Thinking about Incest 9: Avoidance and Taboo

Suppose that some version of the Westermarck Effect exists, so that intimate childhood association inhibits sexual attraction later in life. What is the relationship between such an effect, and the social rules and conventions in place constituting incest taboos? One of the in-house disputes amongst those adopting a biosocial approach to inbreeding, incest, and incest avoidance focuses on just this question.

Westmarck himself seems to think of the relationship as being fairly direct. Summarizing his overall view, he says that

Taking all these facts into consideration, I cannot but believe that consanguineous marriages, in some way or other, are more or less detrimental to the species. And here, I think, we may find a quite sufficient explanation of the horror of incest; not because man at an early stage recognized the injurious influence of close intermarriage, but because the law of natural selection must inevitably have operated. Among the ancestors of man, as among other animals, there was no doubt a time when blood-relationship was no bar to sexual intercourse. But variations, here as elsewhere, would naturally present themselves; and those of our ancestors who avoided in-and-in breeding would survive, while the others would gradually decay and ultimately perish. Thus an instinct would be developed which would be powerful enough, as a rule, to prevent injurious unions. Of course it would display itself simply as an aversion on the part of individuals to union with others with whom they lived; but these, as a matter of fact, would be blood-relations, so that he result would be the survival of the fittest” (Westermarck, History, p.352).

He then considers exogamy to be “a natural extension of this instinct” (p.353), suggesting that incest taboos arise, in some way, from the instinctive aversion, and not vice-versa. Durham puts this in terms of what he calls the expression hypothesis,

the claim that the aversion directly causes the incest prohibitions of human societies; in Westermarck’s words, ‘aversions which are generally felt readily lead to moral disapproval’, and do so by ‘displaying’ or ‘expressing’ themselves ‘in custom and law as a prohibition of intercourse between near kin’” (Durham 2004, p.122, quoting from Westermarck 1922—5th edition of his 1891–, pp.198 and 193, respectively)

Durham takes issue with this aspect of the Westermarck Effect, offering an alternative to this expression hypothesis about the relationship between psychological aversion and incest taboos.
Durham introduces evidential inconclusiveness for the Effect as a reason to be cautious here, but his primary reason for rejecting (or remaining skeptical about) the expression hypothesis is that he thinks there is a more plausible hypothesis, what he calls the Burton problem. Stated as a thesis, this is the claim that while childhood intimacy leading to behavioural and psychological aversion is one causal factor that eventually leads to moral disapproval and finally incest taboos, another is the recognition that inbreeding is rare but often deleterious, and some kind of socially accepted theory about this (see Durham 1990, p.319, Table 6.3). Here feelings of aversion are not direct causes of incest prohibitions, and incest prohibitions not the direct expression of those feelings; rather, there is a two-step cognitive basis for those prohibitions. First, there is the recognition of the higher frequency of “defective infants” born from incestuous sex. Second, there is the attribution of that outcome as punishment from the Gods. In light of these attributions, people come to feel an aversion to incestuous sex, and to morally disapprove of it. Durham recognizes the need to generalize both stages of the Burton process so “(1) that local peoples recognize at least some adverse consequences to inbreeding, and (2) that they culturally interpret them as harmful and potentially threatening to everyone” (2004, p.130). (1) would allow not just heritable effects, but also effects of traumatization on victims; (2) subsumes cultural interpretations as diverse as contemporary theories of genetic encoding and wrath-of-God explanations.

One kind of evidence for the Burton hypothesis is that folk accounts of (2) are widespread. Although this is a cognitive account in terms of the kinds of processes it appeals to, it is also compatible with the resulting theories being wildly mistaken, and also with a mismatch between observable correlations between inbreeding and defect, and the scope and nature of any resulting incest taboo. The real problems for this account are threefold:

  • for the recognition to take place, inbreeding would have to be a, if not the, leading cause of birth defects. Whether this is so depends on a number of factors, including the frequency of inbreeding, environmental tetragens, and other developmental causes of observable defectiveness.
  • Durham and Burton both say that (1) presupposes that infra-familial incest is rare, but then the question is why would that be?—don’t we have to explain that? Circularity #1.
  • the cultural interpretation in (2) seems likely to already presuppose the moral disapproval that it is meant to explain. An undesirable outcome is observed (birth defect). God’s punishment is invoked to explain this outcome, and that punishment is for doing something bad: engaging in incestuous sex. But for an appeal to be made to the moral notion of punishment, the act to be punished has to already been viewed as morally bad, i.e., morally disapproved of. But we’re trying to explain to origins of the moral disapproval of incestuous sex. So this account begs-the-question. Circularity #2.

Can some of this be put in terms of a dilemma on the frequency of incest? If incestuous mating is frequent, then one has lots of birth defects that are a result of inbreeding, but it seems also less likely that (i) people will be able to identify incest as the cause, and (ii) god(s) will be invoked as punishers for everyone. If incestuous mating is infrequent, on the other hand, then we need an explanation of why that is so. It can’t be because there is a taboo against it (since that is what we are trying to explain), and it can’t be because there is an aversion to it (since the rarity is meant to explain the aversion).

There’s also a question about how deep the difference is between the expression hypothesis and the Burton thesis. In his diagram depicting the primary differences, Durham has aversion as an input in the Burton thesis, whereas in quotes from Burton himself, it doesn’t seem that at least those feelings play much of a role, or, if they do, they come in later on. But if psychological aversion features in both hypotheses, is the difference between them just a matter of whether

• aversion → moral disapproval → taboo
• (aversion? →) recognition + cultural interpretation → moral disapproval → taboo
• recognition + cultural interpretation (→ aversion?) → moral disapproval → taboo?

That makes the expression hypothesis, which I take to be captured by the first of these, sound like a straw man position. And the difference between the other two seems fairly subtle.

Conversely, Durham has deleterious effects of inbreeding play a role in the expression hypothesis, but I’m not sure this is right from my reading of Westermarck. In response to the question “Why do humans have an aversion to incestuous sexual relations in their own lives?” the phylogenetic answer is something like “Because that aversion is innate” or “Because we inherited that trait from our early hominid ancestors”; the Westermarck Effect provides an ontogenetic answer to that same question. Whether inbreeding has deleterious effects seems irrelevant to both of these answers, unless it adds to the phylogenetic answer by saying something about why we find incest avoidance in other animals. But I suspect that we really want to focus almost exclusively on nonhuman primates here, since what we want to explain is not just any old aversive mechanism but a mechanism that is recognizably psychological in nature. (If insects avoid close inbreeding, then this behavior is mediated by something other than the kind of rich psychology that primates and humans have.)


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