boinking your sister

heh. several of you were grossed out at the thought — and that’s good! that is as it should be.

i don’t have a brother, so i don’t know what the westermarck effect feels like, but apparently it feels like … ewwwwwwww! and it doesn’t just work for sibilings, but any kids that are raised together. i’ve read many accounts of muslim couples who have been made to marry their first-cousins who complain that they’re not attracted to each other and very often they were raised together (in an extended family household), so they feel like … ewwwwwwww!

robin fox had a lot of interesting stuff to say about the westermarck effect in “The Tribal Imagination” (chapter 6 — and also presumbaly in his book on incest, but i haven’t read that). here’s an excerpt for ya [pgs. 128-31]:

“Taboos on sexual relationship between close kin, between the members of the immediate family, were long thought to be a purely human invention. Animals, it was maintained, had no such inhibitions and mated incestuously. Thus the taboos on such animalistic behavior were thought to be the very foundation of human society; they were the ultimate Drumbeat of humanity, by which, in the immortal words of Levi-Strauss, culture said ‘No!’ to nature….

“The move from nature to culture represented by the imposition of the taboos was seen as precarious and counter to natural motives, which were ineradicable. In consequence, the taboos had to be stern and enforced by constant vigilance. In this traditional view, we all wanted to make love to our nearest kin, but once the momentous leap into culture had been taken, it would have been disastrous to go back into the maelstrom of incestuous animality. Our societies were built on the presumption of mating outside the family, it was the very definition of humanity itself, and so stern taboos, laws, and punishments were needed to keep incest at bay.

“This was a plausible view because societies did almost universally ban sex and marriage within the immediate family, and punishments for breaches of this rule were often severe, including torture and death. In their mythologies, primitive tribes and ancients societies often portrayed incest, and the results of it were usually disastrous. There were exceptions to the rule (and we shall return to these), but they were almost always royal exceptions, and royal persons, as gods on earth, were allowed behavior that was not allowed to ordinary mortals. On the whole, then it was agreed, there was a ‘grisly horror’ of incest (Freud) that universally afflicted people and led them to impose and enforce the taboos, often extending them beyond the family to members of the clan, variously defined. Why, the question went, would we have such strong taboos if we did not have the strong desire in the first place?

“Both popular opinion and the collective voice of the behavioral sciences echoed this orthodoxy. But there was always an undercurrent of skepticism. Why, the objectors asked in turn, do we seem, by and large, not to want to have sex with our closest relations? This would be the common-sense observation. Incest happens, but in proportion to non-incest, it does not happen very often. And most of this avoidance of incest does not seem to result from fear of punishment; there seems to be a genuine aversion to incest. This aversion seems to vary according to the relationship: strongest between mother and son, weakest between father and daughter, variable between brother and sister. But it is there, and usually only breaks down in unappetizing circumstances.

“The orthodox view said that left to our own devices we would immediately resort to incest and so we have to be reined in by strong taboos and sanctions…. The skeptical view says that, on the contrary, left to our own devices we would probably mostly avoid incest spontaneously. The orthodox view asks why, then, if this is so, are there the universal strong taboos?

“The skeptic answers that we often taboo the things that we are averse to, not because we secretly want to do them, but because we disapprove of people doing things that are generally obnoxious to us. We strongly taboo murder, not because we are all given to implacable murderous impulses, but because we are averse to it, so that even if only a few people do it, it offends us. We do, however, understand the temptation to do it; we have all perhaps felt it momentarily. So the subject fascinates us and permeates our legends and stories from the beginning. Sex and violence, incest and murder — often linked in our fantasy productions — persist in our imaginative attempts to interpret ourselves to ourselves….

“[I]n looking at animal behavior under natural conditions, indeed at the behavior of all sexually reproducing organisms, outbreeding seemed to be the rule and incest was rare. This was especially true in our primate relatives, and so by implication in our ancestors during the long haul through the savannas and the ice….

“The origin of this sexual reproduction is still a mystery, but whatever the reason, this new form of reproduction won out over its rival (which is still around) by virtue of its ability to produce instant genetic variability for natural selection to work on. Close inbreeding results in a loss of such variability, hence mechanisms evolved to avoid it. At the same time, if inbreeding becomes too random, then any beneficial genes will be dissipated rather than concentrated and preserved. It is this loss of variation that seems to be at the heart of sexual strategies, not the bad genetic effects of close inbreeding. In small bands these effects would quickly be bred out, and even scattered bouts of outbreeding would reestablish a healthy stock.

