The Editor: Nature or Nurture? 



231 



three times as great as between ordinary 

 children of the same age and sex, 

 brought up under similar environment. 

 There seems to be no reason why twins 

 should be more alike, unless it is due 

 to the power of heredity. The data 

 showed (3) that the twins were no more 

 alike in traits subject to much training 

 than in traits subject to little or no 

 training. Their achievement in these 

 traits was determined by their heredity ; 

 no amount of training could alter these 

 hereditary potentialities. 



"The facts," Thorndike wrote, "are 

 easily, simply and completely explained 

 by one simple hypothesis: namely, that 

 the natures of the germ-cells — the condi- 

 tions of conception — cause whatever 

 similarities and differences exist in the 

 original natures of men, that these 

 conditions influence mind and body 

 equally, and that in life the differences 

 in modification of mind and body 

 produced by such differences as obtain 

 between the environments of present- 

 day New York City public school 

 children are slight." He reached the 

 same conclusion that other studies of 

 this sort have shown, that in the make- 

 up of the individual there are probably 

 nine parts of heredity for every one of 

 nurture or training. 



"The inferences," he says, "with 

 respect to the enormous importance of 

 original nature in determining the 

 behavior and achievments of any man 

 in comparison with his fellows of the 

 same period of civilization and condi- 

 tions of life are obvious. All theories 

 of human life must accept as a first 

 principle the fact that human beings at 

 birth differ enormously in mental capac- 

 ities and that these differences are 

 largely due to similar differences in 

 their ancestry. All attempts to change 

 himian nature must accept as their most 

 important condition the limits set by 

 original nature to each individual." 



Meantime other investigators, prin- 

 cipally followers of Karl Pearson in 

 England, were working out correlation 

 coefficients in other lines of research 

 for hundreds of different traits. It was 

 found, no matter what physical or 

 mental trait was measured, that the 

 coefficient of correlation between parent 



and child was a little less than .5 and 

 that the coefficient between brother 

 and brother, or sister and sister, or 

 brother and sister, was little more than 

 .5. On the average of rnany cases, the 

 mean "nature" value, the coefficient of 

 direct heredity, was placed at .51. 

 This gave another means of measuring 

 the relative importance of nature and 

 nurture, for it was also possible to 

 measure the relation between any trait 

 in the child and some factor in the 

 environment. A specific instance will 

 make this clearer. 



MYOPIA OF SCHOOL CHILDREN 



We know that school children show 

 an appalling amount of eye trouble, 

 particularly short-sightedness. Now 

 suppose it is suggested that this is 

 because they are allowed to learn to 

 read at too early an age. We can find 

 out the age at which any given child 

 did learn to read, and work out the 

 coefficient of correlation between this 

 age and the child's amount of myopia. 

 If we find the relation is very close 

 between them — say .7 or .8 — we will 

 know that the earlier a child learns to 

 read, the more short-sighted he is as he 

 grows older. This will not prove a 

 relation of cause and effect, but it will 

 at least give us great suspicion. If on 

 the contrary we find the correlation 

 very slight, we are forced to admit 

 that early reading has nothing to do 

 with the prevalence of defective vision 

 among school children. If we similarly 

 work out all the other correlations that 

 can be suggested, finding whether there 

 is any regular relation between myopia 

 and overcrowding, long hours of study, 

 general economic conditions at home, 

 general physical or moral conditions of 

 parents, the time the child spends out 

 of doors, etc., and if we find there is no 

 relation of any moment between these 

 various factors and myopia, we are 

 driven to admit that no factor of the 

 environment which we can think of as 

 likely to cause the trouble really has 

 anything to do with the poor eyesight 

 of our school children. This has ac- 

 tually been done, and none of the 

 conditions I have enumerated has been 

 found to be closely related to myopia 



