352 AN INTRODUCTION TO MODERN GENETICS 



Within Western European and American populations, important 

 genetical effects have been ascribed to the differential fertility of 

 different classes. Since age composition and mortality rates are much 

 the same in all classes, these differences in fertility are probably not 

 grossly distorted when judged on the basis of the birth-rates, which are 

 usually the only statistics available. The corrections, if they could be 

 applied, would probably increase rather than decrease the differences 

 which are shown by the birth-rates. 



Many investigators have shown that the birth-rate is highest in the 

 lowest occupational class (unskilled labourers) and falls progressively 



Fig. 145. Fertility and class. — The first column gives the number of children 

 per wife, and the second the net reproductive rate in 1928, for different classes 

 in populations in the United States. 



(Data of Notestein, and Lorimer and Osborn, quoted from Holmes.) 



Net reproduc- 

 Birth rate tive rate 



in higher and higher classes. As we have seen, the I.Q. is highest in the 

 highest classes, so that the consequence which can apparently be imme- 

 diately deduced is that the more intelligent are being gradually swamped 

 by the higher fertiUty of the stupid. Fisher^ has indeed produced an 

 argvmient to show why this must always be so in any society of classes 

 based on wealth. Social advancement is achieved by two sorts of 

 persons, the intelligent and the infertile; by the latter because of the 

 educational and other advantages enjoyed by children who do not have 

 to share the family's resources with many brothers and sisters. In the 

 upper classes, then, the intelUgent mate with the infertile and do not 

 maintain themselves. Fisher supports his hypothesis, which if it is true 

 is of fundamental importance, with considerable evidence. Firstly, 

 studies of genealogies of English peerages show that there is good 

 correlation between the size of a woman's family and that of her mother 



^ Fisher 1930. 



