INHERITANCE OF MENTAL DEFECTS AND DISEASE 61 



severely criticized, and justly so, by Pearson, Heron, Saleeby, 

 and others. Granting that mental defect is transmitted as a 

 single recessive unit character, the mating of a duplex normal 

 with a defective, while producing normal children, nevertheless 

 makes them carriers of the defect. Should two such carriers mate, 

 one-fourth of their offspring would manifest the defect; should the 

 carriers follow the "eugenic rule" and mate with defectives, half 

 of their offspring would be defective. Matings of normal and 

 defective simply sow the seed for future trouble. Should the 

 estimate of some of the workers of the Eugenics Record Office 

 prove correct, namely, that over 30 per cent of the population is 

 heterozygous for mental defect, the direct danger of such matings 

 is very considerable. Certain defects are distributed widely 

 enough as it is, without our advising marriages that would simply 

 make the situation worse. Nothing could be more inconsistent 

 with everything we know of heredity than the ill-considered 

 advice that strength may mate with weakness. 



And besides we have very little assurance that the normal 

 condition dominates mental defectiveness to the extent that is 

 usually assumed. I have been continually surprised in reading 

 papers on the Mendelian inheritance of mental defect to find how 

 placidly and uncritically the assumption is made that normal 

 mentality behaves as a typical dominant. It does not seem to 

 occur to most of those who have treated the subject that the 

 children of a mental defective are apt to be severely injured by 

 the incompletely suppressed traits of that parent, however free 

 from taint the ancestry of the other parent may have been. And 

 this in spite of the fact that Mendelian literature is full of cases 

 of incomplete and variable dominance! Surely from the facts 

 at our disposal no one is justified in feeling very confident of 

 the complete dominance of mental normality. The injury result- 

 ing from the mating of mental soundness with mental weakness 

 may be very direct, manifesting itself in the production of chil- 

 dren mentally inferior or suffering from various neuropathic 

 taints. It is not at all unlikely that many of them would actually 

 be ranked as mental defectives or be caused by untoward circum- 



