Sunday, June 07, 2009

Psychology and Finger Length

There was a lot of interesting stuff going on at the Human Behavior and Evolution Society (HBES) conference held recently at California State University, Fullerton (USA).

You can check out and read the abstracts of the papers presented. It gives you a real flavor for the kind of "science" being done in the name of evolutionary psychology.

There was a session on "Digit Ratio." Apparently this is a new field of research in evolutionary psychology. It attempts to correlate the lengths of your fingers with various behaviors. The most relevant parameter appears to be the ratio of the length of your index finger and your ring finger (2D:4D). In women these two fingers are the same length while in men the fourth finger tends to be slightly longer.

Two of the papers at the HBES meeting were from Gad Saad, an Associate Professor of Marketing at the John Molson School of Business at Concordia University in Montreal, Quebec, Canada. He's the author of a book called The Evolutionary Bases of Consumption.

Gad Saad recently posted on the Psychology Today blog [Can the Length of Your Fingers Affect Your Consumption?]. He said ...
Some have argued that the 2D:4D is nothing more than a "sophisticated" form of palmistry. Others have ventured that it belongs with astrology and phrenology, former scientific fields that are now completely discredited. The reality is that the sheer number of papers that have yielded robust 2D:4D effects in prestigious peer-reviewed scientific journals suggests that it is going to take more than a flippant dismissal, as the means of critiquing this thriving research stream.
That's an interesting argument. It doesn't address the real issue; namely, whether those papers are scientifically valid or not. It merely states that because they are reviewed and accepted by other evolutionary psychologists they must be true. This is, unfortunately, becoming a common excuse these days.

What if peer review is failing to distinguish good science from bad science? I think this is what's happening in most disciplines these days.

Here's the abstract of the presentation given by Gad Saad's group at the HBES meeting.

Finger length ratio and attitudes towards several product categories
Marcelo Vinhal Nepomuceno, Gad Saad, Eric Stenstrom, Zack Mendenhall

The second-to-fourth finger length ratio (2D:4D), a sexually dimorphic trait, is affected by androgen exposure in utero. It has been linked to a wide range of human phenomena including economic outcomes, personality, sexuality, athletic and musical abilities, health status, and occupational interests to name but a few examples. Surprisingly, it has yet to be investigated in the consumption context. Using a sample of 555 university students, we examined if finger length ratio was negatively correlated with products with a male penchant and positively correlated with products preferred by females. Participants responded to several items, which assessed their attitude towards several product categories namely: cosmetics, electronics, pornography, clothing, movies genres (drama, action, science fiction, romance, animation and war), sports (hockey, boxing, synchronized swimming and gymnastics) and genres of video-games (First-person Shooter, Real-time Strategy, Party-game, Platformer and Life Simulator). Two key findings were obtained. First, the length of the index finger relative to the sum of the lengths of all four fingers (2rel) was generally a better predictor of product attitudes than 2D:4D, given that it yielded a greater number of significant effects. Second, we found significant (p<.05) or marginally significant (p<.10) correlations, in the predicted directions, between 2rel and attitudes towards four out of the nine product categories preferred by males and towards five out of the ten product categories preferred by females. The remaining product categories were not significantly correlated to 2rel. This constitutes the first study to demonstrate a link between a morphological trait and attitudes toward specific products.

Could this be scientific evidence that palm reading actually works?

Does this have anything to do with evolution or is the evolution of consumption a separate study?


  1. It merely states that because they are reviewed and accepted by other evolutionary psychologists they must be true.

    I don't think you're addressing the argument as stated: Saad argues only against specifically flippant dismissal. He might perhaps go on to equate avoiding flippant dismissal with uncritical acceptance on the basis of peer-review, but you have not quoted him as saying so.

    That being said, your decidedly non-flippant dismissal, based on a detailed examination of the actual content of the study, appears persuasive.

  2. In this context, would "flippant" mean "flip the bird"? ;-)

  3. Let me correct myself: You don't examine the study in any depth, and it's impossible to determine the validity of a study from the abstract; we need to examine the actual methodology. The abstract can tell us only if the hypothesis is falsifiable, which it does appears to be in the cited case.

