CHAPTER XLII 



INHERITANCE OF MENTAL TRAITS 

 (By Lewis M. Terman) 



Mental as well as Physical Traits are Heritable. 

 — That mental as well as physical traits are heritable 

 is indicated by all the available evidence. This is what 

 we should expect, considering that mental functions 

 are dependent upon physical structures, namely, those 

 of the central nervous system. 



However, we know much less about mental than about 

 physical inheritance. This is partly due to the fact that 

 traits of mind cannot be identified and measured with 

 anything like the accuracy which is possible in the case 

 of height, weight, skin color, etc. Nevertheless consider- 

 able progress has been made in the measurement of in- 

 telligence and of a few of the more specialized mental 

 abilities. 



Amount and Nature of Variability in Intelli- 

 gence. — Mental tests have shown that unselected in- 

 dividuals of a given race differ enormously in their intel- 

 ligence, and that these differences are " continuous. " 

 Figure 149, which gives the distribution of " intelligence 

 quotients " of nine hundred and five unselected Cali- 

 fornia school children, illustrates both of these facts. 

 The intelligence quotient is an index of brightness. The 

 quotient 100 represents average brightness. The highest 

 grade feeble-minded have a quotient of 65 or 70, the 

 merely dull a quotient of 80 or 85. Children brighter 

 than the average test above 100. about one in two hun- 

 dred fifty testing as high as 140. 



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