Taboos are very ancient prohibitions which at one time were forced upon a generation of primitive people from without, that is, they probably were forcibly impressed upon them by an earlier generation. These prohibitions concerned actions for which there existed a strong desire. The prohibitions maintained themselves from generation to generation, perhaps only as the result of a tradition set up by paternal and social authority. But in later generations they have perhaps already become “organized” as a piece of inherited psychic property. Whether there are such “innate ideas” or whether these have brought about the fixation of the taboo by themselves or by coöperating with education no one could decide in the particular case in question. The persistence of taboo teaches, however, one thing, namely, that the original pleasure to do the forbidden still continues among taboo races. They therefore assume an ambivalent attitude toward their taboo prohibitions; in their unconscious they would like nothing better than to transgress them but they are also afraid to do it; they are afraid just because they would like to transgress, and the fear is stronger than the pleasure. But in every individual of the race the desire for it is unconscious, just as in the neurotic.- Freud, "Totem and Taboo"
The oldest and most important taboo prohibitions are the two basic laws of totemism: namely not to kill the totem animal and to avoid sexual (intercourse with totem companions of the other sex.