Why Parents Affect Children Less Than Many People Assume

Despite what a small handful of detractors have had to say, inclusive fitness theory has proved to be one of most valuable ideas we have for understanding much of the altruism we observe in both human and non-human species. The basic logic of inclusive fitness theory is simple: genes can increase their reproductive fitness by benefiting other bodies that contain copies of them. So, since you happen to share 50% of your genes in common by descent with a full sibling, you can, to some extent, increase your own reproductive fitness by increasing theirs. This logic is captured by the deceptively-tiny formula of rb > c. In English, rather than math, the formula states that altruism will be favored so long as the benefit delivered to the receiver, discounted by the degree of relatedness between the two, is greater than the cost to the giver. To use the sibling example again, altruism would be favored by selection if the the benefit you provided to a full sibling increased their reproductive success by twice as much (or more) than it cost you to give even if there was zero reciprocation.

“You scratch my back, and then you scratch my back again”

While this equation highlights why a lot of “good/nice” behaviors are observed – like childcare – there’s a darker side to this equation as well. By dividing each side of the inclusive fitness equation by r, you get this: b > c/r. What this new equation highlights is the selfish nature of these interactions: relatives can be selected to benefit themselves by inflicting costs on their kin. In the case of full siblings, I should be expected to value my benefiting twice as much, relative to theirs; for half siblings, I should value myself four-times as much, and so on. Let’s stick to full-siblings for now, just to stay consistent. Each sibling within a family should, all else being equal, be expected to value itself twice as much as they value any other sibling. The parents of these siblings, however, see things very differently: from the perspective of the parent, each of these siblings is equally related to them, so, in theory, they should value each of these offspring equally (again, all else being equal. All else is almost never equal, but let’s assume it is to keep the math easy).

This means that parents should prefer that their children act in a particular way: specifically, parents should prefer their children to help each other when the benefit to one outweighs the cost to the other, or b > c. The children, on the other hand, should only wish to behave that way when the benefit to their sibling is twice the cost of themselves, or 2b > c. This yields the following conclusion: how parents would like their children to behave does not necessarily correspond to what is in the child’s best fitness interests. Parents hoping to maximize their own fitness have different best interests from the children hoping to maximize theirs. Children who behave as their parents would prefer would be at a reproductive disadvantage, then, relative to children who were resistant to such parental expectations. This insight was formalized by Trivers (1974) when he wrote:

  “…an important feature of the argument presented here is that offspring cannot rely on parents for disinterested guidance. One expects the offspring to be pre-programmed to resist some parental teachings while being open to other forms. This is particularly true, as argued below, for parental teachings that affects the altruistic and egoistic tendencies of the offspring.” (p. 258)

While parents might feel as if they only acting in the best interests of their children, the logic of inclusive fitness suggests strongly that this feeling might represent an attempt at manipulating others, rather than a statement of fact. To avoid the risk of sounding one-sided, this argument cuts in the other direction as well: children might experience their parent’s treatment of them as being less-fair than it actually is, as each child would like to receive twice the investment that parents should be willing to give naturally. The take-home message of this point, however, is simply that children who were readily molded by their parents should be expected to have reproduced those tendencies less, relative to children who were not so affected. In some regards, children should be expected to actively disregard what their parents want for them.

“My parents want me to brush my teeth. They’re such fascists sometimes.”

There are other reasons to expect that parents should not tend to leave lasting impressions on their children’s eventual personalities. One of those very good reasons also has to do with the inclusive fitness logic laid out initially: because parents tend to be 50% genetically related to their children, parents should be expected to invest in their children fairly heavily, relative to non-children at least. The corollary to this idea is that non-parents of the child should be expected to treat them substantially different than their parents do. This means that a child should be relatively unable to learn what counts as appropriate behavior towards others more generally from their interactions with their parents. Just because a proud parent has hung their child’s scribbled artwork on the household refrigerator, it doesn’t mean that anyone else will come to think of the child as a great artist. A relationship with your parents is different than a relationship with your friends which is different from a sexual relationship in a great many ways. Even within these broad classes of relationships, you might behave differently with one friend than you do with another.

