INHERITANCE IN MAN 



321 



Relation of Scholastic Record and Ability to Family Size 

 (Based upon a study of over 3,000 school children in one locality) 



The reasons for the differential birth rate are many, but most or all 

 of them arise from the conditions and demands of modern civilization 

 and particularly of urban life. Among the most obvious causes are the 

 later age at marriage of those who are preparing for professional careers 

 or who desire special and prolonged educational advantages; a sense of 

 obligation on the part of more prudent and responsible parents to have 

 no larger families than can be adequately provided with educational, 

 social, and economic advantages; and the fact that large families are felt 

 to interfere with various professional, social, and economic ambitions. 



Whatever the cause of the differential birth rate, one thing is evident. 

 A larger proportion of each new generation is produced by parents who 

 have shown the least evidence of ability, and a smaller proportion by 

 those who have shown the most — precisely the opposite of the method 

 man has utilized for the continued improvement of his domesticated 

 races of animals and plants. This leads to one more very important ques- 

 tion that we can briefly examine. A question, unfortunately, for which 

 clear-cut human data are very difficult to obtain, and in which many 

 types of personal, religious, and social bias are likely to influence one's 

 thinking. 



Are the differences between men, which result in different grades of 

 achievement and ability, dependent upon "nature" or upon "nurture"? 

 Or, to put it another way, can society safely assume that it can maintain 

 or increase the individual and social worth of its members by providing 

 for the maintenance and improvement of nurture alone? If such dif- 

 ferences as we see in physical vigor, longevity, temperament, mental 

 ability, and social and individual worth can be explained by the differences 

 in nurture, then selection by war, immigration, or a differential birth 

 rate cannot produce any permanent injury to a racial stock. There is 

 abundant evidence that such environmental factors as sanitation, medi- 

 cine, education, material welfare, and a measure of leisure are of great 

 importance, and that inequalities in sharing these advantages explain 

 many of the differences among men. But the indications that the indi- 



