Some Aetiological Problems in Mental Deficiency

Though many classifications of the subject exist, mental deficiency may conveniently be considered in two groups ?minor (high-grade) and severe (low-grade) mental defect. This simple division corresponds, though not exactly, with that proposed by Lewis (1933) and restated by Bourne (1957). Lewis regarded some cases of mental defect as pathological and some as subcultural. The subcultural cases coincide mainly with those of minor degrees of intellectual defect. This thesis, as presented by Penrose (1954a), assumes that some defectives are such because of pathological lesions, and others because of biological variation. In fact, if the biological variation is of such a degree as to produce seriously impaired intelligence, which does not permit the individual to cope with his environment, then the condition must be regarded as pathological. Similarly, it seems reasonable to suppose that many of the pathological lesions responsible for cases of mental defect are one of the means whereby biological variation operates. There is no agreed definition of what constitutes high-grade, or minor, mental defect. Definitions based on intelligence quotient are of little value, as the Scottish Council for Research in Education (1949) showed that 3% of the population score less than an ?.Q. of 70 on a standard test, equivalent to one and a half millions in a population of fifty millions. Definitions based on legal recognition or ascertainment as mental defectives are of even less value, as O'Connor and Tizard (1956) have shown that the average I.Q. of the feeble-minded admitted to mental deficiency institutions is in the region of 70. It seems clear that psychological, educational, and social factors operate in a much more direct manner in producing the problem of minor mental defect. None the less, it should not be assumed that gross pathological lesions are encountered only in idiots and imbeciles and that they are not responsible for numerous cases of minor mental defect. In fact, all the diseases and factors operating in the production of gross mental deficiency may operate in a diluted form to produce minor degrees of mental defect. We will quote some examples of this process before going on to the main theme.