Argument: Incest taboo is due to evolutionary inclination for variation
Larry Arnhart. "So What's Wrong with Incest?". Darwinian Conservatism. October 27, 2006 - In his book The Variation of Animals and Plants Under Domestication, Darwin has a chapter "On the Good Effects of Crossing, and On the Evil Effects of Inbreeding." He surveys the experience of animal breeders in discovering the bad effects of inbreeding. And he wonders whether natural selection could have shaped a natural aversion to incest among human beings. "Although there seems to be no strong inherited feeling in mankind against incest, it seems possible that men during primeval times may have been more excited by strange females than by those with whom they habitually lived. . . . If any such feeling formerly existed in man, this would ahve led to a preference for marriages beyond the nearest kin, and might have been strengthened by the offspring of such marriages surviving in greater numbers."
In Xenophon's Memorabilia (IV.iv.19-23), Socrates identifies the "unwritten laws" legislated by the gods as laws that could not be disobeyed without natural penalties. He speaks of the incest taboo as one of those "unwritten laws," because those committing incest tend to produce defective offspring. Darwin's evolutionary explanation of this would illustrate, then, how Darwinian science might support the traditional idea of "natural law" or "natural right." (I have argued this in a book chapter: "The Incest Taboo as Darwinian Natural Right," in Arthur Wolf and William Durham, eds., Inbreeding, Incest, and the Incest Taboo: The State of Knowledge at the Turn of the Century [Stanford University Press, 2005].)
Edward Westermarck elaborated Darwin's reasoning for the biological evolution of the incest taboo in his book The History of Human Marriage (first published in 1889). Westermarck's theory can be summarized in three propositions. First, inbreeding tends to produce physical and mental deficiencies that lower Darwinian fitness. Second, as a consequence, natural selection has favored an emotional disposition to feel a sexual aversion to those with whom one has been raised in early childhood. Third, this natural aversion to incest creates moral disapproval that is expressed as an incest taboo.
This put Westermarck in conflict with Sigmund Freud's Oedipal theory of human psychology and culture, because Freud insisted that the inclincation to incest was natural to human beings, and that the taboo against incest arose as a purely cultural construction that human beings created to repress their natural desires for incest, this cultural repression of human nature being necessary for civilization. So morality, as Freud understood it, required a conquest of human nature by human culture. By contrast, Westermarck believed that morality was a cultivation of natural human emotions, so that the incest taboo was a cultural expression of a natural human disposition shaped in human evolutionary history.
Allen Koay. "Incest: Age-old taboo". The Star Online. May 5, 2008 - what about the animal kingdom? The famous Jane Goodall observed that chimpanzees exhibited incest avoidance behaviour, where mothers do not allow their male offspring to mate with them, sisters do not mate with their brothers, and females do not mate with older males in their familial group.
Researchers in Britain and Germany found that male hyenas typically are forced to leave their birth group, a move that ensures minimal or no in-breeding.
But according to senior veterinarian Dr P. Vanaja, animals do not recognise the definition of familial lines, and therefore would not know how to avoid incest behaviour.
“Incest happens a lot between animals, especially cats and dogs,” she explained. “Take for example, elephants. They move in male and female groups, and when they come together to mate, they do not know who their parents or siblings are.”
According to Prof Datuk Dr Shamsul Amri Baharuddin, director of UKM’s Institute for Ethnic Research, the creation of lineage and clans is the result of families creating boundaries and avoiding incest.
He said there were tribes and peoples in Papua New Guinea and Africa who practise what he calls “internal reproduction”. This was to ensure the survival of their tribes which were very small in number. “But that was a long time ago, and it was only over a few generations,” he said. “They did not see it as incest. It was more of a way of survival for them. This was especially so with the hunters and gatherers who moved in groups of 20 or 30 families.”