522 INHERITANCE OF MENTAL TRAITS 



ried a woman of good heredity and founded another 

 family of whom 496 individuals have been traced. Prac- 

 tically every member of this branch was normal, nearly 

 all of them being prosperous farmers or successful lawyers, 

 judges, or holders of important public office. Not even 

 a well-controlled experiment could have shown more con- 

 vincingly the important part played by heredity in the 

 causation of mental deficiency. 



Inheritance of Other Forms of Mental Defect. — 

 Both epilepsy and insanity are also known to be heredi- 

 tary, though the laws governing their transmission are 

 not fully understood. It seems to be fairly well-estab- 

 lished that epilepsy is due to a single recessive IMendelian 

 factor. There are many forms of insanity and it is pos- 

 sible that not all of these are transmitted according to the 

 same laws. It has been found that epilepsy, insanity, 

 and feeble-mindedness frequently appear in the same 

 families, often one kind of defect appearing when another 

 would be expected, and it is possible that these various 

 types of defect may be related to each other in some as 

 yet unknown manner. The predisposition to alcoholism 

 is also to some extent hereditary and often appears in 

 families marked by other types of defect. Criminality, 

 as such, is probably not hereditary, but a considerable 

 proportion of criminals are feeble-minded, and feeble- 

 mindedness, as we have seen, is hereditary. 



Figures 150-1-2 are typical family histories of epilepsy 

 and insanity. 



Endowment of the Different Social and Occupa- 

 tional Classes. — What has been said in the preceding 

 paragraphs would suggest that, in general, members of 

 the socially successful classes and higher occupational 

 groups are on the average better endowed mentally than 

 those of inferior social status. That such is the case is 

 indicated by the intelligence of their offspring. Mental ^ 

 tests have brought out the fact that the children of suc- 

 cessful lawyers, doctors, ministers, or college professors 



