Thinking about Incest 8: Primate Evidence and Anthropology

In the last post, I introduced a “Westermarckian cluster” of views about incest and its avoidance, the first two of which were:

  1. incest avoidance is widespread in the nonhuman animal world, including amongst nonhuman primates
  2. humans and their closest primate relatives alike have a natural psychological aversion to incest that plays a causal role in both incest avoidance and, in the case of humans, incest taboos

and which I want to take up in this post. In earlier posts (like Thinking about Incest 5), I have spelled out more precisely how I think 2. should be understood, and here I want to focus on 1. I draw chiefly on three recent reviews of the relevant primate literature: the comprehensive review of Andreas Paul and Jutta Kuester, “The Impact of Kinship on Mating and Reproduction” (in Chapais and Berman, Kinship and Behavior in Primates, 2004), and the more succinct summaries of Anne Pusey “Inbreeding Avoidance in Primates” (in Wolf and Durham, Inbreeding, Incest Avoidance, and Incest Taboos, 2005) and Bernard Chapais, Primeval Kinship: How Pair-Bonding Gave Birth to Human Society (2008).

Primatology is a relatively young discipline, with study of the nature and structure of primate societies being undertaken systematically only in the past 40 years. Since then, documentation of the rarity of reproductive sex and mating between at least some close relatives has provided strong evidence for the existence of one or more mechanisms facilitating this outcome. One of those mechanisms relates intimate association during the pre-reproductive years of at least one member of a dyad to later sexual preferences and behavior during the reproductive years of an individual, i.e., it is just the sort of mechanism posited by Westermarck. Westermarck himself, writing long before anything but anecdotal reports of primate behavior were available, and appealing somewhat casually to other animal data, did not explicitly claim that such a mechanism existed in nonhuman primates. But those who dismissed Westermarck’s claims about incest avoidance in humans did so in part both by professing a confidence about the existence of nonhuman primate incest that was misplaced and, correspondingly, by claiming incest avoidance as a uniquely human innovation that they attributed to one or another specifically social, political, or cultural innovation. Levi-Strauss, writing originally in 1949, tells us, for example, that

[w]hatever the uncertainties regarding the sexual habits of the great apes, and the monogamous or polygamous character of the gorilla and chimpanzee family, it is certain that these great anthropoids practise no sexual discrimination whatever against their near relatives. (Levi-Strauss, Elementary Structures, p.31)

It is unclear on what the “certainty” that Levi-Strauss expresses here is based, and in any case subsequent primatology has shown this view to be badly mistaken.

While Levi-Strauss’s claim here is especially brazen, the view that he expresses about the uniqueness of incest avoidance to hominids (if not to Homo sapiens more specifically) was widespread amongst those theorizing about incest, incest avoidance, and incest taboos in cultural anthropology in particular but in the social sciences more generally until the last 10 years. Operative here was the long reach of the psychoanalytic views of Sigmund Freud, according to whom, famously, the “first choice of object in mankind is regularly an incestuous one, directed to the mother and sister of men, and the most stringent prohibitions are required to prevent this sustained infantile tendency from being carried into effect” (Introductory Lectures on Psychoanalysis, 1920, p.344) and who, like Levi-Strauss, ascribed a special role to incest taboos in marking the transition from creatures with a merely animal nature to those equipped with, and shaped by, full human culture. Repeatedly over the ensuing 50 years, incestuous tendencies were assumed to pervade the animal world, with traces found in primitive forms of humanity, and with the struggle to overcome them representing the eventual rise, triumph, and advancement of civilization and human culture. One pillar on which this view sits—evolutionary primitivism—has long been abandoned in cultural anthropology, but the crumbling of other pillar on which it rests—the assumption that nature is incestuous—has been either band-aided or simply ignored.

Although that assumption is mistaken about large parts of the sexually reproducing world, I am concerned here only with its falsity with respect to that part of the sexually reproducing world phylogenetically closest to Homo sapiens: the nonhuman primates. That a review of the evidence here not only shows this but provides positive support for a Westermackian hypothesis of some kind, is made clear in the summarizing conclusions of the recent literature reviews of both Andreas Paul and Jutta Kuester, and Anne Pusey:

Contrary to earlier assumptions, a wealth of evidence now indicates that kinship plays a profound role in regulating patterns of mating and reproduction among not only human but also nonhuman primates. Given that inbreeding is, under most circumstances, costly, the existence of evolved psychological mechanisms designed specifically to solve the problem of close inbreeding avoidance (as has been first proposed by Edward Westermarck) is not at all surprising. (Paul and Kuester 2004, p.284)

Nonhuman primates provide abundant evidence for an inhibition of sexual behavior among closely related adults. This finding is consistent with the idea that inbreeding avoidance behavior is a naturally selected behavior that was already present among animals before humans evolved. The primate data support Westermarck’s theory that familiarity during immaturity is a major reason for this avoidance. Mating among adults is most inhibited among maternal relatives in species in which these have close associations. (Pusey 2005: 80)

While here Paul and Kuester appeal simply to evolved psychological mechanisms, and Pusey to familiarity during immaturity as major causes of inbreeding avoidance in nonhuman primates, the broader view they sketch of the primate data support more specific claims made about that avoidance. More particularly, that evidence supports the claim that the rule in (*) holds of mother-sibling dyads throughout the primate lineage.

