I.Q. Scores and the Problem of Classification

Since the definition of subnormality in the Mental Health Act (1959) specifically adds to its other descriptive statements, “… including subnormality of intelligence,” as part of the criteria for delineating this type of patient, it is clearly of great importance to come to some decision about the exact meaning of this phrase. One can either insist on a purely intellectual interpretation where the major emphasis is placed on the individual's estimated problem-solving ability relative to the community at large (Castell and Mittler, 1965); or one can broaden the concept to include much more than sheer intellectual ability (Shapiro, 1965) and argue that it also involves a person's social behaviour and his empirically demonstrated failure to adapt to the social and economic environment. If the purely psychometric view is taken, then reliance is placed wholly upon the use of standardized tests which are capable of arranging the population in order of intelligence, and which supply statistically defined cut-off points, according to the percentage of the whole population likely to fall beneath such limits. Conversely, if the clinical viewpoint is adopted, then intelligence levels assume a relatively minor role, and more emphasis is placed on social maturity, and behavioural control. In the absence of properly standardized, and widely accepted maturity scales, the clinical diagnosis is based mainly on subjective impression, supported by case history factors, and the knowledgeable experience of the examining medical expert.