MENTAL DEFICIENCY AND MENTAL DISEASE
The detailed discussion of mental disease and mental deficiency belongs to the extensive topic of abnormal psychology and to psychiatry, not to an introductory text of general psychology. It is important, however, that the student of general psychology should have some conception of the nature of the diseases and deficiencies which are most commonly described, since he necessarily will meet with references to the names of these conditions, even if he does not study abnormal psychology.
The classifications and descriptions of mental diseases are largely the work of psychiatrists, who officially deal with the care of mentally diseased patients. Unfortunately, there are many systems of psychiatry, differing much in their conceptions of the natures and origins of mental diseases, and in their classifications and descriptions. Hence we cannot present here a sketch which will represent a concensus of opinion of psychiatrists, but can merely present certain matters concerning which the differences of opinion are relatively small. We shall approach the subject rather from the point of view of abnormal psychology, which is primarily interested in the analysis of the abnormal mental processes, and in their comparison with normal processes.
We may distinguish between normal and abnormal mental processes in either of two ways. First, we may consider the whole range of variations in some type of response, and measure the range of variations. We may, for example, measure the simple auditory reaction times of many men, and when we have obtained the average time for each man, we may find that these averages range from 90 sigma to 250 sigma. In such a group, we would find relatively few individuals with averages lying between 90 and 100, and relatively few averages between 220 and 250. We would find a somewhat larger number of averages between 100 and 110, a still larger number between 110 and 120, and further increases