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Posted by on Feb 6, 2013 in Evolutionary Psychology | 28 comments

Evolutionary psychology questions answered, Part 1

Back on December 13th I asked readers to submit questions and promised to answer at least the top five (by votes). I am impressed with the great questions that have been posted since and I will try to do them justice.  I will take the first in this post, and only the first because it’s a very broad question.

What are the most notable confirmed and disconfirmed hypotheses from the field of EP, and which of these do you consider most representative of the field operating properly as a scientific endeavour?    7 votes, submitted by Damion Reinhardt.

“Notable” is somewhat subjective, and I will remind readers that “confirmed” is here used in the scientific sense that there is adequate evidence from multiple studies and replications to reasonably take as correct. Biologist Jerry Coyne, known for the Why Evoltuion is True (book and website), makes a fairly good list and I agree to most of these:

  • Incest avoidance, especially in those societies that haven’t made a connection between incest and birth defects.  Also, the proximate cues for avoiding incest, as in the failure of children raised in a kibbutz to marry.
  • Humans’ innate fear of harmful creatures or features, as in spiders and heights, and the lack of innate fears of more modern dangers.
  • The variance in offspring number between males and females in various societies, and the differential “pickiness” of males and females when choosing mates
  • The evolution of concealed ovulation in humans as opposed to other primates.
  • The use of odors and immune-system matching (i.e., MHC genes) as cues for mates.
  • The cause of sexual dimorphisms (e.g., size differences between males and females).
  • The cause of physical and physiological differences between human ethnic groups (was it sexual selection, drift, or something else?)
  • Gene-culture coevolution, as in the evolution of lactose tolerance.
  • The evolution or morality using comparative studies with other primates.
  • Parent-offspring conflict, and cases in which kin are favored over nonkin
  • Why we like food that is bad for us (e.g. fats and sweets), and why we feel disgust at certain foods or odors

Let me expound on a few of these for the unfamiliar. Research shows that a fear of snakes is common across humanity and in many primates. People who grew up in the inner city and have never seen a snake “in the wild” are nonetheless afraid of them. The same humans are not afraid of automobiles, despite them being far more likely to hurt or kill them and despite many having experienced this or know friends who did. This is substantial evidence that some fears are not learned. Experimenters have noted this in labs as well, wherein monkeys raised in said labs are afraid of snake-shaped tubes having never seen a snake in their lives.

Kin selection is an hypothesis that people favor apparent blood relatives over non-blood relatives in a wide variety of ways. This hypothesis has an incredible volume of statistical data in support. Affinity, affection, and the likelihood of bestowing favors are attenuated by apparent genetic relatedness. On the negative side,  murder directed at blood kin is quite rare compared to such acts between non-kin.

Evolutionary psychologist Rob Kurzban produced a list of studies he believes Coyne may have been referring to. Here is Rob’s list for further reading (I would also encourage readers to check out his excellent EP blog):

Incest Avoidance

Fessler, D. M., & Navarrete, C. D. (2004). Third-party attitudes toward sibling incest: Evidence for Westermarck’s hypotheses. Evolution and Human Behavior, 25(5), 277-294.

Lieberman, D. (2009). Rethinking the Taiwanese minor marriage data: evidence the mind uses multiple kinship cues to regulate inbreeding avoidance. Evolution and Human Behavior, 30(3), 153-160.

Lieberman, D., & Symons, D. (1998). Sibling incest avoidance: from Westermarck to Wolf. Quarterly Review of Biology, 463-466.

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. Series B: Biological Sciences, 270(1517), 819-826.

Lieberman, D., Tooby, J., & Cosmides, L. (2007). The architecture of human kin detection. Nature, 445(7129), 727-731.

Fear of Spiders etc.

Gerdes, A., Uhl, G., & Alpers, G. W. (2009). Spiders are special: fear and disgust evoked by pictures of arthropods. Evolution and Human Behavior, 30(1), 66-73.

Nesse, R. M. (1994). Fear and fitness: An evolutionary analysis of anxiety disorders. Ethology and Sociobiology, 15(5), 247-261.

Öhman, A. (1986). Face the beast and fear the face: Animal and social fears as prototypes for evolutionary analyses of emotion. Psychophysiology, 23(2), 123-145.

