Case and kindred in mental deficiency.

detecting a recognizable pattern in the distribu tion of disease may be of value in respect of cause and of prevention. The recognition of classical Mendelian ratios, for instance, is both a partial explanation and an opportunity for genetic counselling which may lead relatives to take eugenic measures. In mentally sub normal populations, however, the occurrence of disease in families often shows more com plicated distributions. From knowledge of the relationship between cause and distribution models may be constructed with which empirical data may be compared, for instance, in mater nal-foetal incompatibility in respect of some antigen, or in polygenically determined diseases showing threshold effects in liability to disease. Before discussing pathological conditions I should like to consider briefly those mentally subnormal subjects who are generally believed to be part of the normal population in respect of inteffigence and are at the lower end of the normal distribution curve judging by the results of intelligence tests. They occupy the IQ range of about 50—70,and the majority, although not all, of those in this range have been referred to as normal variants and by other terms. Im portant evidence for the dichotomy between these more numerous and mildly subnormal subjects and the more rare more severely subnormal subjects afflicted with various patho logical conditions was provided by the Colches ter Survey (Penrose, 1938). In The Biology of Mental Defect (ig@g) Penrose wrote: †̃¿ The frequencies in the two groups are charactdristic. Severe cases are comparatively uncommon, and the events which produce them tend to be relatively rare accidents, rare diseases or rare genetical processes. Conversely, mild cases are common, and most causes of disability here are ordinary events, either environmental or genetical.' Some authors have suggested that a person defined as mentally subnormal is, ipso facto, pathological. To define the word in this way makes this dichotomy impossible and I believe introduces much confusion into the problem. It leads to the use of the word in two different senses, one having regard to the cause, the other to the consequence. The term †̃¿ pathology' has traditionally been applied to underlying morbid processes and is so defined in Butterworth's Medical Dictiona7y (7965). Penrose (ig@@) used the word in this sense when he said †̃¿ There is no doubt that a very large proportion of idiots and imbediles are correctly described as “¿ patho logical―in the ordinary sense of being diseased.' If it is assumed that the distribution of ability on intelligence tests should be Gaussian, the empirical findings fit the assumption down to an IQ of about 50 (Table I); but below that level there is a considerable excess which may be explained as due to pathological causes of mental defect, whereas the intelligence of sub normal subjects with IQ's above 50 is deter mined by the same additive factors (whether genetic or environmental) as those which deter mine the intelligence of the general population. On this hypothesis one expects a considerable degree of familial incidence of mental sub..