I am just finishing up the 30th Anniversary edition of Richard Dawkin’s “The Selfish Gene”. (I believe that I have already finished the original edition, but the 30th ed. has two extra chapters.) Taken purely as a text on evolutionary biology (and nothing more philosophical than that), it is an excellent work. I will probably have more to say about it in a formal book review once I finish it, but I wanted to note one fact that I discovered that I found very interesting. If correct, I think it gives a definitive and rational answer to the question: why not have sex with your relatives?
In chapter 6, titled “Genesmanship”, it discusses, on page 99, and in an endnote on that page, the high probability of brother-sister matings leading to serious genetic diseases for their children. In fact, the footnote says that each person caries, on average about 2 recessive “lethal genes” per person. The gene is “recessive”, which means it doesn’t manifest itself unless you get a copy from both your mother and your father. Normally, because of out-breeding (non-relative breeding), a child will only get one recessive of any given lethal gene, so he will be safe. But, since brothers and sisters have a high probability of getting the same lethal gene from their parents, the chance of a child from a brother-sister breeding getting two copies of the same recessive lethal gene turns out to be about one in eight. That means if you have 8 children, you are likely to have at least one child with a serious genetic problem that will result in early death. (I was vaguely aware of the fact that this was the reason that brother-sister breeding was bad, but I had no idea the probability of death for their children was that high.) To draw an analogy, a brother-sister breeding would be like taking a revolver with 8 chambers, putting a bullet in one of the chambers, and then playing “Russian Roulette” with your kid.
To me, this had to be a major reason for the “incest taboo” in most human cultures. (I say most, because there are examples of brother-sister breeding, such as the Pharaohs in ancient Egypt.) Pre-historic people probably would have “quickly” noticed that brother-sister breeding led to the death of more children than non-brother-sister breeding. (When I say “quickly”, I mean over the course of decades.) (And, yes, I am assuming that people want to have children, but even the minority of people who don’t are going to be raised in a cultural environment that views brother-sister breeding with great revulsion for this reason, and are therefore probably going to view it with revulsion themselves.)
FYI-Dawkins seems to suggest that this fact would lead to an “instinctive” revulsion by people against incest, but I regard human beings as a “tabula rasa” at birth, and I am not sure that this is consistent with the idea of “tabula rasa” at birth, so I think the taboo would have arisen through observation, not due to any sort of “innate knowledge”. (I will probably have more to say on “instincts” and the idea of “tabula rasa” when I write a book review of “The Selfish Gene”.)