350 AN INTRODUCTION TO MODERN GENETICS 



lost.^ Again, children of bargees and gipsies only attend school very 

 irregularly, and while the I.Q. of the youngest tested (four to six years) 

 is only slightly below average (90 instead of 100) the average gradually 

 falls with age and for the group between twelve and twenty-two years 

 is only about 60 per cent of the normal. We have already mentioned 

 (p. 344) the fact that the scores of different American conscripts was 

 correlated with the expenditure of their state on education. Finally, 

 the differences between the averages for the highest and lowest classes 

 (15 to 30 per cent) is not much more than twice the average differences 

 between identical twins (10 per cent) which have been produced by 

 the very slight environmental differences acting on two individuals of 

 the same sex and birth rank in the same family. It is possible, however, 

 that the environmental differences which produce the variation be- 

 tween two identical twins reared together are largely intra-uterine, and 

 it may not be justifiable to compare this with the environmental dif- 

 ferences between the classes. 



We can certainJy conclude from this that the genetic differences 

 between the classes are considerably less than their average perfor- 

 mances would suggest. But they may not be negHgible. Particularly the 

 lowest class of unskilled labourers, who very rarely produce any indi- 

 viduals above the average, may really be genetically slightly inferior. 

 But conditions of labour have so changed recently that many unskilled 

 workers are now recruited from families which used to belong to the 

 skilled class. Perhaps an investigation on children at the same school, 

 some from families which have for generations been imskilled or casual 

 labourers, and others from families which have recently been forced 

 down into this class, would show whether there was a true genetic 

 difference between the two groups. Perhaps also an investigation of the 

 effect of prolonged unemployment of the parents on normal working- 

 class children would reveal the effectiveness of the kind of environ- 

 mental differences which are found in present-day society. But data on 

 these two points are not yet available. 



6. Changes in Population Composition 



There is not space in this book to discuss the general trends of popula- 

 tion change either in total numbers or in age composition,^ and we shall 

 confine ourselves to a sunmiary of views on the changing genetic 

 structure of human populations. With the enormous decrease in death- 

 rates brought about in the last himdred years by advances in medical 

 ^ Gordon 1923. ^ cf. Hogben 1938. 



