358 



TIMBER RESOURCES FOR AMERICA'S FUTURE 



jections commonly accepted 10 to 20 years ago.*^ 

 Instead of leveling off at 165 to 180 million, as then 

 foreseen for the period 1965-75, the population has 

 already passed that lower limit. There is now a 

 practical certainty that 1965 will witness the be- 

 ginning of another surge of births when people 

 bom after 1940 will be having families of their own. 

 The infusion of younger-age persons into the popu- 

 lation structure, which has occurred since 1940, 

 and the second infusion that will begin about 1960- 

 65, will almost certainly keep the Nation's popu- 

 lation on the upward trend until 2000 and beyond. 

 The trend of future population is important be- 

 cause it provides the foundation for estimates of 

 future labor force, future gross national product, 

 future disposable income of individuals, and other 

 elements of economic growth affecting demand for 

 timber products. 



Future Population Depends Chiefly On 

 Future Birth Rates 



In 1955, the Bureau of the Census published 

 four series of population projections for the United 

 States, covering the period 1955 through 1975 

 (table 195). The differences between the various 

 series are due entirely to differing future birth 

 rates. The rates of mortality and net immigration 

 are the same for all four series.^* According to 

 the Census Bureau, all of the future population 

 series are reasonably possible, and none is selected 

 as "most likely." Acceptance of the high series, 

 the low series, or something in between, obviously 

 depends chiefly upon the evidence that would 

 support the various birth-rate (fertility-rate) 

 assumptions.^^ 



" See for example: Davis, Joseph S. The Population 

 Upsurge in the United States. Food Res. Institute, Stan- 

 ford Univ., Palo Alto, Calif. 1949. 



^< Briefly, it was assumed that the age-specific mortality 

 rates would continue to decline until 1955-60 and remain 

 constant thereafter until 1975 — a conservative assumption. 

 Net immigration would continue about the same as it has 

 since the end of World War II. 



85 The method of projection used by the Census Bureau 

 is based on the application of "age-specific fertility rates," 

 or number of births annually per 1,000 women in each 

 5-year age group: Perctntage 



increase 

 1961,-55 

 Age group "Prewar" over 



{years) 1950-65 1951,-55 (approximate) "prewar" 



Under 19 87.9 87.5 61.2 43 



20 to 24 213.4 232.5 152.3 53 



25 to 29 176.0 195.0 135.4 44 



30 to 34 109.5 122.2 81.1 51 



35 to 39 56.2 60.2 38.8 55 



40 and over 17.1 16.5 11.4 45 



The age-specific fertility rate assumptions underlying 

 the projections are as follows: 



Series AA: The 1954—55 rates remain constant from 

 1955 through 1975. 



Series A: The 1950-53 average rates remain constant to 1975. 



Series B: The 1950-53 average rates remain constant 

 until 1965, then drop on a straight line to the "prewar' 

 rates by 1975. 



Series C: The 1950-53 average rates drop on a straight 

 line to the "prewar" rates by 1975. 



Acceptance of a particular popiilation series is also 

 contingent upon at least two theories. One is 

 that fertility rates tend to rise and fall in a pattern 

 of long cycles; the other is that there is a natural 

 propensity for parents to want 3 or 4 children, 

 and that they will tend to have that many if 

 economic and other conditions are favorable. 

 Some evidence by which either of these theories 

 could be supported is to be found in trends in 

 what the population analysts call the "repro- 

 duction rates" (table 196). 



A significant feature of the trend in female gross 

 reproduction rates ®® is that the high rate of 1954 

 was not quite up to the 1905-10 average. This 

 could be interpreted as evidence that the pro- 

 pensity to have children has been no stronger in 

 recent years than it was at the beginning of the 

 century. The prewar decline in the gross rate 

 has been ascribed to the large shift of population 

 from rural to urban. But the same shift has con- 

 tinued while the rate has been rising. The con- 

 temporary shift of population from congested ur- 

 ban districts to suburban areas also may have had 

 some influence. If so, such influence is likely to 

 be permanent as dispersion of congested urban 

 population continues. 



The female net reproduction rate ^'^ since the 

 end of World War II has been much higher than 

 it was at the beginning of the century because of 

 great reduction in mortality rates. The relation- 

 ship of the net rate to the gross rate has changed 

 from 0.745 in 1905-10 to 0.960 in 1954, due en- 

 tirely to the decrease in mortality of females. 

 Some moderate further decrease is to be expected. 



One important question is whether families of 

 the future would willingly support the number of 

 children that each, on the average, would have 

 at the current fertility rate. Such a question is 

 not answerable in any definitive way, but if the 

 1 954 rate is maintained, each woman in the course 



'' The "female gross reproduction rate" represents the 

 number of daughters that would be born to the average 

 1,000 newly born females if (a) none of them were to die 

 before completing their reproductive period of life, and 

 (b) they were subject to the age-specific fertility rates pre- 

 vailing at a specified period in time. A gross reproduction 

 rate of 1,000 means that, under these conditions, the 1,000 

 females would bear just enough daughters to replace them- 

 selves. The gross reproduction rate may be roughly indi- 

 cative of the propensity to bear children and of the changes 

 in that propensity over time. 



'' The "female net reproduction rate" represents the 

 number of daughters that would be born to the average 

 1,000 newly born females if (a) they were subject, from 

 birth to completion of their reproductive period of life, 

 to the age-specific mortality rates prevailing at a specified 

 period in time, and (b) they were subject to the age-specific 

 fertility rates of that same period. Thus a net reproduc- 

 tion rate of 1,000 means that, under these conditions, the 

 1,000 females would bear just enough daughters to replace 

 themselves. At rates of less than 1,000, the Nation's popu- 

 lation — if not replenished by net immigration — would ulti- 

 mately decrease. The net reproduction rate current at a 

 particular time is the effective rate of reproduction implied 

 by the then-current fertility and mortality rates. 



