Developing Intelligence

Your body’s bilateral symmetry statistically predicts your health, probability of schizotypy and depression, number of sexual partners, and resting metabolic rate (particularly if you are male). Bodily symmetry may reflect “developmental stability” – i.e., influences like disease, mutation and stress may cause a developmental divergence from DNA’s symmetric blueprint. Not only do individuals differ in their environmental exposure to these things, but also in their sensitivity to them: a recent Intelligence article claims that “some individuals grow adaptive phenotypes under almost any conditions, whereas others show disrupted development given the slightest perturbations.” In other words, bodily symmetry reflects a combination of developmental history and genetic quality.

Prokosch et al. further suggest that general intelligence – Spearman’s g – is the neural equivalent of bodily symmetry. In other words, cognitive functioning reflects “neurodevelopmental stability“, whereas bodily symmetry reflects “morphodevelopmental stability“.

If true, then cognitive performance and bodily symmetry should predict one another. To verify this prediction, Prokosch et al. measured the right and left foot width, ankle width, wrist width, elbow width, ear width, ear length, index finger length, middle finger length, third finger length, and little finger length of 78 male subjects.

Each subject also completed standard tests of general intelligence, including Raven’s matrices, WAIS III vocabulary, Shipley vocabulary, and digit span (forwards and backwards).

The results showed that bodily symmetry was correlated with all the measures (except for digit span), and that the size of each correlation was related to how strongly each of these measures predits general intelligence. In other words, body symmetry more strongly predicts intelligence than brain size, nerve conduction velocity, reaction time reliability, and a number of other measures. (This result was replicated this year in a study where symmetry of finger length and palm width had the strongest relationship with intelligence.)

The authors suggest that human intelligence may therefore be a “fitness indicator,” like a male peacock’s tail is a sexual advertisement to female peacocks. Prokosch et al argue this is one explanation for why intelligence is so quickly, easily and accurately assessed in social situations, and why it has such a strong relationship with reproductive success. Furthermore, a new study – currently under review – suggests that the heritability of executive functions (which are closely related to gF) may be close to unity.

Intelligence is more than the peacock’s tail, of course – it provides a variety of advantages. But the important thing is that intelligence is not easily faked: the brain is a metabolic “money pit,” consuming a large percentage of our daily calories and requiring a lot of fat. The very fact that intelligence is so difficult to fake, or even to manifest in the first place, makes it a trustworthy signal of evolutionary fitness for the opposite sex.

Related Posts:
Correlating Brains, Bodies and Behaviors
Evolution of the Brain
Symbol Use and Play in Humans, Chimps and Bonobos

Edit: I replaced the link related to the ease with which people can assess intelligence in social situations – I found something better than what was cited by Prokosch et al.


  1. #1 Katherine Moore
    April 17, 2007

    “Your body’s bilateral symmetry statistically predicts your health, probability of schizotypy and depression, number of sexual partners, and resting metabolic rate (particularly if you are male).”

    For number of sexual partners — is a high number good or bad? (I think I know the answer here, but given our societal configuration, I could see either answer being good or bad).

    I find some of this research puzzling. Within the brain laterality — a lack of symmetry — can sometimes be a good thing that makes you better/smarter at tasks. Also, what does this research imply for handedness, if anything? I would think that someone who is very very strongly right handed (or left) might develop bigger right-side appendages over time than left. Then again, I know nothing about anatomy so I don’t know if growth actually ever works that way.

  2. #2 CHCH
    April 17, 2007

    yeah, I’m with you there. The symmetry/laterality literature is a mess – there are so many theories, and so much data, but very few places where they meet up convincingly (as far as I am concerned).

    That said, I am fond of these “developmental stability” ideas, particularly because it that gene-environment interactions are crucial, and that you miss the big picture by examining either in isolation.

  3. #3 Dennis
    April 18, 2007

    too many confounding variables here to show any kind of direct link.
    AND the environmental impact on growth is both too pervasive and too little understood to give us anything that looks like a real correlation here between body shape and intelligence.

  4. #4 brent
    April 18, 2007

    Didn’t Cesare Lombroso try the same type of thing called Phrenology about a hundred years ago? And 78 male subjects doesn’t seem to be a suitable size for analysis. And what were the enthnicity of the subjects? Rumor has it that there might be a little difference between a Kenyan and an Okinawan. 🙂

  5. #5 CHCH
    April 18, 2007

    Brent – these are within-subject asymmetry scores, meaning that any (symmetric) difference in body structure between races would be controlled for.

    Also, when dissing phrenology, consider that there are real and replicable correlations between intelligence and brain volume (although head size is actually not a good predictor of either, one of the many reasons phrenology failed).

    Regardless of whether dennis or others think we know enough about environment impact on development, these data are very “real” according to mainstream scientific standards of statistical significance. So you have to explain them somehow. If you disagree with Prokosch et al’s interpretation, then let’s hear yours!

