HUMAN GENETICS 



349 



do not develop any mode of speech and their intellectual development 

 is very meagre indeed. 



But the environmental disadvantages of the lower classes are cer- 



30 



25 



20 



UJ 

 Ui 



75 



65 



95 



/05 //5 



I.Q. 



I2S 



/35 



145 



Brainwork. 



.... Semi-skilled Handwork. 



Skilled Handwork. 



_._ Unskilled. 



Fig. U4, Intelligence Quotient and Social Class.— The I.Q. was measured for 

 children in elementary and secondary schools in Northumberland, and the occupa- 

 tion of the fathers of the children was noted. The I.Q.'s were grouped in classes 

 centring round values of 75, 85, etc., to U5, and the occupations were grouped 

 in four classes (Brainwork (lower ranks of professions). Skilled Handwork, in- 

 cluding shopkeepers and policemen. Semi-skilled Handwork, Unskilled Handwork). 

 The graphs show the percentage frequency of children with the different I.Q.'s 

 and belonging to the different social classes determined by their father's occupa- 

 tion. Note that the average I.Q. increases towards the higher classes; but note 

 also the enormous overlap between classes. 



(Data of Duff and Thomson.) 



tainly not of this order. We get rather nearer to them in investigations 

 on children who have been prevented by physical illnesses from attend- 

 ing school for the full period; their intelligence test scores are adversely 

 affected more or less in direct ratio to the length of time they have 



