Freud opens by linking the earliest, most rudimentary human societies, contemporary primitive societies, and neurotic individuals (including children). He does this on the basis that these three groups share the same primitive tendencies and repressions. He observes that one may expect such early societies and contemporary 'savages' to indulge freely in all type of sexual relations, but that upon closer inquiry one discovers that this is most emphatically not the case and that these societies have an elaborate system geared towards the prevention of incest:
Indeed, their whole social organization seems to serve that purpose or to have been brought into relation with its attainment.
Freud then goes on to use Australian Aborigine tribes (supposedly the most primitive groups to survive into modern times) to illustrate this important point. He discusses the prevalence of totemism among such tribes. Each tribe identifies with a particular totem, or animal (more rarely a plant) and their ties with this animal are held to be even stronger than blood ties. Sexual contact between individuals with the same totem is strictly forbidden and incurs punishment by the entire community, suggesting that it is regarded in some way as threatening to the whole tribe.
Freud moves on to discuss similar practices among primitive tribes in other parts of the world, such as Africa. He notes at length the avoidance rituals that many of these groups practice among family members. These rules concern men and their mothers-in-law in particular.
Freud ultimately relates all of this back to his own ideas and observations concerning infant sexuality: that infants first turn their desires on members of their own family, particularly boys who focus on their mothers. Such infantile impulses, although repressed, still persist in neurotic adults, who either have never got over such desires or have regressed back to them. The general horror of incest, then, Freud argues, has its origins in the early life of the individual and this has influenced the development of societies.