HUMAN GENETICS 339 



that variability is certainly not abolished in identical twins, and that 

 fraternal twins have a higher correlation than normal sibs, make it 

 certain that environmental differences, even the slight ones between 

 members of the same family, have a recognizable effect in determining 

 scores in intelUgence tests. 



The environmental differences between identical twins brought up 

 in the same family are very small. We might expect to find out more 

 about the possible effects of the environment from a study of identicals 

 which have been reared apart. Comparatively few such cases have been 

 studied yet, but in those which have the average difference is greater 

 than that of identicals reared together and less than that of fraternals 

 reared together. Newman, Freeman, and Holtzinger^ have made a de- 

 tailed study of nineteen pairs. Differences in intelUgence, when they 



Fig. U1. Intelligence Tests on Twins and Sibs. 



(From Herrman and Hogben.) 



occurred, could nearly always be correlated with differences in the 

 amount of schooling or other cultural opportunities which the pairs 

 had received (ii pairs). Moreover, in five out of the six cases in which 

 there had been marked differences in opportunity, there were corre- 

 sponding differences in abiUty; in the sixth case, the twin who had had 

 the greater opportunities had also suffered a severe illness which may 

 well have annulled the cultural advantages she had enjoyed. The 

 authors conclude that their studies provide convincing evidence of the 

 marked effect of education on abiUty. Comparisons were also made of 

 the temperaments of the twins. These are of course much more difficult 

 to study, even with the aid of a whole battery of tests. The interesting 

 point emerged that, while twins reared in different environments might 

 show considerable differences in superficial behaviour, such as con- 

 formity to canons of poHte society, they were often very similar in more 

 fundamental traits. An extreme example was the handwriting of two 

 brothers ; there were differences in most of the usual features reUed on 

 for describing handwriting, such as form of letters and pressure, but 

 both writers suffered from a sUght muscular tremor. 

 ^ Newman, Freeman, and Holtzinger 1937. 



