Have you ever wondered why some people are lefties and some are happy enough using their right hand? Well, handedness in humans is something called a polymorphism – when two or more very different phenotypes of the same characteristic end up manifesting themselves in the main population.
There are many scientifically plausible reasons why some people are left-handed, both genetic and environmental, i.e. nature vs nurture reasoning.
Left-handedness isn’t a new behaviour amongst human populations, and it’s been inferred from cave paintings that lefties have been the minority since prehistoric times. Cultural and religious depictions throughout the ages also point to a preference for the right hand.
From the ‘natural’ reasoning corner for left-handedness, there are several arguments. Looking to families for the answer seems the obvious first place to start – if your parents are left-handed, are you more likely to be? Well, that’s true, and it seems that if your mum’s left-handed, you’re more likely to be too. However, gene effects could easily be muddled with environment here. Mothers generally have a greater role to play in the education and upbringing of their kids, possibly resulting in a learned handedness – not a genetic one.
Studying twins could get around this problem, identical twins are more likely to prefer the same hand than non-identical twins – so it looks like handedness has a stronger case for genetic linkage. But… identical twins don’t always use the same hand as each other, inferring handedness isn’t all genetic.
Using molecular methods to investigate the genome, links between particular regions on the X chromosome and the use of a particular hand have been identified. Both this method and the more simple family based methods of investigation show that being left-handed is definitely in the genes.
Moving onto the ‘nurture’ argument.
There is evidence that a higher proportion of people who have suffered developmental complications or birth stresses are left-handed. Higher than usual counts of lefties have been observed in demographics of people with some nervous disorders and immune problems – the same demographics that are associated with developmental abnormality. This isn’t to say that being left-handed predisposes you to a mental condition or an immune disorder though – more the other way around, being left-handed might just be part of the package.
Some theories about which hand you prefer link it to the more dominant side of your brain. The left side of the brain develops later than the right side and is also more susceptible to stress, so stressful or premature births are more likely to affect this left hemisphere. There is a greater number of left-handers in the population that were born prematurely or with some sort of stress at birth. By stress I don’t mean the feeling you get when a deadline is looming or when you’re frazzled from work. I mean an abnormal birth, for example, when oxygen supply to the baby is limited at some point during labour.
Cultural influences in determining handedness are not to be forgotten. Social pressures to be right-handed when performing certain actions like eating or writing are found all over the world, from France to China, Italy to Brazil. Due to cultural pressures to be right-handed (admittedly moreso in the past than the present), you’d expect there to be less lefties than right-handers. Traditional communities, like Amazonian tribes, also have a tendency to push for right-handedness.
There is also mounting evidence that evolutionary forces are acting on handedness. Left left-handers are not as common as right-handers. This suggests that there might be a cost associated with being left-handed.
It’s true – lefties have shorter life expectancies than right-handers. This could be down to the previously mentioned stressful births, higher rates of brain-associated health problems, reduced immune systems or (this is a bit strange) the fact that left-handed people have more lethal accidents.
Lethal accidents?! I hear you utter in disbelief. Yes – apparently left-handed people have a much higher risk of being killed by machinery and weapons designed for right-handers. These deaths have more of an influence over life expectancy figures than you might think.
However, left-handedness occurs in all human populations, suggesting that there must be some sort of advantage in being a leftie too.
Left-handed people generally have better ‘intermanual coordination’. This means that they can use both sides of their bodies better than right-handers, who show more dominance to the right side. This might be down to the fact that lefties are more likely to have a larger corpus callosum, a part of the brain that links the two hemispheres and allowing greater synchronisation between the two sides of the body. This brain trait can also be linked to better verbal fluency and memory function.
Left-handed men tend to be more creative and some studies have shown that left-handers show more mathematical and musical flair (slightly controversial). One socio-economic study found that left-handed men earn more than their right-handed counterparts but left-handed women earned significantly less than right-handers…
There’s also a high prevalence of left-handed sportspeople at the top of their game – especially in strategic sports like tennis and boxing. This might be because they have an advantage that allows them to ‘surprise’ their right-handed opponents who might be automatically tuned to expect a right-handed move.
All in all, if you’re a leftie, you should be proud, you’re in the minority and might be more likely to have a special creative side. However, there’s always that chance that you have a terrible accident with a blender designed for right-handers…
I read all about the reasons behind being left handed here.