I will add a couple here (-Ed)

D.H. Rakison, J. Derringer (2008). Do infants possess an evolved spider-detection mechanism?
Cognition, 107 (2008), pp. 381–393

M. Cook, S. Mineka. (1990). Selective associations in the observational conditioning of fear in rhesus monkeys. Journal of Experimental Psychology: Animal Behavior Processes, 16, 372–389.

MHC genes and mating

Herz, R. S., & Inzlicht, M. (2002). Sex differences in response to physical and social factors involved in human mate selection: The importance of smell for women. Evolution and Human Behavior, 23(5), 359-364.

Penn, D. J., & Potts, W. K. (1999). The evolution of mating preferences and major histocompatibility complex genes. The American Naturalist, 153(2), 145-164.

Roberts, S. C., Little, A. C., Gosling, L. M., Perrett, D. I., Carter, V., Jones, B. C., … & Petrie, M. (2005). MHC-heterozygosity and human facial attractiveness. Evolution and Human Behavior, 26(3), 213-226.

Sexual Dimorphism

Penton-Voak, I. S., Little, A. C., Jones, B. C., Burt, D. M., Tiddeman, B. P., & Perrett, D.I.(2003). Female condition influences preferences for sexual dimorphism in faces of male humans (Homo sapiens). Journal of Comparative Psychology, 117(3), 264.

Puts, D. A., Gaulin, S. J., & Verdolini, K. (2006). Dominance and the evolution of sexual dimorphism in human voice pitch. Evolution and Human Behavior, 27(4), 283-296.

Thornhill, R., & Gangestad, S. W. (2006). Facial sexual dimorphism, developmental stability, and susceptibility to disease in men and women. Evolution and Human Behavior, 27(2), 131-144

Parent-offspring conflict

Buunk, A. P., Park, J. H., & Dubbs, S. L. (2008). Parent-offspring conflict in mate preferences. Review of General Psychology, 12(1), 47.

Rohde, P. A., Atzwanger, K., Butovskaya, M., Lampert, A., Mysterud, I., Sanchez-Andres, A., & Sulloway, F. J. (2003). Perceived parental favoritism, closeness to kin, and the rebel of the family: The effects of birth order and sex. Evolution and Human Behavior, 24(4), 261-276.

I would add to these Life History Theory and Social Contract Theory. Both require essays of their own to properly explain but I will give them a very brief summary.

Life History Theory (LHT) is a biology theory about how organisms allocate their resources over their complete lifespan. A central idea is the timeline for reproduction based on environmental conditions: is it better to reproduce early, or to shore up bodily resources to do so later? LHT originated in the study of animal behavior, but has tremendous import for humans and human society. It is hypothesized that environmental cues to instability and unpredictability change human behavior such that investment in immediate reproduction becomes a more urgent priority and self-investment such as education or insurance, less so. The logic of this is very simple: why save up for later if you aren’t sure there is going to be a later? This is one of many examples of how tightly integrated the understanding and effects of culture are to evolutionary psychology theory. In recent years, LHT as it applies to evolutionary psychology has been best articulated and developed by AJ Figueredo. Here are two papers on the subject:

Figueredo et al., 2006 Consilience and life history theory: From genes to brain to reproductive strategy. Developmental Review, 26 (2006), pp. 243–275

Figueredo et al., 2005 The K-Factor: Individual differences in life history strategy
Personality and Individual Differences, 39 (8) (2005), pp. 1349–1360 Link

 Social Contract Theory

A part of a cognitive program for successfully navigating a social landscape among others we must both cooperate with and compete with, we should have specializations for social interactions that prevent our exploitation and secure fair outcomes. This is SCT in a nutshell.  SCT has every virtue that I think an evolutionary psychology theory could have:

  • It is grounded in a venerable body of literature about the evolution of cooperation and altruism, see George Williams & game theory
  • It is a clear theoretical model, well defined and easily parsed
  • The implications of the model have been tested, both among Americans and non-western cultures
  • Several alternative explanations for the above results have been tested and ruled out
  • A locus (or rather two locii) in the brain itself has been located that, when damaged, severely impairs SC ability, while leaving almost all other cognitive abilities intact. This points to likely physical locations of the SC module in the brain

See a full discussion extensive citations in chapter 3 of The Adapted Mind by Jerome Barkow, Leda Cosmides and John Tooby.

Disconfirmed hypotheses?