    Of course, the abstract raises some obvious questions: Were actual sex, socioeconomic background and other potentially confounding variables controlled for? If multiple comparisons were made, were the appropriate multiple comparison statistical tests performed and their results reported? Were the conclusions supported evidentially by the results?

    Also: Is it really well-established that relative finger length is sexually dimorphic? It it well-established that it is affected by prenatal androgen exposure? Presumably the citations in the actual study are not reproduced in the abstract.

  4. Barefoot> Presumably, yes, finger length is linked to gender and androgen exposure. If not, then that would be the significant part of the study, not the future cultural impacts of finger length, as the study appears not to control for the gender of the person whose fingers are being measured. If it had been, can you control for the factor that means men with long fingers are physically larger, and then remove the non-Gaussian effects of pro athletes from whatever measure of central tendency you've used? Or is it necessary in this case to include those outliers, which are bound to be extreme, in order to demonstrate the effect the study claims to see?
    No, I can't answer that from the abstract, but the abstracts typically at least mention a confounding variable as large as gender. Most of the abstracts I've read are something to do with ecology, which happens to have significant confounding variables in most cases. Maybe evopsych papers aren't in this habit?

  5. Hi Dr. Moran,

    I stumbled on your post accidentally whilst conducting some Internet searches. My typical position is to not reply to such blog postings but in this case, your post contains so many errors and is so laden with self-righteous arrogance that I thought it fun to reply.

    (1) Unlike what you state, 2D:4D research is not "reviewed and accepted by other evolutionary psychologists." Here is a sample of journals that have published 2D:4D research in the very recent past:

    Archives of Sexual Behavior, Laterality, American Journal of Human Biology, Journal of Autism and Developmental Disorders, Behavioral Neuroscience, Journal of Anatomy, International Journal of Biological Anthropology, Personality and Individual Differences, Arthritis & Rheumatism, Evolution and Human Behavior, Forensic Science International, European Journal of Personality, British Journal of Psychology, Journal of Pain, Social Neuroscience, Hormones and Behavior, Journal of Human Evolution, Proceedings of the National Academy of Sciences, American Journal of Physical Anthropology, European Journal of Applied Physiology, and Early Human Development.

    Most of the reviewers of the latter journals are not evolutionary psychologists. I suppose that all of the reviewers of the latter journals have been lobotomized by the allure of evolutionary psychologists.

    (2) You do not provide any criticisms of 2D:4D research in general and our own work in particular. There might be valuable criticisms of our work, alas it is not forthcoming from you. Instead, you place quotations when referring to the "science" that evolutionary psychologists do, as a means of simply dismissing any research that is conducted by this group of individuals. Frankly, this is unbecoming of a fellow scholar.

    (3) I do not wish to be mean-spirited so I apologize in advance. I did a quick search to explore your contributions to science. Does the lack of any work constitute as scientific progress in your mind? It is somewhat off-putting to have someone stand on a "haughty sandbox" proclaiming that all evolutionary psychologists are fools, to then find out that the glorious "real scientist" has done very little to contribute to science over the past twenty years or so (the time since "fake scientists" such as evolutionary psychologists have been producing thousands of articles).

    Scientific work is messy and hard. At times, scientists take false turns. However, they are in the proverbial boxing ring, fighting it out, and doing their best.

    Might it not be best for you to contribute to the process rather than simply castigating the works of others in a haughty and frankly ignorant manner?

    (4) The title of my book is The Evolutionary Bases of Consumption (not Basis). That said, many thanks for the promotional plug.

    Warm Regards,

    Gad Saad

  6. Here we go thing about good ol' Larry, at least he's CONSISTENTLY dismissive and isn't the least bit bothered by not reading up on those things he's already made up his mind about. He might want to start here: Manning, J.T. (2002). Digit ratio: a pointer to fertility, behaviour, and health. Rutgers U. Press. New Brunswick, NJ.

  7. Nice post, Gad. I think you might know one of my ex-students, Kelly D.?

  8. Many thanks Skeezix. Perhaps you can contact me privately as I am unsure who you are and/or who "Kelly D." is. My email is not hard to track down!