    Other commenters elsewhere have attempted to slander the authors of this and similar papers on the basis that they are motivated by philosophies of racial supremacy. I know of no proof for those claims, and I think these knee-jerk reactions conceal more ulterior motives on the part of doubters than the scientists conceivably have.

  6. #6 Dennis
    April 18, 2007

    The data can be as “real” as you want it to be.

    It is still too confounded to show any causal relationship.

  7. #7 CHCH
    April 18, 2007

    We’re agreed on that! No one ever argued for a causal relationship between bodily symmetry and G; the claim is actually that both bodily symmetry and intelligence are similarly affected by a third variable (developmental stability) and hence covary with one another.

    You’d need to manipulate developmental stability in order to show a causal relationship with intelligence/symmetry. That’s a sufficiently grotesque experimental design for me to accept the limitations of the current evidence, particularly in the absence of a better explanation for this replicable, reliable correlation.

  8. #8 Mr. Gunn
    April 19, 2007

    My God, how many flame wars have been started over the failure to understand that correlation doesn’t equal causation!

  9. #9 MoonShadow
    April 23, 2007

    actually, i think there’s a problem here…

    we know that this is a correlational study. But…how and what are the proper ways to measure “developmental stability”? because it’s somehow in another category of “developmental trends” and there are so many to choose from, and dozens of permutations that could make up a “stable” line of development. And there’s also been findings that to make development healthy you need some ups and downs in there. So..maybe we should define “developmental stability” first..

  10. #10 Chimera Proxyment
    November 21, 2007


    A model or a weight-lifter don’t = High Intelligence.
    If it did then Arnold and Nicole-Smith would be the ones writing E-MC2 and Hawking would never have had a chance.

    Anyone can write something that appeals to the widest audience and by this means make some cash and sell a book. Do you really think just because they have a PHD and no real quality science going on they would just take the high-road and deal in basic lab-work? Of course you know people are looking for superficial validation that they are somehow better than others and they also want to feel their mind is beautiful and a peacock’s tail or something – and symmetry will follow.

    I guess drinking Diet makes you thin like the models on TV – Right?

    Intelligence is hard to measure:
    1. If you measure within a set of ‘knowns’. Then how do you measure the ability to identify real changes in the environment as they come up. With this you very easily miss any ability or trait or fact not identified by the test-maker.

    2. Creativity can be contained by no absolute definition or field.

    3. Synthesis of (Known and Innovative) forms of thinking form the basis for successful adaptation and ultimate survival. Any test maker which imagines that the ‘spatial’ mastery of 3-4 dimensional experience can be somehow summed up in 2D models and by multiple choice is deluded. I know Athletes who work very hard to do things non-athletic scientists simply cannot do. Why do the makers of these tests seem to somehow do so much better at spatial ‘comprehension?’ Does the branch of humanity with better test taking scores really understand 3D relationships in space better than those who work with it to ultimate mastery directly? You would think so; the scores sure say from some dumb series of multiple-choice and by means of the 2D drawings that they can understand and manipulate spatial relations better – the IQ they end up with certainly falls in line with what they would very much like to hear.

    4. Most ‘known’ ideas, thoughts, facts, relevant to any times idea of intelligence is temporally based on that locality. Anyone without the same culture and experiences will inevitably score lower on IQ tests. Give this society 100 years and these tests will look like a horrendous step in the wrong direction and the ideas dated and quant. Instead of trying to Quantify why don’t these ‘great-minds’ figure out how to really teach to the point where students can understand and organize their thinking process?

    I will tell you why: EGO.
    Dogs, Monkeys and small children deal with Ego and work toward its realization. Once past the point of knowing you have one (An Ego) what good does developing more bad science based off of supporting this destructive natural tendency do for anyone?

    Why not a test on how and what one could first profitably learn in order to pursue so-called higher level thought?

    Why do the current tests conform to the idea that the makers of the test and others like them from similar genetic and linguistic-social backgrounds all score highest on the test? Why so much flattery heaped on the choir?

    This ‘Symmetry-thing’ is a trivial format for assessing the merit of intelligence in others – whatever that is. It lines this guy’s pocket green and will get him talked about for like two years and maybe he can come up with a new gimmick. So keep trying to measure intelligence and rely on superficial assessments and samples too small to be realistically considered. This is far easier than acting in a fashion that would really make an impact on the well being of others.

    EXAMPLE: I am some PHD something or other in Psychology and I…

    I will come up with an idea: attention-getting and superficial.
    Bring out the small sample of people such like 100 or so… And presto just like magic I got my name in the press for saying 78% of men and women with superior symmetry like to eat traditional English food. In fact the most symmetrical and probably intelligent ate at least two meals a day but no more than three. I then sellout every London paper with the headlines and come out looking like a researcher with an Idea.

    I do all my research in Wales.

    Now let me write my book and pay me. I don’t care what you really think and I care even less if you prove me wrong. I got my PHD and the status this brings and now I got my name in lights and the big payday. Maybe I can come up with more of this when I need that vacation home.


    If so-called High-IQ men and women fall for these things all I can say is:
    “Impressive teeth, but can you really use them?”

