On Nutrition and Sex-determination in Man. 



267 



(3) Influence of birth-rate on 'proportion of sexes. 



To obtain some estimate of the action of this factor we must 

 know (a) the relative birth-rates in the different groups which 

 will afford some notion of the sizes of the families, and (b) the 

 influence of curtailment of the size of the family on the pro- 

 portions of the sexes. 



(a) The relative birth-rate in groups A, B, and G is com- 

 plicated by the fact that the mortality varies in the different 

 groups. The Census statistics give comparatively few records 

 of marriages before the age of 21. From 21 — 50 years has been 

 taken roughly as the fertile period, and the proportion of the 

 infants under 1 year compared to the number of adults of the 

 fertile period has been taken as the relative birth-rate in the three 

 groups. The following table shews the different proportions in 



the three groups of people at different ages. It will be noticed that 

 the poorer the district the greater the proportion of infants and 

 young people, and the smaller that of adults and old adults. 

 This is no doubt due to the higher mortality in the poorer 

 districts. It is, then, obvious that if we took as our birth-rate 

 the proportion of infants under 1, to the total population, the 

 lessened mortality in groups B and C, as compared with A, will 

 tend to lower the birth-rates in these two groups more than in A. 

 And similarly the birth-rate will be more greatly lowered in C 

 than in B. On the other hand the proportion of youths from 

 1 — 20 is far greater in A than B, and in B than in G. This 

 factor tends to lower the rate in A more than in B and in B again 

 more than in G. On the whole it has seemed safest to compare 

 the number of infants under 1 with the fecund portion of the 

 population in each group — with the following results : — 