We should expect our behavior around these different individuals to be context-specific. What you learn about one relationship might not readily transfer to any other. Though a child might be unable to physically dominate their parents, they might be able to dominate their peers; some jokes might be appropriate amongst friends, but not with your boss. Though some of what you learn about how to behave around your parents might transfer to other situations (such as the language you speak, if your parents happen to speakers of the native tongue), it also may not. When it does not transfer, we should expect children to discard what they learned about how to behave around their parents in favor of more context-appropriate behaviors (indeed, when children find their parents speak a different language than their peers, the child will predominately learn to speak as their peers do; not their parents). While a parent’s behavior should be expected to influence how that child behaves around that parent, we should not necessarily expect it to influence the child’s behavior around anyone else.

It should come as little surprise, then, that being raised by the same parents doesn’t actually tend to make children any more similar with respect to their personality than being raised by different ones. Tellegan et al (1988) compared 44 identical twin (MZ) pairs raised apart with 217 identical twins reared together, along with 27 fraternal twins (DZ) reared apart and 114 reared together. In terms of their personality measures, the MZ twins were far more alike than the DZ twins,  as one would expect from their shared genetics. When it came to the personality measures, however, MZ twins reared together were more highly correlated on 7 of the measures, while those reared apart were more highly correlated on 6 of them. In terms of the DZ twins, those reared together were higher on 9 of the variables, whereas those reared apart were higher on the remaining 5. The size of these differences when they did exist was often exceedingly small, typically amounting to a correlation difference of about 0.1 between the pairs, or 1% of the variance.

Pick the one you want to keep. I’d recommend the cuter one.

Even if twins reared together ended up being substantially more similar than twins reared apart – which they didn’t – this would still not demonstrate that parenting was the cause of that similarity. After all, twins reared together tend to share more than their parents; they also tend to share various aspects of their wider social life, such as extended families, peer groups, and other social settings. There are good empirical and theoretical reasons for thinking that parents have less of a lasting effect on their children than many often suppose. That’s not to say that parents don’t have any effects on their children, mind you; just that the effects that they have ought to be largely limited to their particular relationship with the child in question, barring the infliction of any serious injuries or other such issues that will transfer from one context to another. Parents can certainly make their children more or less happy when they’re in each others presence, but so can friends and more intimate partners. In terms of shaping their children’s later personality, it truly takes a village.

References: Tellegen et al. (1988). Personality similarity in twins reared apart and together. Journal of Personality and Social Psychology, 54, 1031-1039.

Trivers, R. (1974). Parent-Offspring conflict. American Zoologist, 14, 249-264.

What Does Online Dating Tell Us About Racial Views?

Preferences can be funny things, or at least our judgments of them. If I were to state that, “I have no interest in hiring a black person to do this job”, I would receive more than a little condemnation for that view. If I were to state instead that, “I have have no interesting in dating a black woman”, I would likely still receive some condemnation, but probably less than for the first statement. Finally, if I were to state that, “I have no interest in dating a man”, I would receive very little, if any, condemnation for it, even from those who advocate strongly for gay rights. As one of my colleagues recently posed the question, “Why is discrimination based on reproductive / sexual preferences OK, but other forms of discrimination are not?” The issue of discrimination is one I’ve discussed before, considering why discrimination on the basis of standardized test scores is deemed to be appropriate, whereas discrimination of the basis of obesity is often not. So let’s turn our attention towards discrimination in the sexual realm today.

“Free?! I’d have to be an idiot not to find the Asian of my dreams!”

A recent post by Jenny Davis over at the Pacific Standard suggests that “Online dating shows us the cold, hard facts about race in America“. In her article, Jenny discusses some data released from a Facebook-based dating app that figures out which people are interested in which other people on some sexual or romantic level. The data is labeled “unfortunate” in some respects, because there appear to be winners and losers, and those winners and losers seem to break down along racial lines. When it comes to mating, it seems that everyone doesn’t get to join hands and cross the finish line at the same time so that we all end up with equally-high self-esteem (I know; I was shocked too).  To give you a sense for the data (and so you don’t have to click back and forth between links), here’s the breakdown of the response rates for people who are interested.