(*) if you develop care-related attachment through intimate childhood association with an individual, feel contrasexual aversion towards that individual when you are both sexually mature,

where “intimate childhood association” requires only that at least one of the members of the dyad be in childhood or sexually immature.

As Bernard Chapais (2008) has argued in a more general discussion of primate kinship, such maternal dyads are of special significance for extensions of effects they manifest to maternal kind. Of most significance here is that care-based attachment between maternal dyads can serve as the basis for the extension of that attachment to maternal siblings, whose common attachment to the mother through the extended period of their dependence on her significantly increases the chance that they will develop a form of attachment to each other. Since all siblings remain in their natal group for an extended period of time, the conditions exist on the input side of (*) for maternal siblings, as well as for mother-offspring dyads. They are not, in general, satisfied either for paternal siblings or for father-offspring dyads, although there are some instances (e.g., among monogamous gibbons and marmosets) where conditions for both the recognition of paternity and the pair-bonding between adult mates create the possibility of the extension of the attachment relation to these dyads as well. Even where there is polygyny with a single dominating male, such as with gorillas, father-daughter sex is “strongly inhibited” (Pusey, p.66).

What I am claiming here is that (*) applies, in the first instance, to maternal dyads, and then derivatively to maternal sibling dyads, throughout the primate lineage. If this is correct, then running phylogenetically deep in human ancestry is a form of contrasexual aversion due to care-based attachment established through intimate association during the childhood of at least one member of such primate dyads. Moreover, one can readily understand how this dyadically restricted form of the Westermarckian rule expressed in (*) could be readily extended to father-offspring dyads through the advent of certain kinds of innovations to group living, such as pair-bonding, monogamy, and single-male dominance and the corresponding recognition of paternity. While such conditions are occasionally found amongst primates, they are not homologous features of our primate heritage, and because of this, the antecedent of the rule in (*) is not satisfied in general by such dyads in nonhuman primates.

There are several questions about the relationship between this Westermarckian mechanism and a well-established fact about nonhuman primate life at sexual maturity, the first of which poses an objection to the account defended here. One nearly (if not completely) universal feature of the 200 or so species of primates is the geographical sex-based dispersal of offspring from the natal group upon sexual maturity. Such dispersal is not unique to primates but it is especially widespread in the primate lineage. Here most (though sometimes not all) of the offspring of one sex—females in the case of baboons, macaques, and capuchins, males in the case of chimpanzees, bonobos, and spider monkeys—disperse from the natal group upon reaching sexual maturity. Pusey notes that “most biologists believe that this pattern has evolved at least partly as an inbreeding-avoidance mechanism” (p.62). If that is so, then the question is: if that is so, then why would there be a need for a care-based attachment mechanism like that in (*) as well? And with opposite-sex siblings and offspring no longer present in the natal group, what opportunities would that mechanism have to operate, and therefore be selected for, supposing that it emerged in one or more individuals in the population in the first place?

Chapais (2008) suggests that dispersal patterns and “the Westermarck Effect” are “two aspects of the same process” (p.65), which would provide one kind of answer to these questions. But instead of explaining this he enters immediately into a discussion of developmental familiarity, and so the Westermarck Effect, as being a matter of degree (p.66). At one point, he seems to suggest that the preference for outgroup sex partners via the Westermarck Effect “would explain why all males leave their natal group in species where females are resident and why all females leave their natal group when males are resident” (p.66), making Westermarck Effect-generated preferences sound more fundamental than dispersal. I think it’s likely to be the other way round: sex-biased dispersal patterns of behavior would cause and so explain the preference for outgroup sex partners. The problem with it being “exogamy preference first, dispersal as a result” is that it’s hard to see how the knowledge to disperse to other groups could readily come about as a result of the preference for non-local mating partners, together with other knowledge they acquire in their natal group. How does that preference result in that knowledge? Why that action?

If that is right, then the preference to disperse to other groups must be what, in Boundaries of the Mind, I call strongly nativist: the environment plays at most a mere triggering role, and there is a rich internal structure to the mechanism, e.g., it is a module in something like the classic Fodorian sense. But if that is so, then Westermarck Effect-generated preferences for outbreeding could play the role merely of inputs to this innate mechanism. How much this compromises this part of the picture that Chapais sketches is something I’ll have to think about more.


4 thoughts on “Thinking about Incest 8: Primate Evidence and Anthropology

  1. Pingback: Thinking about Incest 11: Saving the Viennese Witchdoctor « What Sorts of People

  2. Sheila,
    Yes, I read this paper by Shor and Simchai not long after it came out. It’s interesting. It’s first (critical) half brings together reservations about both the primatological (briefly) and human correlational studies that have, for the most part, been made by others. The second half, where they interview 60 representative folk from the kibbutz settlements, I need to look at more closely, both for the way in which subjective retrospection is used, and for the support that’s offered for the alternative Durkheimian social cohesion hypothesis. At the time, the paper struck me as worth considering more fully, but I haven’t had a chance to get back to this material in the last 18 months until recently. My hunch is that since Shor and Simchai place more emphasis on *aversion* (rather than, say, indifference or disinterest) than the most plausible versions of the Westermarckian Effect can bear, their critique is not as strong as it might at first appear.

  3. The assertion that primates have an incest aversion seems contrary to our observations of Bonobo sexual expression. The Westermarck effect is only cultural, I suspect, in chimps and humans.

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