148 Discussion 



Olbrich: I don't say "no meaning," only the "ascribed meaning". 



Kallmann : Even if there are concomitant changes in blood supply or 

 blood sugar or some other essential function, the primary basis seems 

 to be some very specific metabolic defect, I would think. 



Olbrich : Do you assume that it is due to fat changes in the brain ? 



Kallmann: Yes, this may be a suitable working hypothesis. 



Danielli: It seems to be difficult to evaluate the twins data that have 

 been presented to us today, since there is a strong tendency for these 

 twins to seek the same sort of environment. The object, presumably, of 

 twin experiments is to say that you have a common genotype, a standard 

 genotype, and that enables you to evaluate precisely the effect of the 

 environmental factor. But this seems to me to be partly vitiated by the 

 fact that the twins do apparently largely seek the same environment 

 and have similar experiences. Now, in the one set of data that was 

 presented, on monozygotic and dizygotic twins, the average deviation 

 from the mean in expectation of life, or accomplishment of life, for the 

 monozygotic was about 5-6 per cent of the total length of life ; in the 

 case of the dizygotic it was about 10-12 per cent. The question that I 

 would raise is : did the experience of life of the monozygotic twin pairs 

 vary by more than 4-6 per cent ? Also, did the experience of life of the 

 dizygotic twin pairs vary by more than 10-12 per cent? If what one is 

 measuring is simply the result of the variation in experience of life, then 

 the data on these twins are more or less meaningless. 



Lorge: There are enough American data to suggest that even under 

 highly diverse environmental conditions the similarity of the mono- 

 zygotics is extremely consistent. My feeling is that the Kallmann data 

 would certainly exemplify, for most of the cases, a homogeneity of 

 environment, whether it is sought that way or whether it is conditioned ; 

 but in those instances where he has been absolutely able to separate 

 out those who have had highty diverse environments, he has a striking 

 homogeneity of results. 



Danielli: That raises the point as to how far these environments 

 really were so diverse. Take human beings living in a civilized society : 

 it is extraordinarily difficult to give them really varied environments, 

 and if they are twins they seek the same sort of environment. I suspect 

 that people working on twins overestimate the amount of diversity ; to 

 succeed in getting 10 per cent diversity of environment is quite a big thing. 



Lorge: I think either way they are a very important methodological 

 issue. What do we mean by "similarity of environment' ' ? This concept of en- 

 vironmental homogeneity requires a considerable amount of investigation. 



Kallmann: It is the objective of twin studies to determine the mode 

 of interaction of genetic and environmental influences, rather than " to 

 evaluate precisely the effect of the environmental factor". The popular 

 notion that the behaviour patterns of one-egg twins resemble each other 

 chiefly because of unusual similarity in their early environments has yet 

 to be substantiated. Actually, if it can be shown that one-egg twins 

 "seek the same environment" more frequently than do sibs or two-egg 

 twins, this piece of evidence would strengthen rather than weaken any 

 correctly formulated genetic theory. 