As anyone can clearly see, there are favorites. When it comes to the highest positive response rate, most women, regardless of their race, appear to favor white men, whereas most men, again, regardless of their race, tend to favor Asian women. In terms of the lowest response rate, women appeared to shun black men, whereas men tended to shun black women. Ouch. Jenny, using what I can only assume is that same “high-powered sociological lens” I’ve encountered before, concludes that this clearly demonstrates that race matters, and serves to counter accusations that we are living in a color-blind, post-racial world. As Jenny puts it we “fetishize Asian women while devaluing blacks”. Now tone doesn’t come across well through text-based communications at time, but neither “fetishize” nor “devalue” sound as if they have a particularly positive connotation to me. It sounds as if she’s condemning other people for their sexual preferences in that respect.

There are many comments to make about this, but let’s start with this one: apparently, there’s something of a no-win situation being erected from the get go. When one group is preferred, it’s a “fetish”, whereas when they’re not preferred, they’re “devalued”. Well, sort of, anyway; if she were being consistent (and who is?) Jenny would also say that women “fetishize” white males. Strangely, she does not. One can only guess as to why she does not, because Jenny makes no apparent attempt to understand the data in question. By that, I mean that Jenny offers no potential alternative explanations through which we might understand the data. In fact, she doesn’t seem to offer any explanation whatsoever for these patterns of responses. If I had to, I would guess that her explanation, if simplified somewhat, would reduce to “racism did it”, but it’s hard to tell.

“But are they the Black singles of my dreams, like the Asians?”

I would like to try and pick up some of that explanatory slack. Despite initial appearances, it is possible that this data has very little, if anything, to do with race per se. Now I happen to think that race likely does matter to some extent when it comes to dating preferences, but the degree of that extent is anyone’s guess. To see why I would say this only requires that one understands a very basic statistical concept: correlation does not equal causation. This is something that I imagine Jenny understands, but it likely slipped her mind in the midst of trying to make a point. There are few examples to consider, but the first is by far the simplest. Most men, if you polled them, would overwhelming respond to women on dating websites, and not other men; women would likely do the reserve. This does not mean, however, that men (or women) “devalue” other men (or women). Similarly, just because people on these dating sites might respond to black people at the lowest rates, it does not mean they “devalue” black people more generally.

But maybe we do devalue certain racial groups, at least when it comes to dating them. This brings us to the second issue: mating decisions are often complex. There are dozens of potential variables that people assess when choosing a mate – such as how much money they have, how much they weigh, how tall they are, their age, their relatedness to us, etc – and the importance of these qualities also varies somewhat depending on the nature of the relationship (whether it is more short- or long-term, for instance). The important point here is that even if people are picking mates on the basis of these other characteristics alone and not race, we might still see racial differences in outcomes. Let’s say, for instance, that men tend to prefer women shorter than themselves as dating partners (the reasons for this preference or it’s actual existence need not necessarily concern us). If that were the case, provided there are any average differences in height among the races, we would still see different response rates to and from each racial group, even though no one was selecting on the basis of race.

Rather than just considering the direction the preferences in the data above, then, let’s consider some of the actual numbers: when it came to response rates, regardless of whether we were considering men or women, and regardless of whether we’re considering the highest or lowest response rates, black individuals seem to respond more often than any other group; sometimes around twice as often. This could be indicative of a number of different factors, though I won’t speculate as to which ones on the basis of the numbers alone. The only point is that those factors might show up in user’s profiles in some way. If other people pick up on those factors primarily, then race itself might not be the primary, or even a, factor driving these decisions. In fact, in terms of response rates, there was a consistent overall pattern: from lowest to highest, it tended to be Latinos, Whites, Asians, and Blacks, regardless of sex (with only a single exception). Whatever the reasons for this, I would guess that it shows up in other ways in the profiles of these senders and responders.

Strangely, I can’t find a picture of a white dating site. Odd…

As I said, I don’t think that race per se is entirely unrelated to mating choices. However, to determine the extent to which it uniquely predicts anything, you need to control for other relevant factors. Does obesity play a role in these decisions? Probably. Is obesity equally common across racial groups? Nope. How about income; does income matter? In some cases it sure seems to. Is income the same across racial groups? Nope. We would likely find the same for many, many other factors.

