The interest that anthropologists have shown in the Incest Taboo has been described as bordering on an obsession. It used to be believed that the Incest Taboo is a universal taboo. Some anthropologists have, however, successfully contested this view, pointing out that what appears universal to human societies is the avoidance of sexual relationships between very closely related individuals. It has been shown that in many cultures in which the Incest Taboo is not formally expressed, close relatives still seem to avoid sexual relationships, especially when such relatives have lived together most of their lives. Anthropologists have noted, also, that human societies tend to invest more effort in spelling out Incest Taboos with respect to what has been referred to as "fictive kin-ships."
The Westermack effect refers to the observed tendency of closely related people, especially closely related people who have lived continuously together, to avoid sexual relationship. The Westermack effect, however, raises some fundamental questions about patterns of heterosexual pairing in pre-historic human societies.
It is estimated that the total global population of human beings was less than 10 000 some 60 000 years ago, having witnessed a significant bottleneck. It is also generally thought that prehistoric human societies were small and scattered with many small societies living, relative to modern day standards, in almost complete isolation. It has been pointed out, by social scientists, that even as late as the 1800s, most people lived and died in the societies in which they were born and hardly ever traveled out. Many modern day rural societies are very small, with populations of just a few hundreds to a few thousands. It is thought that most prehistoric human societies were even smaller and more isolated. This raises the significant question: how did the so-called Westermack effect impact on human heterosexual behavior in prehistoric societies? If pre-historic human population sizes were very small and isolated, how did people avoid incest under the assumed compulsion of the Westermack effect?
It is difficult to believe that the Westermack effect could have been so compelling as to prevent biologically close relatives from inbreeding in pre-historic times. Even in historic times, we find that populations that are socially, rather than physically isolated, tend to interbreed. Incestuous marriages were common among the upper classes of Ancient Egypt, for instance, especially in the royal families. These royal families were not physically isolated from the rest of the population, only social class factors isolated them sufficiently to resort to incestuous pairings. The same pattern is observed among the royal families of Europe.
It would appear, therefore, that isolated human populations resort without compunction to what would be termed incest in the culture of larger interbreeding populations of people. We have enough reason to believe that categories of heterosexual relationships we would now term incest must have been relatively common in prehistoric societies.
There is evidence to believe that mother-son incestuous paring had been part of the fertility magic ritual of prehistoric Paleolithic man, and that such incest was common enough to impact significantly in human evolution, especially with regard to juvenilization of the human specie. The circumstantial evidence for this conclusion is not only in the widespread myths of mother-son incestuous divine pair in early historic Mediterranean world, but also from a detailed examination of the heterosexual dualistic philosophy of fertility cults. It would appear that the intensity of inbreeding associated with incestuous cultic rites had played an important role in the stabilization of the human genetic pool.