As regards the fertility of the human species, the decisive influences are not physiological, but social. The view that higher brain development or prolonged intellectual activity restricts fertility has been rightly contested. Undoubtedly, powerful intellectual activity tends to inhibit the sexual impulse, but this is no less true of great physical exertions. The causes of the high birth-rate among the lower classes of the population are the following:-- (_a_) Members of the proletariat have to marry earlier in life than those belonging to the middle and the upper classes. (_b_) A smaller proportion of the proletariat suffers from venereal infection. (_c_) Owing to the overcrowded condition of their dwellings, those belonging to the poorer classes find it far more difficult to observe “prudential restraint.” (_d_) The poor make less use than the well-to-do of positive methods for the prevention of conception. (_e_) To those belonging to the poorer classes, to have children is often economically advantageous. A child can help in the work of the household, and can early engage in some occupation enabling it to contribute towards the expenses. On the other hand, in the case of the comparatively well-to-do, a large family involves the risk of a depression in the standard of life. (_f_) Women of the middle and upper classes are far more afraid than working-class women of pregnancy and childbirth. They actually suffer more from these eventualities, because their sheltered life makes them weaker and more susceptible; in many cases also they shirk motherhood because they think that pregnancy will interfere with their “social duties.” An excessive number of conceptions, pregnancies, and deliveries is harmful, not merely from the outlook of the domestic economist, but also from that of the political economist. If the aim of the State is to secure a population which is not merely numerous, but also of good quality, care must be taken that the number of conceptions, pregnancies, and deliveries shall not be unduly great; for when the number of births is exceedingly large, it is very likely that the number of those to attain maturity will actually be less than if the birth-rate had been lower. We have to take into consideration, not only the difference between the birth-rate and the death-rate, but also the important matter of the actual number of births and deaths. Although in any two cases the terminal results may be identical, it is a matter of grave economic moment how the figures are comprised by which these identical results are attained. If, to effect a certain increase in population, a comparatively large number of births and deaths has been requisite, there has been an enormous waste of time, energy, and wealth.