CERTAIN DEMOGRAPHIC BACKGROUNDS FOR POPULATION STUDIES 289 



ble to plot a long uninterrupted birth his- 

 tory for this country. The observations on 

 England and Wales start in 1840; the esti- 

 mates on the United States in 1875; and 

 the actual reports in 1915 (Fig. 86). 



Certain conclusions can be drawn from 

 Figure 86. In Sweden, as Lotka points out, 

 the annual birth rate in the middle eight- 

 eenth century was in excess of thirty-four 

 per thousand. This was maintained until the 

 eighteen-sixties. About 1865 a decrease 

 started that was gradual up to the year 

 1920, "... after which a decline so rapid 

 set in that in the course of a single decade 

 the birth rate fell from over 20 to a little 

 more than 15, a decrease by over 25 per 



Specific Birth Rate 



The specific birth rate is defined in the 

 same way as the crude rate except that the 

 numerator, B, signifies the births in a speci- 

 fied class of the population, and the denom- 

 inator, P, those individuals in the total 

 population that are capable of providing 

 the births. Thus, if our interest is in age- 

 specific birth rates, the denominator is the 

 number of women of a given age or within 

 a given age range, and the numerator is 

 the number of infants bom per year to 

 those women in the stated age-class. This 

 statistic measures "fertihty" rather than 

 "fecundity."* 



YEAR 



Fig. 86. Birth rates per 1000 for Sweden (heavy line), England and Wales (broken line), 

 and the United States (estimated 1875 to 1914, dotted line; observed from 1914 on, thin line). 

 (From Lotka.) 



cent . . . . " The picture presented by 

 Sweden is essentially characteristic of most 

 of the leading civilized countries." The 

 birth rate of England and Wales and the 

 United States after 1880 dropped about as 

 did that for Sweden. Thus, we emerge with 

 a generalization about many human popu- 

 lations: during the last seventy-five years or 

 so natahty has been decreasing. There are 

 probably a number of reasons for this of 

 which some are biological and some socio- 

 economic. In part, it is an expression of a 

 shift in age distribution of the type dis- 

 cussed in the preceding chapter ' (see p. 

 281). Modem populations are increasingly 

 characterized by a predominance of middle 

 and old-aged individuals, and this has the 

 statistical consequence of lowering the fer- 

 tihty rate. 



" A notable exception is France, where the 

 major decline started earlier in its history 

 From 1920 to 1925 the birth rate was almost 

 stationary. Since then the decline went from 

 19.0 in 1925 to 15.2 in 1935. 



An illustration of age-specific birth rate 

 is presented in Table 21 (p. 294). There 

 the number of births of daughters per 100,- 

 000 mothers relative to nine maternal age 

 categories are set forth. The essential point 

 that arises from this table is that these 

 particular mothers attained their maximum 

 eflFective fertility rate in the age range 

 twenty-five to twenty-nine years. This is a 

 valuable demographic conclusion. It could 

 not have been reached if the crude rate 

 alone had been computed. 



The relation of age to fertihty usually 

 follows the general form just described. 

 However, the specific birth rate, when ap- 

 plied to other populations, shows that this 

 is not invariably true. In 1917 Knibbs pub- 

 lished a treatise on the population of Aus- 

 tralia in which he showed, as later analyses 

 of Pearl (1940) stressed, that there the 



** We shall use these two terms frequently: 

 fecundity refers to egg production (or sperm 

 production in the male); fertility refers to the 

 nimiber of eggs that develop into living young. 



