The greater the decrease in the birth-rate, the greater the increase in the people's purchasing power.

Now, what has brought about this change in the ratios of increase in population and in food respectively?
Some serious factor, inoperative during the thirty years prior to 1877 must have suddenly been introduced into the social system, to work such a marvellous revolution during the last twenty years.
Some economic writers find it easy here to discover a law, and declare that the birth-rate is in inverse ratio to the abundance of food.
(Doubleday quoted by Nitti, Population and the Social System, p.

55).
Other economic writers of recent date attribute this great change in ratio of increase to economic causes.

Only a few find the explanation in biological laws.
Herbert Spencer is the champion of the biological explanation of a decreasing birth-rate.
With the intellectual progress of the race there is a decadence of sexual instinct.

In proportion as an individual concentrates his energies and attention on his own mental development, does the instinct to, and power of, generation decrease.
It may be true, it certainly is true, that if an individual's energies are concentrated in the direction of development of one system of the body, the other systems to some extent suffer.