Source: On Education, especially in early childhood, 1926, by Bertrand Russell, Part II, Chap. 11.

(For these reasons we should be more concerned to produce sympathetic and affectionate adults than to force a precocious development of these qualities in early years. ... Parental affection, at its best, differs from sex love in this respect. It is of the essence of sex love to seek a response, as is natural, since, without a response, it cannot fulfil its biological function. But it is not of the essence of parental love to seek a response. ... )
( The natural unsophisticated parental instinct feels towards the child as towards an externalized part of the parent's body. If your great toe is out of order you attend to it from self-interest, and you do not expect it to feel grateful. The savage woman, I imagine, has a very similar feeling towards her child. She desires its welfare in just the same way as she desires her own, especially while it is still very young. ...
With the growth of foresight there is an increasing tendency to exploit children's affections for the sake of their help when old age comes. Hence the principle of filial piety, which has existed throughout the world and is embodied in the Fifth Commandment. With the development of private property and ordered government, filial piety becomes less important. ...
This, of course, applies chiefly to the propertied classes ; among wage-earners the older relationship persists. But even there it is being gradually displaced as a result of old-age pensions and similar measures. )