Heredity and Politics by J. B. S. Haldane

Author:J. B. S. Haldane [Haldane, J. B. S.]
Language: eng
Format: epub
ISBN: 9781138955127
Google: cCXrsgEACAAJ
Publisher: Routledge
Published: 2015-09-15T00:02:29.977523+00:00

CHAPTER IV

Differential Fertility and Positive Eugenics

WE must now take up the question of the different rates of increase or decrease in different nations and social classes. We must consider the causes and effects of these differences, and the methods which have been adopted or suggested for controlling them.

If we wish to compare the rates at which two human groups are increasing or decreasing, there are several possible comparisons. We may compare the birth-rates. It is obvious that a population with a high birth-rate is not necessarily increasing in numbers. The birth-rate in China is certainly higher than in most European countries. So is the death-rate. But we do not know whether the population is increasing or not. We may measure the excess of births over deaths. That is valuable. It gives the rate at which a population is now increasing or decreasing, but it affords little guidance as to the future. In order to predict the distance that a motor car will go in the next minute we should know not only the speed but whether the driver has his foot on the accelerator. To predict the trend of the population in the future we need statistics of a rather special character.

Consider one hundred thousand English baby girls born in 1920, and therefore now just coming to the age when a few of them will be producing babies themselves. Somewhat over ninety thousand of those girls are still alive, and most of them will survive until the age of fifty, after which they will not bear any more children. Now the average fertility of women at any given age is known. Fig. 11 shows some American figures. It will be seen that on the average one thousand women between fifteen and nineteen bear something like fifty children per year; between twenty and thirty they bear somewhere between one hundred and one hundred and fifty children per year, and after that, their fertility diminishes. It will also be noticed that the fertility at any given age was diminishing between the years 1920 and 1929. If we take one hundred thousand baby girls and allow for the fact that a certain number of them are dead at any given age, we then ask how many children they will produce, given the standard of fertility of women at each particular age. Clearly, if they have more than one hundred thousand girl babies in the course of their lives, then the population will tend to increase; if they have less than one hundred thousand, it will tend to decrease. The ratio of the number of daughters to the number of mothers is called the net reproductive index. We can make our calculation a little better if we allow for the trend of fertility at different ages. As the fertility of women between twenty-five and thirty is falling1 in England it is reasonable to suppose that ten years hence it will be somewhat lower than it is now. How much, I cannot say. Nevertheless, the

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