  9. Gad Saad writes,

    I do not wish to be mean-spirited so I apologize in advance. I did a quick search to explore your contributions to science. Does the lack of any work constitute as scientific progress in your mind?

    Thanks for correcting the typo, Gad.

    And thanks for the advance apology concerning your ad hominem attack. I suppose that in the mind of an evolutionary psychologist that counts as a valid excuse.

    BTW, I appreciate the fact that you sign your real name to your comments.

  10. The logic behind this line of research goes:
    Finger-length ration (FLR) is affected by androgen exposure in utero. We can therefore use it as a proxy for (otherwise unmeasurable) androgen exposure in utero. What aspects of human behavior and cognition are affected by androgen exposure in utero? That seems a legitimate scientific question that generates lots of testable hypotheses.
    It also seems legitimate (and not even necessarily adaptationistical) to ask evolutionary questions (not so easily testable, perhaps) about why and how certain behaviors and cognitive processes (in humans and/or other animals) are linked to androgen exposure in utero.

  11. I doubt that Larry will read John Manning's book, so here is a brief review of it:


  12. "evidence is persuasive, but not yet definitive, that higher levels of testosterone during this critical developmental stage facilitates the growth of the ring finger, while higher levels of estrogen facilitates the growth of the index finger"

    So, you can't quite just go head and assume digit ratio is a proxy for androgen exposure, can' t you.

    What aspects of human behavior and cognition are affected by androgen exposure in utero, is an entirely separable question from that of finger ratios. You cannot replace one for the other unless you are certain a casual mechanism relates them in first place.

  13. "You cannot replace one for the other unless you are certain a casual mechanism relates them in first place."

    No, that is not the way science proceeds. You don't bail out of hypotheses because you are not yet *certain* of the connection between variables.

    That is *why* you conduct research. If you were already certain... well, not much of interest there.

    Strange way of thinking.

  14. Jeez, lecturing me on how science is done? hehe

    What I say is just logical. Before you use finger ratio as a proxy for androgen exposure, make sure it actually DOES respond to androgen exposure.

    You cannot stay for ever at the level of mere statistic correlations...yet you guys seem to all to happy to forever ignore the actual mechanisms.

  15. Here's one experiment... yet more empirical research that contributes to the corpus of corroborating evidence.

    Effects of prenatal alcohol exposure on forepaw digit length and digit ratios in rats

    Prenatal exposure to alcohol can cause limb and digit defects. Variations in digit ratios in humans are associated with prenatal testosterone exposure. Since prenatal alcohol can reduce testosterone in rats, the effects of prenatal alcohol were measured on rat digit length. Pregnant Sprague-Dawley rats were intubated with 0 g/kg, 4 g/kg, or 6 g/kg of ethanol from Gestational Day 8 (GD8) to GD20. The 0-g/kg group and a nonintubated group served as controls. At postnatal day 31, forepaw digit lengths were measured and digit ratios calculated. Females had smaller digits on both forepaws and higher digit ratios on the right forepaw than males. Rats exposed to 6 g/kg of ethanol had smaller digits than controls on both forepaws and higher digit ratios than controls on the left forepaw. Rat digit ratios differ between the sexes, and prenatal alcohol exposure affects digit ratios. The results are consistent with a perinatal disruption of testosterone levels by alcohol and/or of testosterone's effects on digit length and ratios. An alternate interpretation is consistent with a retinoic acid-mediated effect of alcohol on digit length and ratios.

  16. Alcohol? yeah, it could be the RA pathway. The abstract is mysterious about the sex of the alcohol- treated rats.

    How about an experiment actually injecting or depleting testosterone in pregnant rodents? are digit ratios consistently affected?

    What is the independent indicator for measuring the exposure to prenatal testostetone in humans, such that we can say it assocites with finger ratio? Are blood samples being taken from fetuses, pregnant women, or something like that?

  17. Sure... I'm all for more experimental studies (there may be more extant -- I've not done more than a cursory search).