In addition to determining the extent of how much race matters, one might also wish to explain why race might matter. Simply noting that there appear to be some racial differences doesn’t tell us a whole lot; the same goes for correlations of match percentages and response rates over at OkCupid, which find a similar pattern with respect to race. In the instance of OkCupid, a match percentage of 10% between two people corresponds to about a 25% reply rate; a 90% match percentage gets you all the way up to… a 37% reply rate. Even at around 100% match, the response rate still only lingers at around 50%. There appears to be a lot more that goes into mating decisions than people typically appreciate or even recognize. For what it’s worth, I would rather work to understand those complexities than pat myself on the back for how bad I think racism is.

Which Ideas Are Ready To Go To Florida?

Recently, Edge.org posed it’s yearly question to a number of different thinkers who were given 1,000 words or less to provide their answer. This year, the topic was, “What scientific idea is ready for retirement?” and responses were received from about 175 people. This question appeared to come on heels of a quote by Max Planck, who suggested at one point that new ideas tend to gain their prominence not through actively convincing their opponents that they are correct, but rather when those who hold to alternative ideas die off themselves. Now Edge.org did not query my opinion on the matter (and the NBA has yet to draft me, for some unknowable reason), so I find myself relegated to the side-lines, engaging in the always-fun past time of lobbing criticisms at others. Though I did not read through all the responses – as many of them fall outside of my area of expertise and I’m already suffering from many demands for my time that are a bit more pressing – I did have some general reactions to some of the answers people provided to this question.

Sticks and stones can break their bones? Good enough for me.

The first reaction I had is with respect to the question itself. Planck was likely onto something when he noted that ideas are not necessarily accepted by others owing to their truth value. As I have discussed a few times before, there are, in my mind, anyway, some pretty compelling reasons for viewing human reasoning abilities as something other than truth-finders: first, people’s ability to successfully reason about a topic often hinges heavily on the domain in question. Whereas people are skilled reasoners when it comes to social contracts, they are poor at reasoning about more content-neutral domains. In that regard, there doesn’t seem to be a general-purpose reasoning mechanism that works equally well in all scenarios. Second, people’s judgements of their performance on tasks of reasoning abilities are often relatively uncorrelated with their actual performance. Most people appear to rate their performance in line with how easy or difficult a task felt, and, in some cases, being wrong happens to feel a lot like being right. Third, people are often found to ignore or find fault with evidence that doesn’t support their view, but will often accept evidence that does fit with their beliefs much less critically. Importantly, this seems to hold when the relative quality of the evidence in question is held constant. Having a WEIRD sample might be a problem for a study that reaches a conclusion unpalatable to the person assessing it, but is unlikely to be mentioned if the results are more agreeable.

Finally, there are good theoretical reasons for thinking that reasoning can be better understood by positing that it functions to persuade others, rather than to seek truth per se. This owes itself to the fact that being right is not necessarily always the most useful thing to be. If I’m not actually going to end up being successful in the future, for instance, it still might pay for me to try and convince other people that my prospects are actually pretty good so they won’t abandon me like the poor investment I am. Similarly, if I happen to advocate a particular theory that most of my career is built upon, abandoning that idea because it’s wrong could mean doing severe damage to my reputation and job prospects. In other words, there are certain ways that people can capture benefits in the social world by convincing others of untrue things. While that’s all well and good, it would seem to frame the Edge question is a very peculiar light: we might expect that people – those who responded to the Edge’s question included – tend to advocate that certain ideas should be relinquished, but their motivations and reasons (conscious or otherwise) for making that suggestion are based in many things which are not the idea’s truth value. As the old quote about evolution goes, “Let’s hope it is not true. But if it is true, let’s pray that it doesn’t become widely known”.

