ABSTRACT
Psychological explanations can be subdivided into those which regard delinquency as a symptom of mental abnormality and those which treat it as the normal result of the sort of upbringing that is typical of delinquents' families. To the extent that intelligence can be measured by tests, it is distributed in this way, and there is other evidence that most of the variation between the intelligence of individuals is attributable to polygenic transmission. A type of subnormality which is inherited by means of a single recessive gene is phenylketonuria; but unlike most inherited types it can be corrected – if it is detected very early in life – by a special diet. 'Mental illness' is a generic name for a large variety of abnormalities. They can all be described very generally as abnormalities in the sufferer's perception of his environment or his reaction to it, or in both.
