I 
HEREDITY IN THE CASE OF DEMENTIA PRÆCOX AND RELATED DISEASES 121 
many different forms and degrees. This is for example the case in the 
Swedish Lister family, made famous by LunpBora’s (1913) excellent 
investigations, where along with 195 cases of such psychical taints only 
13 cases of myoklonus-epilepsy occur, 6(8?) cases of other epilepsy, 
7(9?) cases of paralysis agitans and some few cases of various other 
serious nervous diseases. Since moreover a great part of the numerous 
cases of moral and social defects in the family may be referred without 
doubt to the large group of psychical taints, it is clear that this is what 
characterises the family, and that, for example, the accessory special 
factor, which conditions the myoklonus-epilepsy, will scarcely show it- 
self unless it occurs along with the supposed common factor. In any 
‘case, so far as known, there are no other families in which the disease 
appears to be inherited without some other central or peripheral nervous 
diseases also occurring with it. 
Comparing in this way what we know of the manner in which 
Dementia præcox and the diseases related to it are inherited, we come 
_to the view, that the disease depends on at least two cooperating 
factors, one of which is a dominant common factor, another a recessive 
special factor. On an average, therefore, the disease should occur in 
®/isths of the children in the different families; but since it actually 
occurs in a considerably smaller proportion, it must depend either on 
more than these two factors, or on causes of another kind which are 
operative alongside the biogenetic; further than this we cannot go for 
the moment. That such endogenous or exogenous factors are of con- 
siderable importance, there cannot be any doubt, and by continued ra- 
tional study of the causal conditions of the single cases we shall also 
presumably be able to determine what weight should be ascribed to 
them. Even now it may be said, that the study of the inherited con- 
ditions not only throws a new light on the mode of origin of Dementia 
precox, but also on its relation to the other, very serious and 
widespread group of mental diseases known under the name of manio- 
depressive psychosis. This disease also occurs in many different forms, 
which differ quantitatively in greater degree than Dementia præcox and 
are thus much more difficult to deal with, especially as the lighter and 
the simplest degrees are only exceptionally recognised as true diseases. 
There is no doubt that there is a genetic connection between these dis- 
eases. They do not occur so often, that it might be a chance happening 
when they appear so frequently in the same families, as is the case, 
and this not only in the same families but in near relations. It is not 
unusual, that the one of two in a family suffers from Dementia præcox 
