Intellectual disability is "a condition, not a number": Ethics of IQ cut-offs in psychiatry, human services and law

Diagnosis in psychiatry and related fields is complicated by the fact that symptoms overlap across categories, comorbidity of diagnoses is commonplace, and information about a specific physical etiology (critical for diagnosis in medicine) is typically lacking. The field of intellectual disabilities/intellectual developmental disorder (IDD) differs from other psychiatric categories in the historically heavy reliance on IQ cut-offs to create the expectation of a clear demarcation between people who have IDD and those who do not. There is increasing dissatisfaction with this practice, however, as reflected in the move by DSM-5 away from a "disability" and towards a "disorder" emphasis. The May 2014 US Supreme Court majority decision in Hall v Florida which outlawed the use in death penalty cases of a "bright line" (IQ of 70) arbitrary IQ ceiling that ignores the standard error of five points exemplifies this evolving perspective, as reflected in the opinion delivered by Justice Kennedy that "intellectual disability is a condition, not a number". In this paper, we trace the evolving history of our understanding of intelligence (and the increasingly outmoded nature of the entrenched concept of full-scale IQ) and describe the various efforts in three fields-human services, psychiatry, and law-that have been made to go beyond reliance on IQ ceilings and to reestablish a more scientifically accurate as well as clinically appropriate approach to IDD. Three ethical principles-beneficence/non-maleficence, consistency and rationality-are used to indicate why continued reliance on IQ ceilings in diagnosing IDD is a morally questionable practice.