It was August of 1993 when a strapping young baby was sat at small plastic table. His left-handed father took an old fountain pen and placed it carefully in-between the boy’s hands. As legend has it, he instinctively reached out and grabbed the pen with his right, before finding out whether or not it tasted as good as it apparently looked. I’m 22 now, I still use my right hand for most things and I still enjoy chewing my pens. But why is it that only around 10% of us would have picked up the pen with our left?
Evolution of handedness
One theory is that as fine motor and language skills evolved, they were localised to one hemisphere of the brain for the sake of efficiency. It just so happened that it was the left. Because the left hemisphere controls the right side of the body, humans became predominantly right handed.
Another idea is that handedness reflects a trade-off between cooperation and competition. The need to share tools may have made it more efficient for the majority to be right-handed. But during combat, a leftie comes in using different angles and using an unusual stance which the right-hander isn’t used to, giving him an advantage. This is supported by the relative success of left-handed boxers and fencers. And if you look at the correlation between left-handedness and societal violence; you’ll find that left-handedness ranges from ~3% in some pacifist African communities, to ~22% in more violent South American communities.
The most commonly cited genetic model of handedness is Marian Annett’s Right Shift Theory; which posits that handedness is a continuum controlled by a few hypothetical genes. The RS+ (Right Shift) gene controls the development of the motor cortex in the left side of the brain, resulting in a right hand preference or “right shift”. Another allele of the gene is the RS- gene which is carried by lefties. However, the RS- gene doesn’t cause a “left shift”. Instead, it is indifferent to the direction motor dominance.
So left-handers lack a bias towards a dominant left hemisphere (and right hand), because they lack the RS+ (right shift) gene. This means there are 3 combinations of handedness genes: RS+RS+, RS+RS- and RS-RS-, producing a continuum of hand preference. RS+RS+ and RS+RS- people are mostly right-handed, whereas in RS-RS- individuals, childhood pressures may swing hand preference in one direction or the other. This explains the existence of ambidexterity and why left-handed people tend to be more mixed handed. In other words, left-handed people are just ‘not right-handed’.
The model also explains why two left handed parents give right handed children three-quarters of the time (only one of the parents would need the RS+RS- combination). Moreover, seeing as though my Dad is more ambidextrous than left-handed, he may have had the RS+RS- genotype. It’s therefore possible that I inherited two RS+ alleles from my parents, pushing me very firmly in the right-handed direction, hence the right handed grab at the plastic table.
However, it’s also possible that I inherited one RS- allele from my Dad, giving me the RS+RS- genotype. In which case, forcing the pen into my left may have resulted in ambidexterity. It follows from this that, should I father any children with a woman expressing the RS-RS- genotype, there would be a good chance of my children also expressing the RS-RS- genotype and reaching for that pen with their left hand.
The Devil’s servants
But is there any reason one would prefer left handed children? Are there any differences between left and right handed people?
There is a theory that left handed people end up being more creative because ~35% of them use their right hemisphere for language processing. This is supported by studies showing that lefties produce a disproportionately high number of talented musicians and mathematicians.
Perhaps I should be looking for a left handed suitress. Stay tuned for updates…