As an example, let’s consider the reply from Matt Ridley, who suggests that Malthus’ ideas on population growth were wrong. The basic idea that Malthus had was that resources are finite, and that populations, if unchecked, would continue to grow to the point that people would eventually live pretty unhappy lives, owing to the scarcity of resources relative to the size of the population. There would be more mouths wanting food than available food, which is a pretty unsatisfactory way to live life. Matt states, in his words, that “Malthus and his followers were wrong, wrong, wrong”. Human ingenuity has helped come to the rescue, and people have been becoming better and better at using the available resources in more efficient manners. The human population has continued to grow, often unchecked by famine (at least not in most first world nations). If anything, many people have access to too much food, leading to wide-spread obesity. While all this is true enough, and Malthus appeared to be wrong with respect to certain specifics, one would be hard-pressed to say that the basic insights themselves are worthy of retirement. For starters, human population growth has often come at the expense of many other species, plant and animal alike; we’ve made more room for ourselves not just by getting better at using what resources we do have, but by ensuring other species can’t use them either.

As it turns out, dead things are pretty poor competition for us.

Not only has our expansion has come at the expense of other species that find themselves suddenly faced with a variety of scarcities, but there’s also no denying that population growth will, at some point, be checked by resource availability. Given that humans are discovering new ways of doing things more efficiently than we used to, we might not have hit that point yet and we might not hit it for some time. It does not follow, however, that such a point does not, at least in principle, exist. While there is no theoretical upper limit on the number of people which might exist, the ability of human ingenuity to continuously improve the ability of our planet to support all those people is by no means a guarantee. While technology has improved markedly since the time of Malthus, there’s no telling how long such improvements will be sustained. Perhaps technology could continue to improve infinitely, just as populations can grow if unconstrained, but I wouldn’t bet on it. While Malthus might have been wrong about some details, I would hesitate to find his underlying ideas a home on a golf course close to the beach.

Another reply which stood out to me came from Martin Nowak. I have been critical of his ideas about group selection before, and I’m equally as critical of his answer to the Edge question. Nowak wants to prematurely retire the 50 year old idea of inclusive fitness: the idea that genes can benefit themselves by benefiting various bodies that contain copies of them, discounted by the probability of their being in that other body.  Nowak seems to want to retire the concept for two primary reasons: first, he suggests it’s mathematically inelegant. In the process of doing so, Nowak appears to insinuate that inclusive fitness represents some special kind of calculation that is both (a) mathematically impossible and (b) identical to calculations derived from standard evolutionary fitness calculations. On this account, Nowak seems to be confused: if the inclusive fitness calculations lead to the same outcome as standard fitness calculations, then there’s either something impossible about standard evolutionary theory (there isn’t), or inclusive fitness isn’t some special kind of calculation (it isn’t).

Charitable guy that Nowak is he does mention that the inclusive fitness approach has generated a vast literature of theoretically and empirically useful findings. Again, this seems strange if we’re going to take him at his word that the idea obviously wrong and one that should be retired. If it’s still doing useful work, retirement seems premature. Nowak doesn’t stop there, though: he claims that no one has empirically tested inclusive fitness theory because researchers haven’t been making precise fitness calculations in wild populations.This latter criticism is odd on a number of fronts. First, it seems to misunderstand the type of evidence that evolutionary researchers look for, which is evidence of special design; counting offspring directly is often not terribly useful in that regard. The second issue I see with that suggestion is that, perhaps ironically, Nowak’s favored alternative – group selection – has yet to make a single empirical prediction that could not also be made by an inclusive fitness approach (though inclusive fitness theorizing has successfully generated and supported many predictions which group selection cannot readily explain). Of all of Nowak’s work I have come across, I haven’t found an empirical test in any of his papers. Perhaps they exist, but if he is so sure that inclusive fitness theory doesn’t work (or is identical to other methods), then demonstrating so empirically should be a cake walk for him. I’ll eagerly await his research on that front.

