MAN- 241 



due to a single principal unit factor, recessive to what we 

 may call normal mentality. 



There is evidence, however, of other minor factors 

 which modify the grade of feeble-mindedness, and con- 

 siderable reason for feeling that in some similar way cer- 

 tain forms of insanity, epilepsy and other neuroses are in 

 some way related. At any rate, all of these abnormalities 

 are in many cases inherited as recessive traits. 



Here, then, is something wholly undesirable which 

 may be the result whenever the proper unions occur; and 

 as we have seen how inbreeding tends to bring out reces- 

 sive characters, in feeeble-mindedness lies a potential 

 danger. Let us see what this danger is as regards the 

 United States. 



It appears that in our present population of over 100,- 

 000,000 there are something like 300,000 persons who are 

 feeble-minded, epileptic or insane through an hereditary 

 defect, a ratio of 3 per 1000. How many of these defec- 

 tives resulted from a mating wherein at least one of the 

 parents was of the same type is a difficult question and can 

 only be answered with a rough approximation. The sta- 

 tistics at present available are meagre, but from their 

 examination 200,000 may be considered to be above the 

 mark. This leaves 100,000 defectives, then, which have 

 been produced in a single generation by the mating of two 

 transmitters of defective mentality who did not exhibit 

 such defects in themselves.^® 



These 100,000 defectives were produced during a 

 period when there were rather less than 20,000,000 mar- 

 ried couples of reproductive age, by parents who were 

 both heterozygous. But since only one-fourth of the 

 progeny of such matings will be defective, at least 100,000 

 couples of this type were reproducing throughout this 



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