Do some mental disorders occur less frequently among persons with mental retardation?

Distributions of psychiatric classifications among 42,479 persons with mental retardation were reviewed to determine whether relative rates of disorders for persons with both mental retardation and a mental disorder resembled those identified in other studies. Analyses conducted separately for children, adults, by sex, and by intellectual level indicated that rates of reported schizophrenic and psychotic disorders were generally higher than expected relative to rates of personality disorders, affective disorders, anxiety and phobic disorders, and behavior disorders. Factors that may have affected relative rates of identification were noted, and the need to develop more adequate assessment methodologies and conduct valid studies of population samples was underscored.