A Comparative Study of the Causation of Mongolism, Peristatic Amentia, and Other Types of Mental Defect

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Abstract
Mental deficiency has been defined by Tredgold as a state of arrested or incomplete development of the mind. Numerous scientists have for long endeavoured to ascertain the cause or causes of this frequent abnormality. The protagonists of one school of thought made exaggerated claims that the great majority, if not all cases, were due to hereditary factors. Other investigators were able to demonstrate that certain special types of mental defectives owed their deficiency to agents acting after fertilization, and so came to the conclusion that genetic elements were only concomitant and not primary causative factors in many, if not in all types of defectives, and that research on these lines should eventually establish extrinsic causation in all these cases.Thus one has gradually come to distinguish between two main groups of mentally abnormal individuals; those in whom endogenous or hereditary factors could be proved or at least suspected. (Tredgold speaks of primary aments, Strauss of endogenous types, and those in whom exogenous noxious influences could be demonstrated or suspected, and whom Tredgold termed secondary aments, Strauss calls exogenous types.) According to Tredgold and many other investigators, from between 70 to 80 per cent. of all defectives are to be found in the first group, while roughly 20 to 30 per cent. belong to the second. Penrose's figures in support of a hereditary causation are even higher.

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