Most failed hypotheses die a quiet death of attrition because failure to show an effect or failure to replicate usually means a study is not publishable. This is known as the file drawer effect. I can only think of one “notable” such hypothesis. In the late 90′s the notion that homosexuality, particularly among men, might be explained in terms of kin selection if said males invest substantially in their close genetic relatives versus straight males. However, studies failed to find evidence that such a disparity exists. See:

David Bobrow, J.Michael Bailey. 2001.  Is male homosexuality maintained via kin selection? Evolution and Human Behavior 22-5. 361–368.

The hypothesis has been overshadowed by more promising research. For example, check out the current Quarterly Review of Biology which contains a paper about the epigenetic developmental theory of homosexuality. I believe it is freely viewable. Here’s the citation:

William R. Rice, Urban Friberg and Sergey Gavrilets. 2012. Homosexuality as a Consequence of Epigenetically Canalized Sexual Development. The Quarterly Review of Biology . 87-4,  343-368.

Although not “notable” it is worth mentioning that the casting out of evolutionary psychology hypotheses is something that I see all the time. Usually, this happens in early stages, such as when a colleague is proposing a research plan (or when I have) and a cohort either points to contradictory research or finds a flaw in their theoretical approach. Sometimes it happens after data has been collected in an initial “test the waters” study, and the data do not support the hypothesis. Note that all of these are examples of peer and evidence-based rejection before formal peer review comes into play (which only happens after submission to a journal for publication).  The popular notion among inexperienced critics that any “just so story” passes muster is an absurd fantasy.

  • http://twitter.com/iamcuriousblue iamcuriousblue

    The fear of spiders is an interesting one. I’ll have to check your references on this, but I’ll take your word on that being a cultural universal. The thing is, I’ve seen claims that fear or disgust with “bugs” in general is a cultural universal, and I think all one has to do is point to the large number of cultures that consider various kinds of insects to be a delicacy to falsify that point.

    • http://www.www.skepticink.com/incredulous Edward Clint

      My time on reddit.com has led me to believe that spiders are more feared than terrorism, North Korea, and Oprah combined!

    • Peter White

      Phobias can be unlearned. Fear of spiders in my family was so extreme that my older siblings forced me to kill any spiders because they were too afraid. I was just as scared as they were but more afraid of my older brothers.

      When I started high school I knew I would have to study spiders in biology. I discovered an article on self hypnosis and tested it on myself to see if it cured my fear. When the dreaded day came for me to face my fear I found I was more fascinated than afraid. I still laugh when I remember my brothers running out of the bedroom in a panic if they saw a spider.

      • kiiski

        There could be a fine line between fear and fascination as far as brain processes are concerned:
        http://www.scientificamerican.com/article.cfm?id=fascinated-by-fear

        An innate fear of heights is probably less controversial than that of spiders, but many still seek out the experience for fun on roller coasters, mountains, and so on. Maybe the fascination some of us have with heights, spiders, or snakes partly stems from a milder form of the “fear” reaction?

        • http://www.www.skepticink.com/incredulous Edward Clint

          Anxiety/apprehension might be on the continuum as well. I know people (myself) that do not feel “fear” re: spiders… but I don’t know of any for whom spiders are as dull and uninteresting as seeing a moth or grass.

  • http://www.facebook.com/Scott1328 Tim McCollough

    You know that EP critics are going to pick through this list you produced and say that the results you point to are not properly the results of Evolutionary Psychology, rather to some other field such as neurobiology, ethology, developmental biology. In effect, you will be left to refute the position that whatever hypothesis are well-founded are not *really* EP and that you are cherry picking from other fields to pad EP’s resume. Of course, they will allow EP to keep the less well-founded and discarded results so that they can deride it for its dearth of well-founded results.

    • http://www.www.skepticink.com/incredulous Edward Clint

      I will be very amused to see anyone claim that research appearing in publications like “Evolution and Human Behavior” or “Ethology and Sociobiology” is not EP. Ditto for well-known EP’gists like Tooby, Cosmides, Symons, Fessler, et al.

  • NotthefakeLevi

    Another hypothesis that was tossed around in the 90s was the prominent role of pheromones in human attraction when further evidence has suggested that the role either much more complex or non-existent.

