262 GENETICS 



cases examined by Newman were treated by uniform meth- 

 ods, so that they may be compared in such a way as to bring 

 out general relations. 



The intelligence quotients for the two members of the 

 nine pairs are as follows (Stanford-Binet scores in points) : 



Pair IQ Difference 



I 97 and 85 12 



II 67 and 78 II 



III 99 and loi 2 



IV 106 and 88 18 

 V 93 and 89 4 



VI 102 and 94 8 



VII 106 and 105 I 



VIII 92 and 77 15 



IX 102 and 96 6 



The mean difference in intelligence quotient for these 

 nine pairs of identical twins that have lived apart is 8.6. 

 This is considerably greater than the mean for identical 

 twins that have lived together, which was 5.3. So far as it 

 goes it indicates that difference of environment has a con- 

 siderable effect in increasing the mental difference between 

 twins. In cases I, II, IV and VIII, the difference between 

 the twins was very marked, but is similar to the differences 

 between the five extreme cases of identical twins that had 

 lived together (12.6, 12.9, 13.0, 13.9, 16.0). This empha- 

 sizes the fact that it is not possible to be certain what dif- 

 ferences are due to something that happened at the division 

 of the egg, and what are due to later environmental differ- 

 ences. The mean difference between identical twins that 

 have lived apart (8.6) is nearly the same as that for fra- 

 ternal twins that have lived together (9.9). 



In addition to the studies of Intelligence, Muller and 

 Newman made extensive examinations designed to test 

 other features of personality. One set was designed to test 



