Difficult to Secure Accurate Data.—It is obvious, of course, that in tabulations such as these there may lurk considerable margins of error. Notwithstanding our Binet-Simon and other tests for feeble-mindedness, for example, there is yet much to be desired in the way of accuracy. Many cases just bordering normality are by no means easy to decide. Then again in most human records, when one gets back beyond the third or, at most, the fourth generation, the investigator has to depend on the hearsay evidence of relatives, friends or neighbors, and how vague this generally is can only be appreciated by those who have themselves tried to collect such data. But in spite of all the difficulties, there is little doubt that the more carefully prepared records are sufficiently accurate to establish the fact beyond dispute that defective tends in large measure to breed defective.
Fig. 34
Inheritance of feeble-mindedness (after Goddard); symbols same as in Fig. 33, [p. 236].
One serious drawback in making a study of the inheritability of insanity and other nervous disorders is that so far we have dealt mainly with mass effects rather than specific neuroses. But even when the latter is attempted we are confronted by the fact that there are various intergradations of the recognized types of defect, that because of varying degrees of defect in the same type a standard is hard to establish, and above all that what appears as a specific mental malady in one individual may crop out in his descendants in an entirely different guise. Moreover, not only the predisposition of the individual, but age and precipitative cause enter as factors in determining the ultimate symptoms.
Fig. 35
Inheritance of feeble-mindedness (after Goddard); symbols same as in Fig. 33, [p. 236].
Feeble-Mindedness and Insanity Not the Same.—Authorities make a sharp distinction between insanities on the one hand and feeble-mindedness on the other. According to Goddard, not only is there no close relationship between the two conditions, but in reality they stand at opposite ends of the psychical scale. In general, insanity is a degenerative process, whereas feeble-mindedness is an arrest of development. In the first case the victim loses part of the mentality he once had, in the second he stops short of normal development.
Many Types of Insanity.—The commonest manifestations of insanity are undue depression, apathy, excitement, instability, obsessions, hallucinations and delusions. Some mental disorders are associated with recognizable structural changes in the nervous system, but the structural basis of many is not known.