    See: Warren S. T. Hays (2003). Human pheromones: have they been demonstrated?. Behavioral Ecological Sociobiology 54 (2): 98-97. and Taymour Mostafa (2012). Pheromones in sex and reproduction: Do they have a role in humans?. Journal of Advanced Research 3 (1): 1–9.

  • http://twitter.com/Eshto Ryan Grant Long

    I am happy that kin selection theory for homosexuality turned out to be unsubstantiated since I loathe children and have no desire to help my relatives raise them.

    • http://www.www.skepticink.com/incredulous Edward Clint

      and if it hadn’t then you’d be obligated to babysit?

  • Metalogic42

    Something I’ve always wondered about fear of snakes/spiders/other creepy crawlies:

    I’m not afraid of them in the slightest. Perhaps that’s a result of environmental conditioning, but I can’t remember *ever* being afraid of them. Is the universal fear of such things just “on average, in every society” (allowing for a percentage of outliers), or is it supposed to be truly universal, with outliers considered to have some sort of mental pathology?

    • http://www.www.skepticink.com/incredulous Edward Clint

      Good question. They tend to be fears that exist in every society, but not necessarily in every individual. Individuals vary for many reasons, and in fact that’s normal for most traits. Some people are hyper-afraid of snakes, others not so much at all.

      • Dharlette

        As someone who is also not afraid of snakes/spiders, I’d have to say that I think the argument that this is a cultural fear is far more likely. It doesn’t make much sense to be afraid of spiders, for example, when the vast majority of them are not dangerous and when it’s incredibly rare for a spider to be at all agressive towards a human. I took a course last summer on spiders, and spent a day in the field trying to catch black widows; I feel quite confident in saying they are absolutely not out to get you and are far more likely to retreat and hide than to attack. Even the stories we frequently hear about “brown recluse” bites are usually false. Most brown recluse bites are misdiagnosed by doctors, and are actually just bacterial infections.

        It seems to me that a paralyzing fear of a creature that is probably harmless would be far more harmful than helpful in pre-modern times; spiders are *everywhere* and a fear of them would be crippling if you lived outdoors. Likewise, many of the people who are afraid of spiders are also afraid of roaches, grasshoppers, and other completely harmless insects that couldn’t possibly hurt them.

        This seems more likely to be a cultural thing than an adaptive one.

        • http://www.www.skepticink.com/incredulous Edward Clint

          There is evidence that infants selectively respond more to spider images, and that other primates do. Africa, the home of the ancestors of all of us, has 6,000 species of spiders including many deadly species.

          Cross-cultural studies such as Davey et al (1998) have found spider fear is common, but varies, with cultures. The EP biological preparedness theory would predict this sort of variation.

          • Dharlette

            Given that there are around 37,000 species of spider worldwide, 6000 species is pretty poor levels of diversity for a continent as huge as Africa. Given that Africa is 3x the area of the U.S., which has 3,500 spider species (Roth 1993), that’s a pretty disappointing figure. It would suggest that there are actually shockingly *few* spiders in Africa. To find that a fear of spiders is common (but not universal) and varies greatly with culture as the Davey et al study does is far more in keeping with my hypothesis that fear of spiders is cultural than your assertion that it is genetic. The work with infants is hypothetically more compelling…until I actually looked up that study, and found out that it did not actually study whether infants were afraid of spiders. What the authors say about their study is this:

            “We used the preferential looking and habituation paradigms in three experiments to investigate whether 5-month-old human infants hava a perceptual template for spiders that generalizes to real-world images of spiders.”

            So, they found that babies could recognize spiders easier than they could flowers. That’s not the same thing as being afraid of spiders. It could just as easily suggest that people used to eat spiders and have an innate recognition of them so that they could seek them out! We are a long way off from having confirmed this idea.

          • http://www.www.skepticink.com/incredulous Edward Clint

            I’m not sure why being comparatively speciose is critical. Even a handful of dangerous species that are common enough in your ancestral neighborhoods could be enough to make an intrinsic fear adaptive. 6,000 is quite a lot of species, no matter how big Africa is, or how many more other lands have per square kilometer.
            A purely cultural explanation would predict no differences in the speed or ease of learning. Since there is a difference, this has to be explained beyond culture.

          • Dharlette

            Why would a cultural explanation not predict a difference in the speed or ease of learning to be afraid of spiders? If it’s cultural, you would expect there to be vast differences in the speed/ease of learning in cultures where people are afraid of spiders vs. ones where they aren’t.

