The standard argument for why there is aversion to incest among humans as matter of innate disposition is the Westermarck effect, which is a model where aversion to mating emerges if you are raised with an individual of the opposite sex. Some basic illustrations are sketched out in The Blank Slate: The Modern Denial of Human Nature. But some comments below make me wonder if there is are alternative explanations. Robin Fox has made the claim, repeated in many places, that cousin marriage was ubiquitous in the human past:
As an anthropologist I am forced to face the fact that for the vast majority of our existence as a species close cousin marriage must have been the norm, if for no other reason than that most of the time there was no one but cousins to marry. Indeed I have spent much of my professional life analyzing the complexities of ...
