On Being Handed
There’s a box of matches on the table in front of you. You pick up the box, choose a match, and strike it against the side of the box. Which hand did you use to strike the match?
Chances are, you used your right hand. According to most estimates, roughly 90% of the human population is right-handed. Somehow, 90% of us ended up this way, but how? Would it be more beneficial if we were all ambidextrous, perfectly capable of using either hand in any situation? Well, maybe your parents had a dominant hand, and therefore taught you to do everything with that hand. Or perhaps you were hard wired with a dominant hand from birth, and very little training would have changed your innate preferences. As with many questions about human behavior, it’s almost impossible to detach nature from nurture. Luckily for those of us curious about this one-handed mystery, there are a few ways to poke at the nature vs. nurture dilemma with developmental, neural, and evolutionary approaches.
Were you born handed?
The first question we can investigate is how early handedness develops. As you might imagine, this is difficult to study. We can’t give an infant a survey to fill out, but we can observe their hand and arm movements, postural control, and head turning. Researchers have looked for signs that babies have some sort of stable preference, but the jury is out (Provins 1992). It looks like handedness doesn’t stabilize until age three (McManus et al., 1988) or possibly even age eight (Fennell et al., 1983), and continues to intensify in degree throughout childhood as motor programs become engrained. Interestingly, men are more likely to be left-handed than women (Papadatou-Pastou et al., 2008), which suggests it might be more innate, but you can also imagine that social and cultural gender biases may shape early handedness.
What’s going on in your brain?
In our era of sending people into an MRI scanner, many studies have attempted to understand asymmetries in handedness at a neural level. Several brain functions are known to be lateralized, or processed primarily on one side of the brain. The most researched of these is language – since Paul Broca’s studies in the 19th century, it has been consistently shown that right-handed people process language in the left-hemisphere. This has led many researchers to wonder where left-handed people process language. A recent study used a non-invasive measure of brain activity (functional transcranial Doppler ultrasonography – yes, it sounds like something a weather forecaster and/or dolphin watcher would use) to investigate this question and found that the results were not as clean cut as one might hope. Indeed, left-handed people were more likely to have right language dominance, but the lateralization was not as strong as in right-handed people and most left-handers still had left-language dominance (Knecht et al., 2000). Going back to our question of child development, it looks like these relationships between handedness and language lateralization are also present in children (Szaflarski et al., 2012). Some researchers argue that handedness is a marker for how symmetric your brain is organized – if you’re extremely one-handed, you probably have a very asymmetrical brain.
What about our ancestors?
Oddly enough, primates show hand preference. As you see in the figure at left, chimpanzees are primarily right-handed, just not as strongly as humans. Since monkeys can’t really use Survey Monkey, researchers observe monkeys as they manipulate objects or communicate. Some researchers have suggested that early tool use and communicative gestures may have shaped brain and hand asymmetries (Cochet & Byrne, 2013), but these theories are difficult to test. In addition, archaeological studies of Neanderthal art and tools suggests that right-handedness was established in their culture (Cashmore et al., 2008). The evidence isn’t conclusive, but the evolutionary approach suggests that there is a conserved set of genes or behaviors that have led us to have a dominant hand, most often the right.
Whether it’s genes, culture, or a balanced combination of both, somehow I ended up at the south(paw) end of the phenotype pool. Perhaps when we identify a set of right-leaning genes, there will be some sort of gene therapy to reverse left-handedness. Until then, there’s this.
Feeling empty handed?
If you want to get your own handedness score, take the Edinburgh Handedness Inventory.
For a more detailed article with a complete set of references, check out this review.
Read one researcher’s opinion in a Scientific American article, here.
Or maybe you’re wondering if left-handedness is really caused by an early bacterial infection in your brain. You may be right.
Cashmore, L., Uomini, N., Chapelain, A. (2008) The evolution of handedness in humans and great apes: a review and current issues. Anthropol Sci, 86, 7-35.
Cochet, H., Byrne, R.W. (2013). Evolutionary origins of human handedness: evaluating contrasting hypotheses. Anim Cogn, 16, 4, 531-542.
Fennell, E. B., Satz, P., & Morris, R. (1983). The development of handedness and dichotic ear listening asymmetries in relation to school achievement: A longitudinal study. Journal of Experimental Child Psychology, 35, 248–262.
Knecht, S., Drager, B., Deppe, M. Bobe, L., Lohmann, H., Floel, A., Ringelstein, E.B. Henningsen, H. (2000). Handedness and hemisphere language dominance in healthy humans. Brain, 123, 12, 2512-2518.
McManus, I. C., Sik, G., Cole, D. R., Mellon, A. F., Wong, J., & Kloss, J. (1988). The development of handedness in children. British Journal of Developmental Psychology, 6, 257–273.
Papadatou-Pastou, M., Martin, M., Munafo, M.R., Jones, G.V. Sex differences in left-handedness: a meta-analysis of 144 studies. Psychol Bull, 134, 5, 677-99.
Provins, K.A. (1992). Early infant motor asymmetries and handedness: critical evaluation of the evidence. Developmental Neuropsychology, 8, 4.
Szaflarski JP, Rajagopal A, Altaye M, Byars AW, Jacola L, Schmithorst VJ, Schapiro MB, Plante E, Holland SK. (2012) Left-handedness and language lateralization in children. Brain Res, 1433, 85-97.
Vauclair J, Cochet H (2013) Speech-gesture links in the ontogeny and phylogeny of gestural communication. In: Botha R, Everaert M (eds) Oxford Studies in the evolution of language: the evolutionary emergence of human language. Oxford, Oxford University Press, pp 160–180.
Note: This post appeared in a preliminary form on Ashley’s blog, Scrambling for Significance. Totally not trying to pull a J&^*# L%&#$@.