British historical drama “the King’s Speech”, recent winner of seven BAFTAs, included a reference to the childhood left-handedness of King George VI and how he was educated to use his right hand instead. Humans are predominantly right-handed and the film’s subplot reflects historic cultural discrimination against left-handedness. Besides humans, other great apes also exhibit handedness, as documented recently in the most comprehensive survey of great ape handedness to date (article in press in the Journal of Human Evolution). A team led by William Hopkins, a professor of psychology who divides his time in Atlanta between Agnes Scott College, Emory University and Georgia State, surveyed handedness in nearly eight hundred gorillas, chimpanzees, orangutans and bonobos. Handedness was assessed using the aptly named TUBE task, which is literally a manual task involving a small tube (plus peanut butter and a great ape finger). Apes are given a 15cm tube of 2.5cm diameter with tasty peanut butter smeared inside. The trick is that to get at the peanut butter beyond the immediate rim, which can be licked, the ape must poke a finger inside and scoop out the peanut butter. The dexterous task of finger poking will be assigned to the hand preferred for motor actions, whilst the subordinate hand will be employed in the simpler role of holding the tube.
Results showed that gorillas and chimps were mostly right-handed, like humans, but that orangutans were mostly left-handed. Bonobos showed no significant tendency towards either left- or right-handedness. The authors explain that this fits the postural origins theory of handedness, which proposes that tendency to left-handedness is correlated to the amount of time that a particular type of great ape spends amongst the trees. This theory assumes that right-handedness is inherently predominant but that the more tree-dwelling apes are left-handed for manual tasks because they use their inherently dominant right hand to hang on to trees. The evolution of a terrestrial lifestyle, as with the ground-dwelling gorillas, allows the left hand to revert to its inherently subordinate role and thus explains why gorillas exhibit the most robust right-handedness of the non-human great apes. An inherent tendency to right-handedness is indeed suggested by patterns of left hemisphere dominance for language in human brains. That is the reason why Professor Hopkins, who works on the hemispheric specialisation of higher cognitive processes, is interested in handedness. Left hemisphere language dominance is seen in 96% of right-handed individuals but only 70% of left-handed individuals. The inherent tendency to left hemisphere language dominance indicates tendency to right-handedness by association.
The subtle importance of King George VI's left-handedness, not spelled out explicitly in the film, is that forcing children born left-handed to “learn” right-handedness has been connected to the development of stutters. This may be because disturbing the hemispheric connection interferes with development of language function, although the potential mechanism is as yet unidentified. Perhaps the most interesting results, therefore, are seen with the bonobos. These apes have a statistically significant difference between a weak tendency to left-handedness in sub-adults and a weak tendency to right-handedness in adults. This population level move from left- to right-handedness with maturity would make bonobos an interesting subject for future study. Learning more about the controls on handedness and its links to hemispheric language dominance will help us understand more about how higher cognitive functions are organised and interconnected within the brain. Work on handedness may also help speech therapists better understand the causes of speech impediments; and protect a future left-handed king from being hindered in his public speaking by disrupted hemispheric specialisation.
[image from Hopkins et al (in press)]