    So we've come along way from Larry's first assertion that we could dismiss this digit ratio line of research as nothing more than palm reading.

  18. No, we have NOT come a long way.

    The two questions I've made are pretty obvious steps that sould follow. Instead of checking these basic points, you guys are all about fishing for stuff like statistical correlations between car brands and the digit ratios of their owners.

  19. Try to get a study doing experimental androgen interventions on human fetuses... best of luck getting your IRB to sign off on that.

    Correlational research in some areas of science is the only way to go. However, some advanced statistical techniques (path analysis, etc.) help to tease out rival hypotheses.

    Let's all start smoking again, because, after all, there have been no randomized, double blind experiments on humans that have shown a causal relationship between smoking and disease.

  20. Look Mike, if you are a fan of digit ratio, you SHOULD know what indicator was used for androgen exposure, that was foudn to be correlated to digit ratio in the first place. As you say, i doubt it was experimentla manioulation of androgne levels. So, what then?

  21. ...a very large body of correlational research produced by many independent investigators that generally corroborates the hypothesis. The exact proximate mechanism(s) by which androgens affect digit ratio would be great to know.

    The exact proximate mechanism(s) by which smoking causes lung cancer in humans would be great to know too. But the conclusion that smoking causes cancer is based on correlational, not experimental, research.

  22. Jeez! I'm not asking for experiments, even. My question is pretty simple. To argue androgen exposure is involved, they correlated finger ratio with...what? You should know that, Mike

  23. ...with other variables that presumably may also be influenced by fetal androgen levels. In other words, it is of particular interest because digit ratio may be an easily observable assay of fetal androgen exposure.

    The point is that there is an increasing number of studies that generally corroborate this association.

    If the association is correct, by observing digit ratio one might be able to get a (very) rough estimate of risk of heart attack, breast cancer, fertility, spatial ability, etc.

    Also, such correlational studies may be heuristic. They suggest areas of profitable future research, and also suggest what variables might be examined in experimental research (with animals) or quasi-experimental research (with humans) to nail down the causality.

  24. I correlate digit ratio with yet another "presumed" indicator of androgen exposure, huh? It gets pretty circular and non-serious.

    Male-female dichotomies (such as those found in many consumer options) may show some correlation with some testosterone-influenced traits(chest hair? penis size?), but, it is hardly a deterministic relationship. All we can say is that some of thses correlations, if replicable (many aren't!!) suggest testosterone exposure may have small, clearly overridable influece on male-female choices. and the explanation of this small effect may be largely due to fulfilling cultural associations, and no so much with the biological traits themselves ( ahairy guy may like a pink car but will buy a blue one so his friends won't tease him about being gay)

    It's not more xeciting than that. You can ditch right away those visions of people taking decisions as if they were hard-wired in their androgen exposure. That's bullshit.

  25. "with no more data than that gleaned from a passing glance at your hands, a stranger might infer whether you are likely to have homosexual inclinations, are highly fertile, may eventually suffer from a heart attack or breast cancer, have musical aptitude or sporting prowess, and a surprisingly long list of other characteristics"

    This, my friend, is bullshit.

  26. ...not if you are an empiricist, are familiar with the scientific literature, and understand the word "likely."

    All evolutionary psychologists are culture/biology interactionists. However, the mind/brain is a product of evolution, cultures do not arise de novo, and our evolved psychological adapatations are likely to be manifest in cultural norms and behavior.

    The ability to learn a language as a toddler with virtually no training is an evolved psychological adaptation. That there is cultural variability in what language is spoken as an adult doesn't negate the existence of this psychological adaptation.

    Steven Pinker presents a good overview of these topics. See his book "The Blank Slate: The Modern Denial of Human Nature."

    Also, see his talk re this book on

  27. Hahaha
    I'll just say that if you guys are not palm-reading,'re doing an excellent job of looking like palm-reading.,

  28. I think I'll start smoking.

    Can't stand these "just so stories" of the epidemiologists -- this "supposed" relationship they have between cancer and smoking is all just correlational bullsh*t. Palm-reading.