“So nature aims for a middle ground: organisms breed out to avoid losing variability, but not so far out that they dissipate genetic advantages. In human terms this means that the immediate family is taboo, but that marriage with cousins should be preferred. This is exactly what we find in human history until the dramatic growth and disruption of human populations upset the natural balance of the traditional society.”

actually, this is exactly what we find in human history until the roman catholic church started fiddling with mating patterns in europe in the early middle ages (see Inbreeding in Europe series down below ↓ in left-hand column).

“[I]f inbreeding becomes too random, then any beneficial genes will be dissipated rather than concentrated and preserved.”

think about that in terms of altruism genes.

(note: comments do not require an email. don’t panic!)


  1. I don’t have a sister and it still grossed me out. It was especially the guy’s vivid description of the sheer lust he felt for his sister that got to me.

    I guess it’s not surprising that father-daughter is slightly less repellent than the others. For the father, the daughter can be kind of like a younger, hotter, more fertile version of his wife…but still yech!


  2. Jonathan Haidt (professor of psychology at the University of Virginia) wrote in his book ‘The Happiness Hypothesis’ that he likes to use the question of what is wrong with two consenting adult siblings having sex with eachother, to provoke discussions about moral arguments and moral judgement among his students. I didn’t read the original post but maybe that’s what the poster was trying to achieve?

    Also, evidence supporting the Westermark effect comes from studies of Israeli Kibbutzim. It seems that in the early Kibbutzes all children were raised together, apart from their parents, in some sort of communal children’s societies. It was hoped that when they grew up, the kids would marry other kids they grew up with, and in doing so, enhance group solidarity. But when the kids did grow up, even though they were not related to each other, according to interviews with these kids, they didn’t find each other attractive, and subsequently married outside the group.

    OTOH, full siblings raised apart are actually more likely to be attracted to each other at sexual maturity than non-relatives, as they have more physical and behavioural traits in common. Rather like in Daniel Defoe’s book ‘The Fortunes and Misfortunes of the Famous Moll Flanders’ where Moll goes from England to Virginia and marries her half brother by accident.

    See Evolutionary Psychology I – The Modern Scholar.


  3. @the colonel – “OTOH, full siblings raised apart are actually more likely to be attracted to each other at sexual maturity than non-relatives, as they have more physical and behavioural traits in common.”

    yes, indeed.


  4. @ihtg – “For the father, the daughter can be kind of like a younger, hotter, more fertile version of his wife…but still yech!”

    i keep thinking that when i (inadvertently) read something about morgan freeman and his step-granddaughter. he obviously thought her grandmother was hot … and now he thinks the granddaughter is hot.

    but still, like you say, yech. (at least they’re not really related, though.)


  5. expanding on what I said previously

    the comments he made which especially stood out:

    “What as your first experience like?

    “We didn’t immediately start having sex or anything. It was pretty gradual like any other relationship is, especially as teens. My first experience with just physical contact (kissing and touching) was pretty wild. There was definite shame involved. I felt scared of what “God” would do to us, but being a horny teenager I managed to overlook it somehow. ”

    “As adults did you ever feel uneasy about it, or think it was wrong?

    “As adults, no. At least, I haven’t. Looking at it logically and ignoring the “ick” factor that so many people place on it, I see no reason to be ashamed. We used protection, both consented, and loved each other.”

    The ‘ick’ factor some people place on it?

    No one has to place any importance on it–the disgust is innate, instinctual. This guy, like Freud, just doesn’t understand what everyone else is reacting to. He rationalizes the Westermarck effect as religious guilt, but this is completely missing the point. I don’t know if it is because of biology or his upbringing, but he has clearly never developed the psychological ward which most people have. He felt that somehow he was violating societal taboos, but doesn’t realize that most people don’t need the taboos to avoid banging their sis or their mom. We don’t do it because it is totally disgusting.


  6. If Westermarck was “only” a social taboo, believe me, the left would already be screeching about how illogical it is and how manifestly evil Western countries are for not allowing siblings to marry


  7. re: the thing about more related people being more attractive (Westermarck notwithstanding)

    I wonder if this is a matter of inclusive fitness more than anything… anecdotal I can say that the women I am most attracted to share a lot of superficially features with me… I wonder if they share a lot of genes with me, too? do we instinctively lust after mates that will increase our inclusive fitness? it seems to be the logical explanation


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