            The key reason for my argument against fear of spiders being evolved is that it is an inherently irrational fear. Spiders really aren’t all that dangerous. Most things diagnosed as “spider bites” aren’t actually spider bites at all, and the vast majority of spiders have no reason whatsoever to bite a person. Being afraid of spiders in the wild would be more of a liability than a benefit.

          • http://www.www.skepticink.com/incredulous Edward Clint

            Infants are studied because researchers want to get an insight into behavior before culture has had a large impact. It is unlikely or impossible for every infant to have had time or ability to learn much by cultural osmosis.

            re: is fear of spiders useful?
            I think this is a good question. The issue is not so much what happens in the United States/Europe, but in Africa where our ancestors lived for most of the time our species has existed. How likely were they to be injured or killed by spiders? I don’t know enough about African arachnids of the Pleistocene to say. Evidence that they posed little danger would be a considerable criticism of the theory.

          • Dharlette

            Alright, having read the study more thoroughly now, here are some of my concerns:

            1)They had the infants positioned on their parent’s laps, and the parents could see what was on the screen. It is very, very easy in that position for a parent to unintentionally pass information on to their child that could easily affect their behavior. Essentially, this is not a blind test.

            2)They had a very small sample size to begin with, and then threw out a fifth of their data for being too far from the mean, and even more for “fussiness” and experimenter error. All told, they threw out about a third of their data. Frankly, this smacks of data manipulation to me.

            3)I think their proxy for innate fear of spiders (namely, looking at images of spiders for longer than other things) is not a very good one. How long infants stare at things is usually associated with things that infants love, not things they’re afraid of. Faces are the classic example here.

          • http://www.www.skepticink.com/incredulous Edward Clint

            1. Possible, though suggesting that parents are capable of reliably controlling precisely how many seconds their infant spends looking at a thing is a more extraordinary claim than the experimenter’s hypothesis.

            2. This is actually evidence of good research practices, unless more information is known. A scammer is unlikely to report throwing data out to begin with. Reducing an already small-ish sample size is something any researcher avoids unless they can’t because it reduces the statistical power of their result. Ultimately, these are reasons why any one study is not considered conclusive of a finding. Replication is necessary.

            3. This research technique was developed by Baillargeon et al (1985) and has been in regular use as a viable research method at least the last 20 years over dozens of papers and many research topics. I don’t happen to know of any reviews of the method, but all told I’d consider it robustly tested at this point.

          • Dharlette

            I completely disagree with you on #1. There’s a reason why double-blind is the standard; it’s because people are very, very good at reading one another. Water dowsers get statistically significant results if they have single-blind studies. Horses can do arithmetic in single-blind studies. And babies are very well in tune with their parents, who have had plenty of time to be conditioned to cultural notions about spiders.

            You and I are in agreement here; this needs replicating.

            The research technique is old and well-established, yes, but it’s usually not applied to negative stimuli. Typically, it’s applied to things that babies love, like faces. If your argument is that there is an inherent *fear* of spiders, then you shouldn’t be using the same metric as babies use for things they love. At most, this gets you to the idea that babies find spiders fascinating.

  • http://twitter.com/narjahanam mUhamMAD?

    I’d say I’m pretty much committed to a sociobiological view of things, as summarized in “Too Smart for Our Own Good – The Ecological Predicament of Humankind”; coarse psychological traits have been selected for (and are now maladaptive).

    That said, evo psych claims that are grossly inconsistent with the fact that the neocortex does not develop in a mosaic fashion should be chucked without question.

    • David Pinsof

      The Panksepp paper is misguided in many ways. For an evopsych model of brain specialization that is thoroughly consistent with existing neurobiological data, check out the following:

      http://www.sscnet.ucla.edu/anthro/faculty/barrett/barrett%202012%20PNAS.pdf

      • http://www.www.skepticink.com/incredulous Edward Clint

        Thanks, David. I agree. Clark Barrett is a colleague of mine, and a brilliant guy.

        • http://twitter.com/narjahanam mUhamMAD?

          Well you’re welcome to elaborate.

      • http://twitter.com/narjahanam mUhamMAD?

        “The Panksepp paper is misguided in many ways.”

        Bare assertion.

