52 



The Journal of Heredity 



fluctuations in mental capacity of one 

 kind or another. 



Our belief in the variations in human 

 intelligence rests not only upon deduc- 

 tions from the general principle of 

 variation, but also upon direct obser- 

 vations on the human mind itself. 

 Yerkes° and his associates organized 

 and conducted a prodigious experiment 

 in mental testing in the United States 

 Army during the World War. A 

 sample, consisting of over 162,000 men, 

 was selected for analysis from the 

 nearly 2,000,000 psychological record 

 cards sent in from the various army 

 camps. The analysis revealed great 

 variation in mental ability. Much of 

 the fluctuation may have been due to 

 differences in educational opportunities ; 

 part of it was probably caused by in- 

 born differences in ability. This is 

 shown by a comparison between 660 

 officers, none of whom had gone be- 

 yond the eighth grade, with 13,943 

 native white drafted soldiers of high 

 school or college education. Though 

 the best educated officer in the group 

 had received less education than the 

 least educated recruit, the average rat- 

 ing on examination alpha for the offi- 

 cers was slightly higher than that for 

 the recruits. Apparently the inferior 

 training of the officers was offset by 

 an average inborn intelligence superior 

 to that of the recruits. 



Goddard" has demonstrated the in- 

 heritance of feeblemindedness. Degrees 

 of feeblemindedness range from idiocy 

 and imljecility through the higher 

 grades (morons) to types which we 

 would characterize as mentally dull. 

 There is no sharp boundary between 

 the feebleminded and the "normal" 

 categorie-s of persons. Therefore, if 

 the lower ranges of the scale of in- 

 telligence are often inherited, why not 

 the higher also? There is no little 

 evidence to support such a conclusion. 

 Galton's classic studies revealed that 

 British men of genius were almost 

 without exception descended from 

 families in which great ' intellectual 

 ability was the rule. 



Frederick Adams Woods in his fas- 

 cinating book, "Mental and Moral 

 Heredity in Royalty,"' has shown that 

 both mental and moral qualities are 

 inherited. 



The reader who is interested in ex- 

 amining a very good summary of the 

 evidence that mental capacity is in- 

 herited, will find it in Popenoe and 

 Johnson's "Applied Eugenics," chapter 

 4.** Briefly, part of the evidence as 

 presented is as follows : 



(T) Mental traits (insanity and 

 feeblemindedness) segregate in suc- 

 cessive generations much like other 

 traits which are known to be inherited. 



(2) Resemblances in twins persist in 

 spite of differences in the environment ; 

 unlike qualities in twins persist in 

 spite of sameness in the environment 

 (Galton). 



(3) Older twins do not show greater 

 resemblance than }'ounger ones, though 

 the former have been subjected to the 

 same environment longer than the 

 latter (Thorndike). 



(4) Thorndike's experimental work 

 also shows that, "Equalizing practice 

 seems to increase differences. The 

 superior man seems to have got his 

 present superiority by his own nature 

 rather than by superior advantages of 

 the past, since, during a period of 

 equal advantages for all, he increases 

 his lead." 



(5) Pearson finds that the average 

 coefficient of correlation with respect 

 to several mental traits in brothers and 

 sisters is about .5. The same degree 

 of resemblance exists for physical traits 

 known to be inherited. 



The facts amply justify the view that 

 men differ greatly at birth in their 

 inherited mental endowments, and that 

 these differences exert a profound in- 

 fluence upon their lives. 



It is therefore important to deter- 

 mine whether mentally superior stocks 

 are holding their own numerically in 

 our increasing population. If unusuallv 

 good families die out or decrease it 

 is not probable that their loss can be 

 made good by the substitution of in- 



