An Intro to Handedness

While many species of animals on earth exhibit individual variance in handedness, humans are among the few to show a statistically significant and specific preference for a particular hand. As research in this field grows and our understanding of handedness becomes less anthropocentric, scientists have been able to find more and more evidence of handedness in the creatures around us. For us at least, these preferences appear to go back in time, as archaeological records of both humans and Neanderthals seem to suggest that about 10% of both were left-handed, which corresponds to the rough proportion we see today. How then is such a trait so remarkably well conserved over such a long period?

Predicting Handedness

Handedness seems to be predictable from a young age, even before birth. In one study, researchers looked at a few hundred baby scans when the fetus was sucking its thumb. Of the 90% of those who sucked their right thumb, almost all later became right-handed by the time they were 12. Conversely, of the 10% of those who sucked their left thumb, a staggering 75% of them became lefthanded.

And, as expected, genetics plays a role in determining handedness. Having a left-handed parent increases the chances that a baby will be left-handed, while having two left-handed parents increases the chances further to a whopping 25%.

The Biological Basis for Handedness

The main genetic theories of handedness attempt to explain the overwhelming majority of right-handedness and the persistent prevalence of left-handedness. They begin by asserting that through evolutionary natural selection, most individuals have speech and language control in the brain’s left hemisphere. Since the left hemisphere also controls the movements of the right hand, which are needed to produce written language, it follows that evolutionary development caused a genetic bias toward individuals with left hemisphere speech/language and right-handedness.

At the genomic level, one frame of examining handedness considers two alleles: the D (meaning dextral/right) allele and the C (meaning chance) allele. It is noted that D, which promotes right-handedness, is much more common in the population and thus is more likely to be found in most individuals. Conversely, C, which randomly selects handedness based on other factors, is much rarer in the population. Comparing these two genes, it could be thought that D is dominant over C, as the DD genotype results in a strong preference for right-handedness, the DC genotype results in a high likelihood of right-handedness, while the CC genotype results in an equal chance of right-handedness or left-handedness.

It is interesting to note that, using Mendelian Genetics, these theories do not fully align with the prediction aforementioned that a child born to two left-handers would have a 25% chance of being left-handed (we would expect 50% owing to the expected CC genotype). This then suggests that other cultural and environmental pressures are at play, which further decrease the prevalence of left-handedness and help explain why left-handedness can arise in a family of right-handers and vice versa.

These theories also imply that left-handers’ brains are somehow wired slightly differently to compensate for the aforementioned difference. To this, scientists at the University of Oxford claimed in a study that left-handers’ brains show better connectedness and coordination, especially in language processing areas, as compared to right-handers’. While in popular culture, it has been claimed that this lends left-handers skills and creativity that right-handers lack, there is still insufficient evidence to prove a causative effect.

As an aside, it has also been noted that sex may play a role in handedness as well, with males being more likely to be left-handed on average than females. Moreover, it has also been established that handedness is much more likely to be a polygenic trait influenced by multiple genes, rather than the simple DC hypothesis discussed earlier.

Evolutionary Causes

When considering cultural and environmental pressures at play, which cause deviation from Mendelian predictions, experts tend to consider the theory of social cooperation. Communities of people (i.e. populations) that share the same spaces, tools, and living conditions have recognised that using the same hand gives them a selective advantage which then increases their fitness. Thus, over time, right-handedness was selected for and became the norm expected in communities, which represses the left-handed phenotype in CC individuals.

Cultural Impact

In many cultures, left-handers were historically forced to use their non-dominant right hands over their left, which is still practised up to today. Even when this is not enforced, there can be inherent biases against use of the left hand. For example, the left hand is still considered dirty or rude to use in some traditions. This is even conveyed through language, where in French, for example, “gauche” means either “left” or “clumsy”. English is no better: “left” is derived from the Anglo-Saxon “lyft”, meaning “weak”; “right” also means “to be correct”; “sinister” is derived from Latin “sinister”, meaning “left”.

Culture and language aside, many parts of our material world are simply not designed well for left-handers. Think of things like scissors, spiral notebooks, can openers, and lecture seat tables as some instances where left-handers are unfairly prejudiced against.

However, with increasing acceptance of left-handedness, more people appear to be freely admitting to it, which could explain the increase in left-handedness noted in some studies.

Conclusion

Handedness is definitely a much more complex topic crossing into Genetics, Ecology, Evolution, and Culture, among others, and this piece simplifies many of these complexities for brevity. If you are looking for a much more balanced and well-informed view of handedness, do consider reading (or asking ChatGPT to summarise) this amazing review paper on handedness by Llaurens et al. (2008) which is linked in the references below!

References

1. De Kovel, C. G. F., Carrión-Castillo, A., & Francks, C. (2019). A large-scale population study of early life factors influencing left-handedness. Scientific Reports, 9(1). https://doi.org/10.1038/s41598-018-37423-8

2. Left-Handers Day: Amazing facts about lefties. (2024, August 13). BBC Newsround. https://www.bbc.co.uk/newsround/53739189

3. Llaurens, V., Raymond, M., & Faurie, C. (2008). Why are some people left-handed? An evolutionary perspective. Philosophical Transactions of the Royal Society B Biological Sciences, 364(1519), 881–894. https://doi.org/10.1098/rstb.2008.0235

4. What causes some people to be left-handed, and why are fewer people left-handed than right-handed? (2024, February 20). Scientific American. https://www.scientificamerican.com/article/what-causes-some-people-t/