I’m sure they’ll be here any day now…

While this only scratches the surface of the responses to the question, I would caution against retiring many of the ideas that were singled out in the answer section. Just as a general rule, ideas in science should be retired, in my mind, when they can be demonstrated without (much of) a doubt to be wrong and to seriously lead people astray in their thinking. Even then, it might only require us to retire an underlying assumption, rather than the core of the idea itself. Saying that we should retire inclusive fitness “because no one ever really tested it as I would like” is a poor reason for retirement; retiring the ideas of Malthus because we aren’t starving in the streets at the moment also seems premature. Instead of talking about what ideas should be retired wholesale, a better question would be to consider, “what evidence would convince you that you’re mistaken, and why would that evidence do so?” Questions like that not only help ferret out problematic assumption, but they might also help make useful forward momentum empirically and theoretically. Maybe the Edge could consider some variant of that question for next year.

What Makes Incest Morally Wrong?

There are many things that people generally tend to view to be disgusting or otherwise unpleasant. Certain shows, like Fear Factor, capitalize on those aversions, offering people rewards if they can manage to suppress those feelings to a greater degree than their competitors. Of the people who watched the show, many would probably tell you that they would be personally unwilling to engage in such behaviors; what many do not seem to say, however, is that others should not be allowed to engage in those behaviors because they are morally wrong. Fear or disgust-inducing, yes, but not behavior explicitly punishable by others. Well, most of the time, anyway; a stunt involving drinking donkey semen apparently made the network hesitant about airing it, likely owing to the idea that some moral condemnation would follow in its wake. So what might help us differentiate between understanding why some disgusting behaviors – like eating live cockroaches or submerging one’s arm in spiders – are not morally condemned while others – like incest – tend to be?

Emphasis on the “tend to be” in that last sentence.

To begin our exploration of the issue, we could examine some research on some cognitive mechanisms for incest aversion. Now, in theory, incest should be an appealing strategy from a gene’s eye perspective. This is due to the manner in which sexual reproduction works: by mating with a full sibling, your offspring would carry 75% of your genes in common by descent, rather than the 50% you’d expect if you mated with a stranger. If those hyper-related siblings in turn mated with one another, after a few generations you’d have people giving birth to infants that were essentially genetic clones. However, such inbreeding appears to carry a number of potentially harmful consequences. Without going into too much detail, here are two candidate explanations one might consider for why inbreeding isn’t a more popular strategy: first, it increases the chances that two harmful, but otherwise rare, recessive alleles will match up with on another. The result of this frequently involves all sorts of nasty developmental problems that don’t bode well for one’s fitness.

A second potential issue involves what is called the Red Queen hypothesis. The basic idea here is that the asexual parasites that seek to exploit their host’s body reproduce far quicker than their hosts tend to. A bacteria can go through thousands of generations in the time humans go through one. If we were giving birth to genetically-identical clones, then, the parasites would find themselves well-adapted to life inside their host’s offspring, and might quickly end up exploiting said offspring. The genetic variability introduced by sexual reproduction might help larger, longer-lived hosts keep up in the evolutionary race against their parasites. Though there may well be other viable hypotheses concerning why inbreeding is avoided in many species, the take-home point for our current purposes is that organisms often appear as if they are designed to avoid breeding with close relatives. This poses many species with a problem they need to solve, however: how do you know who your close kin are? Barring some effective spatial dispersion, organisms will need some proximate cues that help them differentiate between their kin and non-kin so as to determine which others are their best bets for reproductive success.

We’ll start with perhaps the most well-known of the research on incest avoidance in humans. The Westermarck effect refers to the idea that humans appear to become sexually disinterested in those with whom they spent most of their early life. The logic of this effect goes (roughly) as follows: your mother is likely to be investing heavily in you when you’re an infant, in no small part owing to the fact that she needs to breastfeed you (prior to the advent of alternative technologies). Since those who spend a lot of time around you and your mother are more likely to be kin than those who spend less time in your proximity. That degree of that proximity ought to in turn generate some kinship index with others that would generate disinterest in sexual experiences with such individuals. While such an effect doesn’t lend itself nicely to controlled experiments, there are some natural contexts that can be examined as pseudo-experiments. One of these was the Israeli Kibbutz, where children were predominately raised in similarly-aged, mixed-sex peer groups. Of the approximately 3000 children that were examined from these Kibbutz, there were only 14 cases of marriage between individuals from the same group, and almost all of them were between people introduced to the group after the age of 6 (Shepher, 1971).

