Developing Intelligence

Several have criticized my post on handedness by pointing to evidence (or hearsay) that animals do have handedness. This evidence comes in several forms:

Anecdote: “My cat plays with its right paw

Individual or Activity-Specific Cases: “Horses reliably pick one leg to lead their galloping” or “Chimps prefer to fish with their left hand

Marginal asymmetry: “X chimps are right-handed for every Y left-handed chimps.” [where X/Y < 9]

Let's be clear: handedness in humans is immensely asymmetric (9:1) at the population level, and holds across multiple tasks within individuals (most people are right-handed not just for writing, but for throwing, batting, fishing, etc).

So while anecdotes are entertaining, activity-specific cases are interesting, and marginal asymmetries are common (after all, motor programs such as galloping usually become automatized and require that some leg be used first), none of these traits alone can be used to demonstrate handedness as it exists in humans, since humans manifest all these traits in combination.

Why shouldn’t non-human animals have handedness to the extent humans do? Corballis argues it has to do with the development of language – but that’s the previous post.

Comments

  1. #1 Sven DiMilo
    October 17, 2007
  2. #2 CHCH
    October 17, 2007

    Thanks Sven, but your example

    1) Is activity-specific (toads show “pawedness” for the underwater but not paperstrip test)

    2) Is only weakly asymmetric (at most, 7:3 in favor of righties)

    So I contend this doesn’t contradict the premise of Corballis’ argument. Very interesting though.

  3. #3 Simon G.
    October 18, 2007

    Mike and a few colleagues of mine have pointed out that the New Caledonian crow is handed (or “beaked”) when they use their tools: PDF , and this is covered in a number of related papers .

    The general argument is that handedness is an outcome of lateralisation (i.e. certain tasks are processed in certain hemispheres of the brain, such as language being predominantly processed in the left hemisphere in humans). This is important, as Mike (& others) have argued that this allows rapid processing of intensive tasks like language, tool use, etc.

    If Mike’s correct then, we should only see laterality and handedness in cognitively complex animals – chimps, dolphins, and crows being the obvious ones to look at.

    –Simon

  4. #4 Sven DiMilo
    October 18, 2007

    curses…foiled again

  5. #5 Drugmonkey
    October 18, 2007

    CHCH, you have some very credulous and illogical thinking going on here. or at least Corballis has some illogical thinking which you appear to be echoing, unexamined.

    “As Corballis mentions in his 2003 BBS article, even the great apes – our closest relatives in the animal kingdom – tend not to show a right-hand preference unless raised in captivity, suggesting handedness is learned through imitation of caregivers. So why should humans be the only species to show clear manual asymmetry, 9:1 in favor of righties?”

    there were also some comments following your post underlining this point. WTF? What on earth relevance does it have through what developmental environments the nonhuman primate “learned” laterality? The question at hand is “are their brains capable of supporting unimanual dominance”! His whole line of discourse revolves around the available left-side wetware of the brain and whether communication lateralization drove manual lateralization, vice versa or whether they co-evolved. He’s arguing the first and fails to make a convincing case. Likely because of the unsupported a priori…

    Getting to some specifics, the contrast with humans is idiotic since you could similarly argue that humans acquire the lateralized behavior in imitation of caregivers. sorry but there are simply too few “wild boy of aveyron” types to draw any other conclusion.

    nevermind the point that many nonhuman animals express clear laterality even absent human caregivers. much of which I will note that Corballis actually discusses. The point for the casual reader here who has not made it over to the Corballis paper is that it is VERY clear that this paper is an academic exercise in the worst sense, not a highly scientific review, IMO. There are many places, even conceded by the author, in which the argument is highly speculative. Count up the number of times “may” appears in his arguments.