  29. Go ahead and smoke. You won't necessarily die. Cancer is a complex trait. Smoking may help trigger it, but it does not guarantee you'll get cancer, unlike say, drinking plutonium.
    Same thing about digit ratio (though I honestly doubt the statitsics are nearly as close as good as those for smoking and cancer). The fact that you have a given digit ratio does not even remotely guarantee what your car choice will be. It's a complex choice, and many other factors weigh in. It's not a deterministic relationship.

    The problem is that evolutionary psychologists drool thinking that our decisions are highly deterministic and biologically "hard wired", which is bullshit. Looking at someone's hand is NOT a reliable, determinidtic indicator of any of its correlations. To think this way is indeed as false as believing in palm reading.

    Once again, I must insist on the point of replicability of statistic corrlations. With tobacco, there is replicability, but this is seldom the case for studies of evo psych. What this menas is that there is simply NOTHING there, not even as un undeterministic factor that may help trigger a given behavior. .

  30. "...evolutionary psychologists drool thinking that our decisions are highly deterministic and biologically "hard wired" "

    You don't know much about evolutionary psychology... NONE of them think behavior is determined only by genes. Straw man there. All are nature-nurture intereactionists.

    As with most critics of evol psych, you have a sophomoric understanding of the field. Suggestion: read The Blank Slate by Pinker, at least for a start, then come on back with a more informed perspective on the field.

  31. You're not facing the reality of your field: that you're often caught with your pants down making hypotheses of gene selection, when no genetic variation is involved.

  32. Hahaha. Yeah, I wish I knew any evo psych that took the environment seriously (other than nod acknowledging its existence)

    This happens often, when you can only think evolution in terms of gene-reductionism and selection, be it in psychology or in behavioral ecology.

    Behavioral ecology has the same problem than evolutionary psychology: statistic correelations, a fairly "economic" explanation, all very darwinian... and little regard for organismal reality. That is when closer study reveals no genetic variation for that trait and they get caught with their pants down.

  33. Gosh... you've convinced me, and the rest of the idiots in this field, that our research program is without merit.

    We'll switch our research soon to accommodate you.

    And, those damn epidemiologists really piss me off too. I hople they all see the light as well.

  34. Mike says,

    Gosh... you've convinced me, and the rest of the idiots in this field, that our research program is without merit.

    The research program is called adaptationism.

    It bothers me that evolutionary psychologists are so committed to it because I'm a pluralist. But that's not the major problem I have with evolutionary psychology. The major problem is that evolutionary psychologists don't even seem to realize that there's another way of thinking about evolution.

    They don't seem to be adaptationists by choice—which is bad enough—but by ignorance.

  35. We must thank evo psych, thopugh for putting it up so clear, for all too see, what is wrong with unrealistic emphasis of the gene and adaptation that bypass mechanisms at the organismal level and gets caught with its pants down all the time. Not precisely "elegant" science.

    The attitudes of its followers say as much, too.
    Mike's sarcasm "We'll switch our research soon to accommodate you" is for real. Adaptationists are never going to change their happy ideology.

    If you want a more serious, REAL evolutionary psychology, go to the developmental psychologists (Gottlieb, for instance), not the Dawkins fans.

  36. You may wish to familiarize yourself with Tinbergen's four questions. That may calm your nerves.

    Different specialties focus on different questions -- none of which alone are sufficient to provide a complete description of behavior.

    If you are not an adaptationist, that leaves creationism. Best of luck with that research paradigm.

  37. "If you are not an adaptationist, that leaves creationism"

    You REALLY think so, huh. GAWD you guys are fucked up! I'll do you the favor of briefly explaining, out of the goodness of my heart.

    Gene selection is actually only part of explaining the evolution of adaptation and complexity. It's necessary , but it's insufficient. I don't have any problem in saying this because I am familiar with the other mechanisms and I understand how they are different form selection and how is it that they are relevant (drift, phenotypic, plasticity, exapation)

    Adaptaionists like you are squeezing it all into selection just to fight damned dumb creationists...It's like if therer were anyhting else than selection you'd have to run into a church...I'm sorry I can't take it anymore. You're too stupid. Good bye! (end of conversation for this endpoint)

  38. We're way ahead of you.


    Adaptations, Exaptations and Spandrels, by Buss, Haselton, Shackelford, Bleske


    How can we identify psychological adaptations?

