202 GENETICS 



different. What is the situation as to their mental charac- 

 teristics? 



Several cases of this sort have been very carefully studied 

 by Muller and by Newman. We shall have occasion to look 

 at the results of this study in some detail later (see Chapter 

 X). Here we may summarize briefly from our present point 

 of view what was discovered. One-egg twins that have lived 

 apart show the same close resemblance in physical char- 

 acterization that is found in such twins that have lived 

 together. They show also resemblance in mental character- 

 istics: in emotions, temperament, and in "intelligence" as 

 measured by psychological tests. In certain cases, there are 

 found some dissimilarities in mental and emotional charac- 

 teristics, depending on differences in education and experi- 

 ence in the separated twins — a matter to which we return 

 in a later chapter. Thus it is clear that (i) the genes have 

 great influence on mental characteristics, emotions, tem- 

 perament and the like, but that (2) these things may be 

 modified by different education and experience. 



An illuminating study of the mental and moral character- 

 istics of twins was made by Johannes Lange. He found in 

 the prisons of Germany, or in the prison records, thirty 

 persons, each of whom was a member of a pair of twins, 

 the two members in each case being of the same sex. He 

 traced and examined the other twin of each pair. Thus one 

 twin was chosen because he was a criminal; the other twin 

 was taken as he chanced to be. As it turned out, thirteen 

 of the pairs were single-egg twins, while the other seven- 

 teen were two-egg twins. 



Of the thirteen pairs of one-egg twins, one of which was 

 known in each case to be a criminal, it turned out, upon in- 

 vestigation, that the other also had a criminal record in 10 

 out of the 13 cases. It is clear that if one such twin comes in 

 conflict with the law, the other one, identical in original 



