Low Grades of Intelligence
The most striking example of individual deviation from the average grade of intelligence is to be found in the cases of those unfortunates who continue throughout life to be deficient because they have underdeveloped nervous systems. As a result of heredity or pathological conditions in early childhood a certain number of persons, conservatively estimated as two in every thousand, are permanently subnormal. These cases vary in degree. The lowest grade defectives, known as idiots, are defined in the Report of the British Royal Commission on the Feeble-minded as persons “so deeply defective in mind from birth or from early age that they are unable to guard themselves against common physical dangers.” The less defective are classed as imbeciles, feeble-minded, and morons, each class representing a further approach toward normality.
The lower grades of defectives are so dependent on the care of others that they do not reach the school at all, but the higher grades either escape detection until they try to learn reading and arithmetic or through the persistence of parents are brought to school in the hope that their defectiveness may be temporary. Some of the highest grades succeed in learning enough so that they pass out of the first grade. They do not master reading, but they learn to repeat the words sufficiently to deceive the teacher with the appearance of having recognized the printed symbols.