Tredgold,[346] in discussing the etiology of mental deficiency, divides the causes into factors indicative of, or producing, a variation of the germ plasm and those acting directly upon the offspring. The former include neuropathic inheritance, alcoholism, tuberculosis, syphilis, consanguinity and the age of the parents. Among the latter are abnormal mental and physical conditions of the mother during pregnancy, or injury to the fœtus; abnormalities of labor, primogeniture and premature delivery; and after birth—traumatic, toxic, convulsive and nutritional factors. He found neuropathic inheritance in over eighty per cent of the cases studied. In 64.5 per cent the heredity took the form of mental defects, insanity or epilepsy, and in eighteen per cent paralysis, cerebral hemorrhage, neuroses of various kinds, or psychoses. There was a history of alcoholism in 46.5 per cent of the series investigated. Tuberculosis occurred in the families of thirty-four per cent, syphilis in 2.5 per cent, consanguinity in five per cent, and a marked disparity in the ages of the parents in four per cent. Factors acting directly on the offspring, either before, during or after birth, were found to be present in sixty-five per cent. Goddard[347] in a study of 327 cases found a history of inherited mental deficiency in fifty-four per cent, probable heredity in 11.3 per cent, neuropathic ancestry in twelve per cent, accidents of various kinds in nineteen per cent, and no ascertainable cause of any kind in 2.6 per cent of the total number.
The definition of a feebleminded person, proposed by the Royal College of Physicians of London, and subsequently adopted by the English Royal Commission, reads as follows:—"One who is capable of earning a living under favorable circumstances, but is incapable, from mental defect existing from birth, or from an early age, (a) of competing on equal terms with his normal fellows; or (b) of managing himself and his affairs with ordinary prudence." The English Mental Deficiency Act of 1913 included the following definition:—"Persons in whose case there exists from birth or from an early age mental defectiveness not amounting to imbecility, yet so pronounced that they require care, supervision, and control for their own protection or for the protection of others, or, in the case of children, that they, by reason of such defectiveness, appear to be permanently incapable of receiving proper benefit from the instruction in ordinary schools." It will be noted that imbeciles and idiots do not come within the scope of these definitions. This is due to the fact that the term feeblemindedness as used in England includes only the High Grade Amentia of Tredgold or the Morons as defined by Goddard. The classification of the latter is as follows:
1. High Grade Morons—Those that can do fairly complicated work, with only occasional or no supervision, run simple machinery or take care of animals, but are unable to plan.
2. Middle Grade—Those capable of doing routine institution work only.
3. Low Grade—Those who are only capable of running errands, doing light work, making beds, scrubbing or caring for rooms—if there is no great complexity of furniture.
Tredgold describes imbecility as Medium Grade Amentia and idiocy as Low Grade Amentia.
The Mental Deficiency Act of England defines idiots as "persons so deeply defective in mind from birth, or from an early age, as to be unable to guard themselves against common physical dangers." It also refers to moral imbeciles as "persons who from an early age display some permanent mental defect coupled with strong vicious or criminal propensities on which punishment has had little or no deterrent effect." The imbecile as defined by the Royal Commission of England is "one who by reason of mental defect existing from birth or from an early age is incapable of earning his own living, but is capable of guarding himself against common physical dangers."
Tredgold classifies either feeblemindedness, imbecility or idiocy if due to pathological germinal variations (caused by alcoholism, tuberculosis, syphilis, etc., and manifested by amentia, insanity, epilepsy, etc.) as being either simple, microcephalic, or Mongolian. He describes those which represent somatic modifications due to gross cerebral lesions as syphilitic, amaurotic, hydrocephalic, porencephalic, sclerotic, paralytic and other toxic, inflammatory or vascular forms. The somatic modifications due to defective cerebral nutrition he divides into epilepsy, cretinism, nutritional forms and isolation (sense deprivation).
The classification of mental defects used by Fernald at the Massachusetts School for the Feebleminded and based on mental ages is as follows:—Idiot,—low grade, less than one year; middle grade, one year or more; high grade, two years. Imbecile,—low grade, three and four years; middle grade, five years; high grade, six and seven years. Moron,—low grade, eight and nine years; middle grade, ten years; high grade, eleven and twelve years. Fernald calls attention to the fact that the diagnosis cannot be based on the mental age alone. The intelligence quotient must be taken into consideration. This is determined by dividing the mental by the physical age. It is a comparison of the average intelligence of the child, using the normal as a standard. The diagnosis cannot be definitely made until the age of sixteen, or until the probable mental age at sixteen is determined.
The following definitions are used by the American Association for the Study of the Feebleminded:—"An idiot is a mentally defective person having a mental age of not more than 35 months, or, if a child, an intelligence quotient of less than 25. An imbecile is a mentally defective person having a mental age between 36 months and 83 months inclusive, or, if a child, an intelligence quotient between 25 and 49. A moron is a mentally defective person having a mental age between 84 months and 144 months inclusive, or, if a child, an intelligence quotient between 50 and 74."