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When I tell people that I ‘do psychology' I typically get one of three reactions. 1) People ask if I can read their thoughts. No, unless you're a drunken guy in a bar, in which case, gross. 2) They begin to tell me about their current psychological troubles and parental issues, to which I listen sympathetically and then make it clear that I got into experimental psychology because I didn't want to have to listen to people's problems (sorry). Or 3) they ask me a very astute question about the brain that 9 times out of 10 I can't answer. This last option is by far the most preferable and I've had several very interesting conversations come out of these interactions.
One such question I received recently was where does handedness come from in the brain? While initially a basic-seeming question, I quickly realized that I had no idea how to answer it without dipping into pop psychology tropes about right- and left-brained people that I definitely wanted to validate before I started trotting them out.
So what exactly is handedness? Does it really reflect differences in dominant brain hemispheres? Is it underlying or created, and what happens if you switch back and forth? Can a person truly be ambidextrous?
Going beyond basic motor differences, handedness has been loosely related to the more general functions of each brain hemisphere as well. The left hemisphere is typically associated with more focused, detailed and analytical processing, and this type of thinking may be reflected in the precision movements utilized by the more typically dominant right hand. Conversely, greater spatial awareness and an emphasis towards systems or pattern-based observations are thought to reside primarily in the right hemisphere. (I highly recommend Iain McGilchrist's RSA animation on the divided brain for a great overview.) However, it is important to note that these types of thought and behavior are by no means exclusive to one hemisphere or another, and the different areas of the brain are in constant communication with each other via signals sent through white matter tracts that traverse the brain, like the corpus callosum that connects the two hemispheres.
In the 1960s a simple test was devised to empirically determine a person's dominant hemisphere in terms of language. This, the Wada test, involves injecting sodium amobarbital into an awake patient in an artery traveling up to one side of the brain, temporarily shutting down that hemisphere's functioning. This allows neurologists to see which abilities are still intact, meaning that they must be controlled by the opposite side. This test is especially important in patients undergoing neurosurgery, as ideally you would operate on the non-dominant hemisphere to reduce possible complications in terms of movement, language and memory. The Wada test revealed that many left-handers are actually also left-brain dominant in terms of language, and that in only a small proportion does language reside in the right hemisphere. Still other left-handers share language abilities across the two hemispheres.
So where does this appendage preference come from? Handedness is thought to be at least partially genetic in origin, and several genes have been identified that are associated with being left-handed. However, there is evidence that it is possible to switch a child's natural preference early in life. This often happened in cultures where left-handers were perceived as 'evil' or 'twisted', and attempts were made in schools to reform them, forcing them to act as right-handers. As mentioned above, when motor movements (and their underlying synaptic connections) are practiced, they become stronger and more efficient. Thus, individuals who were originally left-handed may come to favor their right hand, particularly for tasks like writing, as they were forced to develop these pathways in school. These same individuals may still act as left-handed for other motor tasks though, simultaneously supporting both the nature and nurture aspects of handedness. Notably, this mixed-handedness is different from ambidextrousness, as both hands cannot be used equally for all actions. True ambidexterity is extremely rare and has been largely under-studied to date. However, it has been theorized that in ambidextrous individuals neither hemisphere is dominant, and in some cases this has led to evidence of mental health or developmental problems in children.
So the next time you meet a psychologist in a bar, instead of challenging them to guess what you're thinking, ask them the most basic brain-related question you have. It will undoubtedly lead to a much better conversation!
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McGilchrist, I. The Master and His Emissary: The Divided Brain and the Making of the Wester World. London: Yale University Press, 2012.
McManus, C. Right Hand, Left Hand: The Origins of Asymmetry in Brains, Bodies, Atoms and Cultures. Boston: Harvard University Press: 2002.
Medland, SE et al. Genetic influences on handedness: Data from 25,732 Australian and Dutch twin families. Neuropsychologia 47: 330-337 (2009).
Rodriguez, A et al. Mixed-handedness is linked to mental health problems in children and adolescents. Pediatrics 125: 340-348 (2010).
Image credit: Paty montano and Was a bee (via Wikimedia Commons) and my personal photography skillz (thanks Martin O'Neill and Al Power)
