Are there any moral arguments against non-coercive incest between adults?

There is, of course, the genetic issue. So, sexual relations between close relatives that lead to procreation are unwise. Incestuous relations with one's underaged children are, of course, by definition non-consensual. One also finds the same argument that is deployed against homosexual marriage used to justify incest prohibitions, namely that incest would undermine the institution of marriage, and that the institutions of heterosexual, non-incestuous family and marriage possess value that trumps the value of legitimating incestuous as well as homosexual unions. Many have come to think that it is false that homosexual marriages would undermine the institutions of marriage and family. That's an empirical question rather than a philosophical question, and I tend to think the reformers are correct an that family and marriage will in fact flourish when homosexuals are included. Some think undermining the institution of marriage may be a good thing. For myself, I think marriage has value, but I also think alternative forms of association may serve many people just as well. Time may come when people reach similar conclusions about incest. I do have some concerns that legitimating incestuous relations among consenting adults may damage the institutions of family and marriage. In fact, incest prohibitions seem to be essential to defining family. But, again, the extent to which family can be modified before it collapses is empirical, not strictly a philosophical question. The philosophical principles operant here, as I see them, are these: 1. The Liberty Principle: the liberty of consenting adults to engage in whatever conduct, including sexual conduct, ought to be maximized. 2. The Social Goods and Virtues Principle: those goods and virtues that are cultivated by the institutions of family and marriage are indeed valuable things and ought to be cultivated (e.g. intimate bonds of affection, the love, security and guidance of children, care for the sick and elderly). Family and marriage are not, in other words, intrinsically valuable. They are valuable because of the goods and virtues that they sustain. It is possible that other forms of social relation or expanded definitions of family and marriage may also sustain those goods and virtues, but I think we do need to respect the fact that family and marriage has proven successful over time and space in delivering the goods. So, my conclusion is that the Liberty Principle ought to be followed maximally, though constrained by the limits established by the Social Virtues Principle. My sense is that both marriage and family can be modified and expanded in the interests of the Liberty Principle beyond their current forms and that they can be supplemented by alternative forms of association, perhaps even by some forms of incest. Nevertheless, I suspect that some limits to the Liberty Principle's application will prove prudent and proper.

A footnote to Peter Fosl's sensible response.

The trouble, of course, is in the talk of 'non-coercive' incest. Where different generations are concerned -- father and adult daughter, for example -- it would be naive to suppose that the younger party, who may think she is freely consenting, isn't in many cases subject to subtle coercion. And even if, leaving the issue of potential offspring out of it, there is nothing morally wrong with genuinely non-coercive incestuous relations between adults, it could well still be a bad thing if people generally believed that to be so (for the belief, by relaxing the received taboo, could have the bad effect of creating a context in which subtly coercive incestuous relations become very much more common).

This is an interesting phenomenon in moral thought more generally, it seems. There can be cases where it might be permissible to do X but it would be a bad thing if people (including ourselves) generally thought it is permissible to do X -- e.g. because we just can't be sufficiently trusted to judge when something is a case of X of the permissible sort. Think about e.g. examples where doing X involves acts of euthanasia.

Back to incest: there however surely can be genuinely non-coercive cases. For example, the brother and sister separated near birth, brought up by different adoptive families, who later meet and -- ignorant of their blood relationship -- start sexual relations. It happens. Their initial incestuous relationship surely didn't involve any wrong-doing on their part. And what if they continue, after the discovery, in a knowingly incestuous relationship (though, now appreciating the genetic dangers, abandoning plans to have children together)? I can see no decisive reason for supposing that this is wrong, or indeed for supposing that there is a danger in thinking this sort of really rather special case of an incestuous relationship isn't wrong.

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