    Adaptationism does not suggest that everything is an adaptation. However, without natural and sexual selection, not much functional complexity would evolve over time. Genetic drift and phenotypic/neural plasticity alone just don't cut it in describing the foundations of either human physiological nature or psychological nature.

    New methods of fMRI can identify what object people are thinking about -- even different people. This doesn't happen without an evolved human neurological/psychological nature.


  39. Mike says,

    We're way ahead of you.


    Adaptations, Exaptations and Spandrels, by Buss, Haselton, Shackelford, Bleske

    I discussed this paper before. The term "drift" does not appear anywhere in the paper. Instead, they talk briefly about "noise."

    The authors give no indication that they are aware of the real problems with the adaptationist program. If you read the discussion at the end of the paper you will see that the main emphasis is on different forms of adaptation and the idea that a trait may be non-adaptative and not a by-product or a spandrel isn't even mentioned. That's why the paper is titled, "Adaptations, Exaptations, and Spandrels."

  40. Adaptation gives direction to evolution.

    Do you really believe that non-adaptive genetic drift, non-adaptive byproducts, and non-adaptive random noise are more important in the determining the design features of an organism than natural and sexual selection?

  41. Mike asks,

    Do you really believe that non-adaptive genetic drift, non-adaptive byproducts, and non-adaptive random noise are more important in the determining the design features of an organism than natural and sexual selection?

    Depends what you mean by "design features." Does it include things like junk DNA, back problems, the ability to roll your tongue, hair color, blood type, and the epicanthal fold?

    Mike, you keep missing the point. It's not a question of whether natural selection is responsible for adapations or not. The real question is whether it is an adaptation (design feature?).

    If you just ASSUME that everything you see is an adaptation then you'll automatically conclude that it is due to selection. You never address the real question, which is whether the feature is really a adaptation or not.

  42. We don't assume everything is an adaptation (what Gould called "pan-adaptationism").

    If we did, we would simply make up another "just so" story, declare it true, and do no empirical research. There would be no need to test any hypotheses with empirical research.

    Instead, we see suggesting that supporting the hypothesis that a trait is an adaptation, in the words of George Williams, is an onerous process. Adaptations show signs of functional design, reliability, and complexity.

    Several evolutionary psychology hypotheses have been falsified and abandoned because of disconfirming empirical evidence.

    The articles published in the popular press may give simplified, breathless versions of what we do -- but they do that will all science.

  43. "Adaptation gives direction to evolution"


  44. A. Vargas said:


    Actually, the bovine gastrointestinal system is a bundle of adaptations, too, with a very interesting phylogenetic history.

    Gee whiz, "A"! You might be on to something here! Or, is it all just drift...

  45. I bet you guys think high vegetation is all there is in the giraffe's homeland. Did you know sauropods cannot lift their necks, too.

    Since adaptation guides evolution I guess whales will be sprouting gills pretty soon, huh.

  46. As a crazy pan-adaptationist, I wouldn't doubt it. Look what happened to Dumbo.

  47. I think it is interesting to notice that the links between '2D:4D finger length' & various types of social behavior ... are even confirmed in multiple species of primates!

    See this study published by Nelson & Voracek:

    PRIMATE HANDS: 'Finger Length linked with Social Behavior!'

    (the ABSTRACT is available at: Heritability of digit ratio (2D:4D) in rhesus macaques)

    But one should notice here that the effects are not very large, so the basic problem is that these concern only 'statistical patterns', which can hardly be applied to individuals.

  48. I don't get this digit ratio thing, I have heard it correlates with aggressive behaviour and mathematical thinking etc. I was one of the biggest trouble makers in school, got expelled for more fights and lunacy than the rest of my year combined, I joined the marines at age 18, left 5 years later and have since become a maths teacher, I only heard about 2d4d recently and upon inspecting my own hands realized that as a male breed my index fingers on both hands are longer, damn!!! what does this mean? should I have been a woman??