As Frazer says: "There is no law commanding men to eat and drink or forbidding them to put their hands in the fire. Men eat and drink and keep their hands out of the fire instinctively."
If men and women avoided incest instinctively no legislation would be needed compelling them to avoid it.
Indeed the confessions received by psychoanalysts reveal that the first sexual desires of the young are directed toward children of the opposite sex within the family circle. The many slight or serious indiscretions of an incestuous nature in which neurotic brothers and sisters indulge in infancy and childhood are generally "forgotten," that is, repressed, in later years, but analytic probing brings a great amount of such repressed material to the surface.
Since neither animals nor human beings experience any natural fear of incest, why is it that all races are officially so afraid of it?
Inbreeding. It cannot be due to the fear of race deterioration consequent upon inbreeding. Inbreeding is not necessarily a harmful process of reproduction as East and Jones have shown in their book on "Inbreeding and Outbreeding." It seems to have, at times, for instance in Athens during the classic age, led to the production of many very superior individuals.
Furthermore the primitive savages who punish incest even among domestic animals have no conception of such eugenic theories. Some of them, incredible as it may sound, do not even realise the relation of cause to effect which exists between intercourse and pregnancy.
Freud offers an explanation based upon the Darwinian hypothesis of the primal horde in which the old father kept all the females for himself and drove away the growing sons.
This state of affairs has been observed among herds of wild cattle and horses. It generally leads to the killing of the oldest bull or stallion by the younger males.
The Primal Horde. Freud assumes that this must have been the usual occurrence in the primal horde. One day the sons joined hands and killed the father.
"Though the brothers had joined forces in order to overcome the father, each was the others' rival among the women. Each one wanted to have them all to himself like the father, and in the fight of each against the others the new organization would have perished. For there was no longer any one stronger than all the rest who could have successfully assumed the role of the father. Thus there was nothing left for the brothers to do, if they wished to live together, but to erect incest prohibitions, perhaps after many difficult experiments, in the course of which they may all have renounced the women whom they desired."