490 EVOLUTION, GENETICS, AND EUGENICS 



and the environment the same, twins tend to become more similar 

 than when the environment is different and the heredity is the same. 

 Extreme environmentalists may see in this situation evidence that en- 

 vironmental differences are more powerful in determining mental ca- 

 pacity than are hereditary differences. I do not believe that this is 

 true, and for several reasons. In the first place, if environment were 

 more powerful than heredity, identical twins reared together should 

 not be twice as similar as fraternal twins reared together; yet this is 

 the case. In the second place, all the tests used to determine mental 

 ability, except possibly the International test, measure largely the ef- 

 fects of training. This tends to mask or cover up hereditary resem- 

 blances and differences, increasing the difference in cases where educa- 

 tional experiences are widely divergent and decreasing the differences 

 (in fraternal twins reared together) where educational experiences are 

 alike. Thus the tests used tend to exaggerate the potency of environ- 

 mental differences and to minimize the potency of hereditary differ- 

 ences. 



In spite of these weaknesses in method, however, the data at least 

 warrant the conclusion that both hereditary differences and environ- 

 mental differences play important roles in determining mental charac- 

 teristics. In so far as a person's mental status is measurable in terms 

 of ability to perform well or poorly on intelligence tests, it seems fair 

 to conclude from our data that differences in heredity and differences 

 in environment are about equally responsible for the mental status of 

 an individual. On the other hand, since a large part of what is meas- 

 ured by intelligence tests is merely achievement and not real mental 

 capacity, some will say we are not really measuring mental status at all. 

 There is doubtless some justification in this criticism of intelligence 

 tests, yet I cannot help but believe that one's individual mind, his 

 mental rating, is at any time the product of his hereditary make-up 

 and his training. If this be true, it is only fair to rate him according to 

 this product. 



In spite of all this apparent evidence that hereditary differences 

 and environmental differences are of about equal potency in determin- 

 ing mental status, I have a deep-seated conviction, which I am unable 

 completely to justify at this time, that differences in heredity are con- 

 siderably more influential, perhaps twice as influential, in determining 

 one's mental status, as are differences in environment. 