Which is probably why this seemed like a good idea.

The effect of being raised in such a context didn’t appear to provide all the cues required to trigger the full suite of incest aversion mechanisms, however, as evidenced by some follow-up research by Shor & Simchai (2009). The pair carried out some interviews with 60 of the members of the Kibbutz to examine the feelings that these members had towards each other. A little more than half of the sample reported having either moderate or strong attractions towards other members of their cohort at some point; almost all the rest reported sexual indifference, as opposed to the typical kind of aversion or disgust people report in response to questions about sexual attraction towards their blood siblings. This finding, while interesting, needs to be considered in light of the fact that almost no sexual interactions occurred between members of the same peer group; it should also be considered in light of the fact that there did not appear to exist any strong moral prohibition against such behavior.

Something like a Westermarck effect might explain why people weren’t terribly inclined to have intercourse with their own kin, but it would not explain why people think that others having sex with close kin is morally wrong. Moral condemnation is not required for guiding one’s own behavior; it appears more suited for attempting to guide the behavior of others. When it comes to incest, a likely other whose behavior one might wish to guide would be their close kin. This is what led Lieberman et al (2003) to deliver some predictions about what factors might drive people’s moral attitudes about incest: the presence of others who are liable to be your close kin, especially if those kin are of the opposite sex. If duration of co-residence during infancy is used a proximate input cue for determining kinship, then that duration might also be used as an input condition for determining one’s moral views about the acceptability of incest. Accordingly, Lieberman et al (2003) surveyed 186 individuals about their history of co-residence with other family members and their attitudes towards how morally unacceptable incest is, along with a few other variables.

What the research uncovered was that duration of co-residence with an opposite-sex sibling predicted the subject’s moral judgments concerning incest. For women, the total years of co-residence with a brother was correlated with judgments of wrongness for incest at about r = 0.23, and that held whether the time period from 0 to 10 or 0 to 18 was under investigation; for men with a sister, a slightly higher correlation emerged from 0 to 10 years (r = 0.29), but an even-larger correlation was observed when the period was expanded to age 18 (r = 0.40). Further, such effects remained largely static even after the number of siblings, parental attitudes, sexual orientation, and the actual degree of relatedness between those individuals was controlled for. None of those factors managed to uniquely predict moral attitudes towards incest once duration of co-residence was controlled for, suggesting that it was the duration of co-residence itself driving these effects of moral judgments. So why did this effect not appear to show up in the case of the Kibbutz?

Perhaps the driving cues were too distracted?

If the cues to kinship are somewhat incomplete – as they likely were in the Kibbutz – then we ought to expect moral condemnation of such relationships to be incomplete as well.  Unfortunately, there doesn’t exist much good data on that point that I am aware of, but, on the basis of Shor & Simchai’s (2009) account, there was no condemnation of such relationships in the Kibbutz that rivaled the kind seen in the case of actual families. What their account does suggest is that more cohesive groups experienced less sexual interest in their peers; a finding that dovetails with the results from Lieberman et al (2003): cohesive groups might well have spent more time together, resulting in less sexual attraction due to greater degrees of co-residence. Despite Shor & Simchai’s suggestion to the contrary, their results appear to be consistent with a Westermarck kind of effect, albeit an incomplete one. Though the duration of co-residence clearly seems to matter, the precise way in which it matters likely involves more than a single cue to kinship. What connection might exist between moral condemnation and active aversion to the idea of intercourse with those one grew up around is a matter I leave to you.

References: Lieberman, D., Tooby, J., & Cosmides, L. (2003). Does morality have a biological basis? An empirical test of the factors governing moral sentiments relating to incest. Proceedings of the Royal Society of London B, 270, 819-826.

Shepher, J. (1971). Mate Selection Among Second Generation Kibbutz Adolescents and Adults: Incest Avoidance and Negative Imprinting. Archives of Sexual Behavior, 1, 293-307.

Shor, E. & Simchai, D. (2009). Incest Avoidance, the Incest Taboo, and Social Cohesion: Revisiting Westermarck and the Case of the Israeli Kibbutzim. American Journal of Sociology, 114, 1803-1846,