Consequently, the pathogenesis and treatment of psychopathology, emotional disorders, and behavior problems experienced by individuals with mental retardation have not been the purview of mainstream clinical psychology. This article initiates a special section on mental retardation and mental illness to provide up-to-date summaries of various key clinical and research issues regarding this population (i.e., individuals with "dual diagnoses").