CHARACTERISTICS: RELATION TO ENVIRONMENT 257 



marked differences in the physical characteristics of one-egg 

 twins. Komai and Fukuoka ^ have described two fifteen- 

 year-old Japanese boys that are obviously one-egg twins; 

 they show the usual close resemblance in form, features, 

 coloring and the like. But they now differ greatly in size. 

 One is taller by 14.8 centimeters, or about 6 inches, and 

 his weight is greater by 10.4 kilograms, or about 23 pounds. 

 .The two were at first of the same size, the difference gradu- 

 ally arising during growth. The smaller of the twins is af- 

 fected by diabetes insipidus; this is probably connected with 

 his decreased growth. The larger twin is right-handed, the 

 smaller left-handed. Whether the difference in health and 

 growth is in some way connected with what occurred at di- 

 vision of the egg is uncertain. 



A comparable case is described by Siemens.^ Two sisters, 

 clearly one-egg twins, were as usual nearly identical in most 

 of their physical characteristics till the age of 10. At this 

 time one of them became alifected with a severe lateral cur- 

 vature of the spine, and from this time their development 

 was very different; at 16 the healthy twin was 4.8 inches 

 taller than the other. The origin of the defect that pro- 

 duced curvature of the spine and changed development is 

 unknown. 



Identical twins that have lived together sometimes show 

 considerable psychological differences. Newman has made 

 extensive psychological tests on fifty pairs of identical or 

 one-egg twins, and for comparison with these, on fifty pairs 

 of fraternal twins. The results of these examinations have 

 not yet been published in full, but certain data from them 

 have been published. By the Stanford-Binet tests, the in- 

 telligence quotients (IQ) and the "mental age" were de- 

 termined for the two sets. The two classes of twins are best 

 compared by the differences between the scores made by the 

 two members of each pair. These differences were as fol- 

 lows: 



