BIOLOGY IN HUMAN AFFAIRS 



of course, proves nothing either way. The correlations 

 can be accounted for on either the nature or the nurture 

 hypothesis. Facts which seem to me to render the nurture 

 hypothesis untenable are the following: 



1. Although twins of two-egg origin resemble each other 

 in intelligence no more than do ordinary siblings, twins of 

 one-egg origin (having presumably identical heredity) 

 are in the vast majority of cases of almost identical intel- 

 lectual ability. In fact no very large differences have ever 

 been reported for identical twins who have been reared apart. 



2. Foster children adopted in the first year of life have 

 been shown by Burks to yield almost a zero correlation 

 with their foster parents in Stanford-Binet intelligence scores . 

 Burks' data indicate that the contribution of heredity 

 to variational differences among children of modern 

 American communities is about five times as great as the 

 contribution of the nurture factors connected with the home. 

 It is true that Freeman's study of foster children was con- 

 siderably more favorable to environmental influences, but 

 the children he studied were in most cases adopted at 

 later ages, when selective influences could have entered to 

 distort results. Obviously the only way to avoid the selec- 

 tive factor in such a case is to study only foster children 

 who were adopted so early in life that it would not be 

 possible for the more intelligent parents to select the more 

 intelligent children. 



3. Children of widely different grades of intelligence 

 are found in exactly the same environment. Siblings, for 

 example, are found to differ over a range from an intelli- 

 gence quotient of 50 or less to one of 150 or more. 



4. Individual differences in intelligence are evidenced 

 from very early years. They are readily detectable by the age 

 of twelve months and at thirty-six months are about as 

 marked as they are at any later age. One would certainly not 

 expect this to be the case if nurture factors were paramount. 



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