        “or an evopsych model of brain specialization that is thoroughly consistent with existing neurobiological data, check out the following:”

        Granted I just skimmed this but nothing I see here is really at odds with what I said, or what Panksepp has said about the brain. Or for that matter, the Bates / Elman et alia collaboration in developmental connectionist psychology (← that trigram would make most evo psychs’ skin crawl). If it’s coarse traits you think are being selected for, like connectivity or development timing factors, or cellular biases to operate with certain inputs rather than others, then sure. Otherwise, no.

  • http://twitter.com/narjahanam mUhamMAD?

    From Jaak Panksepp:

    We believe that some currently fashionable versions of evolutionary psychology are treading rather close to neurologically implausible views of the human mind. Although the lower reaches of all mammalian brains contain many intrinsic, special-purpose neurodynamic functions (e.g., basic motivational and emotional systems), there is no comparable evidence in support of highly resolved genetically dictated adaptations that produce socio–emotional cognitive strategies within the circuitry of the human neocortex.

    Although there is substantial neuro–evolutionary evidence for the emergence of certain special abilities such as language (DEACON 1997; PINKER 1997), we still do not know with any assurance that such uniquely human abilities emerged from de novo genetic shaping of cognitive structures or from the re-molding of preexisting adaptations (i.e., exaptations). It is certainly possible that language emerged within a spandrel of evolving multimodal brain complexities selected to generate internal imagery necessary for reflective consciousness, rather than being a deeply ingrained adaptation that emerged from evolutionary sculpting of the fine details in cortico–communicative neural circuits (GOULD 1991). The availability of extra general-purpose ‘computational space’ in the cortex may have been sufficient to allow language to ‘emerge’ without the guidance of specific evolutionary selective pressure. As CLARK (1997) put it, language may have adapted to the emerging complexities of the brain rather than the other way around. Although once language did emerge, it most certainly provided new social environments for both biological and cultural evolution (DEACON 1997). However, the result of such selective pressures, we would assume, was to dedicate more cortical tissue towards general-purpose symbolic processing. Once there was an urge to communicate and some new intersubjective pragmatics (e.g., social intent and social gestures), language may have been guided by cultural evolution as much as by natural selection. In any event, the traditional view that language is a discrete, genetically-dictated specialization residing unconditionally in BROCA’s and WERNICKE’s areas (a simplification that CHOMSKY himself never accepted) has been crumbling for some time now (DEACON 1997). The reason the receptive aspects of speech find their natural home in WERNICKE’s Area may largely be because that part of the neocortex is simply the brain’s most multimodal area for integrating information coming from the external senses.

  • http://twitter.com/narjahanam mUhamMAD?

    Another nugget from Panksepp’s “Seven Sins” paper:

    Although functional specialization must have emerged as cortical columns proliferated and interconnected in increasingly complex ways, there is presently little empirical data, aside from certain perceptual and motor/action processes (e.g., VANDERHAEGHEN et al. 2000), that any types of functional psychological strategies emerged in the cortex via natural selection. Perhaps the identification of the learned inputs into the headwaters of the fear circuitry in the amygdala (LEDOUX 1996), and the reward-association learning circuits of the frontal lobes (ROLLS 1999) might be taken as exemplars, but they may also reflect general learning principles operating in higher brain areas to which intrinsic subcortical emotional circuits project (PANKSEPP 1998a). Put another way, the relatively homogeneous columnar organization of the neocortex is not straightforwardly compatible with any highly resolved, genetically-governed, modular point of view. Indeed, functional studies suggest a vast plasticity in many of the traditionally accepted cortical functions. For instance, visual cortex can be destroyed in fetal mice, and visual ability will emerge in adjacent tissues (see DEACON 1990, 1997). Accordingly, the heteromodal cortex of the human brain may be better conceptualized as a general purpose cognitive-linguistic-cultural ‘playground’ for regulating the basic affective and motivational tendencies that are organized elsewhere. In this view, cognitive processes are ‘tools’ or ‘handmaidens’ for helping regulate the more basic life concerns.

    ^ I would add to this that even CROSS-SPECIFIC transfers of neural tissue in sufficiently immature subjects have been documented (search for “fetal pigs”):

    http://crl.ucsd.edu/bates/papers/pdf/bates-elman-johnson-karmiloff-smith-1998.pdf