258 



GENETICS 



Mean difference 

 in Mental Age 



Mean difference in 

 Intelligence Quo- 

 tient 



Identical Twins 



50 Pairs 

 Fraternal Twins 



50 Pairs 



8 years 4 mo. 5.3 points 



15 years 9 mo. 



9.9 points 



Thus the average difference between fraternal twins, 

 having somewhat diverse genetic constitutions, Is nearly 

 twice as great as that between Identical twins, in which the 

 genetic constitutions are alike. 



But It is Important that the Identical twins that have Hved 

 together also showed differences between members of the 

 pairs. In different pairs there was much variation in the 

 amount of diversity. Of the 50 pairs there were five In 

 which the two members gave identical scores in the intelli- 

 gence tests; the difference of their scores was o. At the 

 other end of the scale were five pairs that showed respec- 

 tively the following differences (points In the Stanford- 

 Blnet scale) between the two members: 12.6, 12.9, 13.0, 

 13.9, 16.0. 



All these five differences are considerably greater than the 

 average difference between the genetically unlike fraternal 

 twins. It Is clear that in some cases even Identical twins liv- 

 ing together differ much In mentality. Whether the differ- 

 ences are the result of original diversities consequent on 

 the method of division of the egg, or have been produced 

 In some way by different experiences of the two, Is not 

 known. 



For comparisons as to the relative effects of genetic and 

 environmental diversities on mental characteristics, valuable 

 data have been obtained from the examination of Identical 

 twins that have lived apart under different environments. 



