INHERITANCE OF MENTAL DEFECTS AND DISEASE 57 



forms of insanity was found to be higher (90 per cent) among 

 brothers than among sisters (70 per cent) or between brother 

 and sister (68 per cent) . Where insanity occurred in twins it was 

 of the same type whether the twins were of the same sex or not. 

 {Zeit.f. Psychiat. 66, 514-541, 1909). Similar findings have been 

 recorded by H. Krueger {Zeit. f. d. gesamte Neurol, u. Psychiat. 

 24, 113, 1914). 



Is insanity transmitted as a typically recessive trait? In 

 Huntington's chorea it is generally conceded that we have a 

 character that usually behaves as a typical dominant. But most 

 of the writers who have considered insanity from the Mendelian 

 standpoint conclude, often in a guarded and tentative manner, 

 that most forms are recessive. One fact that on the face of it 

 indicates that such is the case is that insanity and other neuroses 

 frequently arise in families in which the parents are normal or 

 slightly neuropathic, and that the frequency of such cases is 

 increased when the presence of insane or neuropathic relatives 

 points to the heterozygous constitution of the parents. When, 

 however, we are dealing with a character so protean as the 

 "neuropathic constitution" is commonly assumed to be, this 

 evidence becomes somewhat less convincing. 



The neuropathic constitution may take a relatively mild form 

 in the parents in which it escapes being recognized, while in the 

 offspring it may take the form of insanity. A trait essentially 

 dominant Vv'ill, if highly variable in its manifestations and es- 

 pecially if the degree of its manifestation is largely dependent 

 upon environmental factors, closely simulate a recessive trait in 

 its mode of occurrence. 



To speak of insanity as a defect and as, therefore, due to the 

 loss of one or more determiners in the germ plasm is misleading. 

 Properly, in our view, it is neither the one nor the other. It 

 is more probable that the hereditary basis of insanity is something 

 positive, a definite pathological factor or factors working havoc 

 with the normal development of the organism, and which may be 

 kept from exercising to the full its deteriorating effects by an 

 admixture of healthy germ plasm. How far insanity is the prod- 



