INHERITANCE OF MENTAL DEFECTS AND DISEASE 57 
forms of insanity was found to be higher (90 per cent) among 
brothers than among sisters (7o 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, 213, 1014). 
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 will, 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- 