    All the bipedal / free the hands / gestural language crap is unsupported in Corballis’ arguments merely by taking a slightly different (and IMO more supportable) view of the underlying facts of lateralized behavior in nonhuman primates.

    ahh, but it is not lateralization, but 90% right dominance in the population that is the key, you say. so what does that tell us about the lefties and ambidextrous humans? are they language impaired? no? how could those genotypes or developmental errors have been sustained? why would we expect the “tolerance” of 10% human lefties to be a different phenom than the “tolerance” or something other than the 9:1 ratio in other primates?

    the argument just doesn’t hang together when examined.

    it does, however make some very interesting psycho-archeological predictions however which would be interesting to see tested…

  6. #6 CHCH
    October 18, 2007

    Hi Drugmonkey – I’ll try not to be too defensive here…

    “What on earth relevance does it have through what developmental environments the nonhuman primate “learned” laterality?”

    The relevance is that Corballis assumes humans have innate predispositions for right-handedness, at least at the population level, and he argues that this is somehow related to language. So clearly it is a problem if nonhuman primates show handedness – *unless it’s learned*.

    “The question at hand is “are their brains capable of supporting unimanual dominance”!

    I think that’s an interesting question, but my perspective is that the real issue is why nonhumans animals don’t show the same degree of handedness as humans.

    “Getting to some specifics, the contrast with humans is idiotic since you could similarly argue that humans acquire the lateralized behavior in imitation of caregivers”

    You could argue this, but it’s a premise of Corballis’s piece, meaning he takes it for granted. I didn’t bother getting into that argument because I’m not very interested in that question (I take it you aren’t either :).

    “ahh, but it is not lateralization, but 90% right dominance in the population that is the key, you say. so what does that tell us about the lefties and ambidextrous humans? are they language impaired? no? how could those genotypes or developmental errors have been sustained? why would we expect the “tolerance” of 10% human lefties to be a different phenom than the “tolerance” or something other than the 9:1 ratio in other primates?”

    I think you’re being disingenous here, but since I have liked your comments in the past and love having thoughtful people here, I’ll bite: I never suggested that 90% was a magic number or key, but rather that animals do not show handedness in the way humans do, and that the 9:1 ratio is one way in which they fail to show human handedness.

    You have also missed the point that Corballis’s argument is about phylogenetic relationships between handedness and language, not ontogenetic relationships.

    I agree that there are interesting questions about people who may be ambidextrous, as well as predictions for cognitive archeologists, but I don’t pretend to have answers (nor do I necessarily endorse Corballis’s view).

    I’m getting the feeling that you wished I’d written a more critical review of the paper, rather than merely summarizing it. I would have been more expressly critical if I had more time, and a summary seemed a better way to use that time since a summary may be more useful to readers (and me) than my current opinion.

  7. #7 michel Thiery
    January 10, 2008

    If you wish to understand anything in handedness please look at my blog both in French and English :

    http://bolechette.blogspot.com

    and especially the book therein : right hand, left hand, what significance ?

  8. #8 Holger Schneider
    June 27, 2009

    Your argumentation is based on the premise that hand preference is linked to the language center in the brain which is in the left hemisphere. On this basis your arguing that most people have to be right-handed. This is wrong. There is clear scientific proof, e.g. by MR-scans, that handedness is linked to the motor cortex (Gyrus praecentralis and Gyrus postcentralis). In right-handed persons, the left motor cortex is dominant and vice-versa. William D. Hopkins, primalotologist an the Berry College and Yerkes National Primate Researsch Center shows that in great apes -especially primates- there are 50 % right-handers and 50 % left handers. Primate brains are very similar to that of human beings especially concerning the lateralisation of the motor cortex. So why shouldn’t there be 50% left- and 50% righthanders in humans if there are no cultural influences. You are argumenting that 90% of humans are righthanded. Do you know how these statitics are made. The scientists ask people whether they are right or left handed. But in our society many lefthanders are trained right-handed from childhood on. They don’t know that they’re lefthanded. Asked of their hand-preference they would describe themselves as righthanded. Do you get the clue?

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