the evolution of moral taboos

A piece in The Times of July 15th 2008 was headed "I used to have sex with my brother but I don't feel guilty about it."
"Incest is seen as bad," it opined, "but that isn't always the case."

Then on July 22nd, a follow-up article, "Sibling incest: what you think" canvassed the newspaper's readers. "Good on you!" said one. "This story highlights the absurd and arbitrary moralising taboos that our society likes to construct around sexual relations."

It is often thought that moral taboos, such as the incest taboo, are arbitrary social constructs with no adaptive significance, a form of social control that post-dates social organisation.
Example: Consensual homosexuality does no harm to anybody and its persecution is thus morally indefensible. The function of the taboo seems to be to enforce a particular conception of 'normal' sexual and social arrangements, often citing the approval /disapproval of a supernatural deity for its ultimate authority. But it is not a coincidence that homosexual sexual activity does not produce children.
Obviously, any ancestral society of homosexuals would have died out after one generation. So in any species that proliferates by sexual union between two genders, homosexuals must always be a mathematical minority. The inevitable way that ‘not being homosexual’ will manifest itself to the bearer of that trait is in a sense of physical recoil. Just as inevitably, this will be codified in brains that do not perceive themselves as machines but as thinking agents as a moral objection.

We can choose to dismantle some taboos, but we are mistaken if we think either that taboos are merely the random, deliberately nasty products of censorious religious minds, or that we can extrapolate from the case of homosexuality or any similar example the maxim that all taboos are worthless and should be consigned to the same fate.
The thing to realise is that moral taboos are metaphors for facts of evolutionary biology.
Many, many children would die without evolved instinct, if their parents had to tell them everything – don’t touch fire, don’t play with spiders, don’t kiss crocodiles. Phobias and instincts are instant, evolved parents.
Taboos work the same way. Some we have outgrown, and can lay down. But a lot of conditioning needs to be overridden, and we should perhaps be more generous to those of earlier generations who find this hard.

It should be obvious that to pre-Darwinian cultures with inherited evolutionary instincts but no scientifically accurate means of rationalising them, the obvious, perhaps only way of doing so was to make ‘sins’ of non-beneficial traits and practices.
The situation is clearer if we look again at the taboo of incest.

This is instructive in that it is still standing, which is to say the idea of it still provokes distaste and unease even in such sections of society for whom opposition to homosexuality is considered unacceptable in any degree. But viewed objectively, the two are identical: both minority sexual practices which, if pursued consensually, do no immediate harm to those who practice them or anyone else.
We may account for why some people are repulsed by both, and for why some people are repulsed by neither. But why should some be repulsed by one and not the other? This is the value to us of the incest taboo: we have caught it in a moment of transition, at a point where evolved, 'superstitious' objection is having to hold its ground against rational interrogation. In this collision we see the way in which morality and sociobiology have always worked in secret alliance.

Incest can produce children, but the genetic similarity of the parents vastly increases the risk of mutation, genetic defects and inherited abnormalities and diseases. Thus, since surviving members of species will come overwhelmingly from parents who chose their partners outside the family circle, so the desire to avoid incest will proliferate within a species, since the ‘non-incestuous gene’ proved the more advantageous and ‘won’ the competition between the two kinds of sexual desire.
But it is important to remember that while the result was determined by a mere impersonal quirk of biology (inbreeding reduces reproductive success), the actual ‘battle’ for reproductive success and therefore proliferation was between kinds of desire, not views on genetic evolution. So what is being handed down from generation to generation is the tendency for the organism to consciously reject incest, not an unconscious prohibition of it at the genetic level.

Thus, inevitably, we arrive at level two: the instinct by which this trait manifests itself to the bearer - as a physical, almost emotional recoil from the thought of incestuous sexual relations.
This instinct, divorced entirely from any awareness of its scientific raison d’etre, which the bearer at most stages of human history would in any case have been completely unable to grasp, thus becomes self-justifying, and is formalised as level three: the moral taboo: incest is wrong. (This has the additional effect of providing a strong social disadvantage – stigma – to prevent even those who do desire it from acting upon their desires.)
At this conscious, unscientific level, as usual, God is evoked. It is a sin to lie down with one’s mother or brother. What other way could a ‘sin’ be justified if not by appeal to external authority?

The second half of the twentieth century has seen war declared on both stigma and taboo. Interestingly, unlike homosexuality, the view that incest is morally wrong, even when strictly consensual and among adults, persists even in secular ethics. (This is what makes it so valuable as a demonstration model). But it is hard to see it standing for long against a sustained campaign that argues it is an unconscionable oppression to restrict the sexual freedoms of consensual adults who can now, of course, start families in one of numerous artificial ways. As with homosexuality, the old moral taboo is left, as it were, hanging in mid-air, no longer supported by biological necessity. 
What objection remains other than that nasty old free-floating taboo? If we are honest, we have to say nothing, and if that still leaves us feeling uneasy, we have to own to that, too.