characteristics: relation to environment 259 



The first case of this kind was carefully studied by Muller, 

 and nine pairs have since been fully examined by New- 

 man.^® In each of these ten cases the twins were separated 

 in infancy, being adopted into different families; they have 

 then lived to adult life under diverse conditions and influ- 

 ences. In addition to careful observations as to character 

 and temperament, they were subjected to psychological and 

 temperamental tests by the best standardized methods 

 available. 



The type of results reached by these studies will best be 

 appreciated by first looking at the detailed resemblances 

 and differences in some particular cases. In the first such 

 study, by Muller, there were twin sisters that had been 

 separated when two weeks old, and that did not see each 

 other till they reached the age of 18; from that time until 

 the age of 30 they lived apart more than nine-tenths of 

 the time. Physically they showed the extreme similarity in 

 characteristics that is usual in identical twins. Both "have 

 always been intellectually active," "both have been ex- 

 tremely energetic, capable and popular, and they have been 

 prominent in all sorts of club work in their respective com- 

 munities" (Muller). "Both have had two or three attacks 

 of tuberculosis, almost simultaneously." The usual intelli- 

 gence tests gave results very closely alike for the two twins. 

 But "the non-intellectual tests — of motor reaction time, as- 

 sociation time, 'will-temperament,' emotions and social at- 

 titudes — gave results in striking contrast with those of the 

 intelligence tests, in that the twins gave markedly different 

 scores in all these tests." The differences were on the aver- 

 age greater than those between two individuals taken at 

 random, and seemed "to be correlated with salient differ- 

 ences in their past experiences and habits of life." 



Thus this first study of such a case indicated that the 

 different environments and experiences of the two Individ- 