  11. #11 Chimera Proxyment
    November 21, 2007

    So if my ideas cannot so easily be understood here is some background:

    My family came from a background where the father did not graduate High-School.

    I went through public schools and received only opposition from the individuals who were set with the purpose to teach me. What was encountered however consisted of selective attention on ‘good’ students and a preoccupation with hammering down any questions ‘inappropriate’ or supposedly tangential to the subject at hand. They treated me like a second-class citizen. I received minimal quality learning and had no family involvement to indicate to me the importance to successfully navigate this systemization of my thought process, logics, and ability to retain facts deemed indicative of high-mental performance by the established hierarchies of that time.

    I was supposed to learn in a way analogous to a machine and allow for being ignored while others received undo attention when they were an ‘A’ student or a ‘problem’. I quickly learned to distrust these authorities and in fact just ignored the arbitrary facts these ‘teachers’ attempted to ‘force-feed’ me. Answers to questions important to my full understanding were neglected.

    I personally consider art, philosophy, and eastern-studies my forte’. I studied and have taught martial-arts. I play chess in my free-time and listen to everything from techno to classical music.

    Admitting that I did not respect the words coming from the mouths of my teachers and saw no superiority among my peers it ended up that my progress would be broken completely when put into a class for the impaired. At the time it was not PC to call these people ‘retarded’ but that is what I was called by my peers. My IQ score came out with a rating of about 90.

    So white middle class educated elite decide to make these tests and design learning/teaching programs – a huge responsibility.

    So, while I do possess talents that have performance values exceeding the median within those select activities. I do not own high-level performance in the areas that our intellectuals deem corollary to deep understanding and / or profound in the values of cognitive functional performance that they deem intelligent.

    My ‘inner-language’ is different. I rejected at an early age anything associated with that important to my teachers. I carried no respect for the system, (most but not all) the teachers, and often wondered how my peers understanding and judgments carried such certitude along with the name calling for being ‘slow’. Needless to say I did not want to speak with any substance with people set to browbeating me every-time I opened my mouth.

    Funny: by high school they told my parents that I was some kind of ‘extra-intelligent’ individual and I thought my classes we too slow for me after I had a meeting with him in reference to my attendance and discipline.

    I still score a 90 on a general IQ test. Personally I lose my patience and cut out of the extra investment of time and just skip all the garbage when it gets tedious.

    I personally live an alternative lifestyle and do not buy into the whole: having a high IQ means something substantial, that going into a high paying job indicates deeper understanding and intellect.

    In fact I carry little regard for the whole monstrosity of socially defined values and systemization without reflection; especially when an intellectual system appears to me only in many cases to go about supporting the self-centered aims of the small group in power calling the shots.

    Is it really true that a small group of highly systemized upper-middle-class white men and women carry ownership on truth, the meaning of life and the capacity to evolve oneself within the framework of an ever changing universe?


    Why then do these tests now support the Elite of another race entirely? Asians tend to score better when they do take the test. Coincidence? China will soon it appears take the place of world-power. When they possess ‘ownership’ of the best jobs, define what the meaning of a quality education consists of and dominate politics and economics do you really think the tests will reflect them as dumb, slow, or ‘retarded’?

    Ancient Greece: center of learning and brilliance of the age. What happened?
    Moorish Spain? London? The industrial revolution of the East-Coast of North America? And so on through to the Nerds and computer software revolution right after the aerospace revolution where rocket engineering was top-dog for the greatest minds? What happens? Did the Greeks get somehow ‘dumber’?


    Time – Place definitions about what constitutes intelligence just do not hold up and seem to have nothing to do with Genes. Without the contribution of the 0 from the East-Indian Mathematicians from antiquity we would not have had the tools to calculate without an abacus and Europe would not have reached its pinnacle the way in which it had.

    I am all for Chinas ascendency to power. I hope their methods of selection for quality minds exceeds the performance of American and European work. Afterall as it stands we just can’t keep up now.

    Maybe we all are slow here in the West.

    Maybe we just don’t have the same cultural values.

    Are we that arrogant? What about the Genes? Sounds like the ‘search for the superman’ blond haired and blue-eyed; or had that changed lately? Certainly Africa is in trouble.

    So why concern ourselves with subjective beauty, symmetries and mentations?

    Would not the underprivileged and even our current power base do well just to concentrate ever more diligently on developing an understanding on how we learn and can teach in tandem in actuality?

    Instead all I see is throwing money at the problems. Deffinitions of intelligence that give no regard to potential and instead merely stratify classes and separate races. This all strikes me as not merely some ‘vague’ moral wrong but rather a misunderstanding by our intellectual elite bordering on stupidity and possibly even myopic variations of skewed perspectives that create systems that function on irrational biases.

  12. #12 erik
    February 7, 2009

    Yeah, how weak, I’m glad everyone here feels the same way. I’m bashing this ‘study’ even though my fingers are all very symmetrical! Stable development doesn’t equal intelligence later. The reason we even became more intelligent was to adapt to unstable environments. Totally weak!

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