I recently read an interesting article by Michael Corballis, Professor Emeritus of Psychology, at the University of Auckland in Auckland, New Zealand.¹ Michael Corballis is one of the world’s eminent neuropsychologists who has written many books and articles dealing with handedness, laterality and the brain. I spent a day with Michael Corballis when I visited New Zealand in 2015. We talked about topics related to my book, Laterality: Exploring the enigma of left-handedness, which I was writing at the time. Visit Michael Corballis’s website to appreciate the breadth and depth of his writings and interests. Michael Corballis

Michael Corballis writes that during the 19th century, the brain was considered symmetrical with the left side of the brain mirroring the right. A paper presented at a conference in 1836 argued that the brain is not symmetrical because the left hemisphere is dominant for language. However, this idea was not accepted until the 1860’s when further evidence showed that comprehension and production of speech are impaired after damage to the left hemisphere.

Functional differences between the two sides of the brain led to more dramatic ideas of brain asymmetry during the 20th century, influenced by the Nobel Prize winning work of Roger Sperry. Sperry developed a surgical technique to alleviate symptoms of severe epilepsy. He severed the bundle of neurons, the corpus callosum, connecting the right and left hemispheres. This operation reduced the occurrence of seizures, but it disconnected the two sides of the brain. Researchers could now test the thinking capacities of each hemisphere independently after the individuals recovered from surgery. These split-brain studies confirmed that the left side of the brain is responsible for speech while the right side specializes in processing nonverbal, spatial and imagery material. A dramatic duality between the brain’s right and left sides developed. Various authors proposed that individuals were ‘left-brained’ or ‘right-brained’ thinkers depending on their mastery of certain cognitive skills. For example, a highly verbal person, like a lawyer, is left-brained while a visual artist falls into the right-brained category. It is currently an ingrained part of popular culture to talk in these terms.

Where does handedness fit in? Michael Corballis points out that there is a low correlation between handedness and language asymmetry. Brain imaging studies indicate that 88% of right-handers and 78% of left-handers show the typical pattern of left hemisphere dominance for language. Handedness is also not correlated with structural asymmetries in the brain. The amount of neural matter in certain brain areas differs on the right and left sides but these differences are not related to the side of handedness. A 2019 study parceled the brain into 180 areas and found that 71% of them were larger on one side of the brain as compared to the other side. The larger areas were equally distributed between the left and right sides of the brain, again showing an asymmetry not consistent with one hemisphere or related to handedness.  The right hemisphere controls movements of the left hand while the left hemisphere controls movements of the right hand, but one cannot say that left- and right-handers differ when it comes to ‘brainedness’. As Michael Corballis states, “To speak of individuals as “left-brained” or “right-brained”…makes little sense.”

¹Corballis, M.C. (2020). Bilaterally symmetrical: To be or not to be? Symmetry, 12, 326; doi:10.3390/sym12030326.