See Westermarck, Origin and Development of the Moral Ideas, vol. i, pp. 386-390, 522.
Westermarck, Origin and Development of the Moral Ideas, pp. 9, 159; also the whole of Ch. VII. Actions that are in accordance with custom call forth public approval, actions that are opposed to custom call forth public resentment, and Westermarck powerfully argues that such approval and such resentment are the foundation of moral judgments.
This is well recognized by legal writers (e.g., E. A. Schroeder, Das Recht in der Geschlechtlichen Ordnung, p. 5).
W. G. Sumner (Folkways, p. 418) even considers it desirable to change the form of the word in order to emphasize the real and fundamental meaning of morals, and proposes the word mores to indicate "popular usages and traditions conducive to societal reform." "'Immoral,'" he points out, "never means anything but contrary to the mores of the time and place." There is, however, no need whatever to abolish or to supplement the good old ancient word "morality," so long as we clearly realize that, on the practical side, it means essentially custom.