Differential diagnosis of mental subnormality and abnormality: the contribution of psychometrics.

In a psychiatric population, cognitive abnormality impacts on intellectual functioning and is often misconstrued as subnormality, i.e., mental retardation. Standard IQ and neuropsychological tests contribute little to the differential diagnosis, which hinges on the question of developmental failure. A developmentally rooted psychometric battery, one that assesses conceptual, perceptual-motor, and social maturity, is proposed as an objective diagnostic method. Research confirms the validity of this approach for distinguishing between psychosis with severe functional vs. developmental impairment, even when matched for IQ. Diagnostic and treatment implications are discussed.