It's a question that's surprisingly hard to answer: why are most of us right-handed, some of us left-handed, and even fewer ambidextrous? Can we point to our genes, or is it an environmental phenomenon?
A new genome-wide association study of over 1.7 million people can't give us all the answers, but it is bringing us closer to understanding the combination of factors that helps produce our preference for a dominant hand.
"Although there is an enduring fascination with why some people are left- or right-handed or both, understanding why some people are left-handed and others right-handed is also an important research question because handedness can influence brain structure and the way different functions are located within the brain," says geneticist Sarah Medlan from the QIMR Berghofer Medical Research Institute in Australia.
In what the researchers claim is the largest study of its kind to date on handedness, the team found 41 single DNA base pair changes that are linked to a person's chance of being left-handed, while seven others were linked to being ambidextrous.
Unfortunately, though, that's definitely not the whole story. The team found that these base pair changes only conferred around 12 percent of the variance in 'handedness', meaning there's something else (or multiple things) that are making up the bulk of our left- or right-hand preferences.
"The results from our analyses suggested that genetic factors could only account for a small amount of the variation in handedness, whereas environmental factors were likely to play a much more important role," explains one of the researchers, University of Queensland geneticist David Evans.
"This percentage was similar for ambidexterity, meaning factors such as injuring a hand or training by playing sport or musical instruments are likely to have a strong role in a person's ability to use both hands equally well."
In a genome-wide association study, researchers investigate large numbers of genomes, analysing which small DNA changes are linked to a particular trait – like heart disease, or, in this case, left-handedness.
In this study, the researchers took data from the UK Biobank, 23andMe, and the International Handedness Consortium to create a mega database of genome variants – 1,766,671 individuals' data to be exact.
"These large numbers of participants provide the statistical power to detect the effect of genes that have even very small effects on handedness," says 23andMe statistical geneticist Gabriel Cuellar-Partida.
"This also highlights that large studies are needed to understand the genetic factors influencing other neurological traits and conditions and why participation in research studies is so important."
People who are left-handed don't always have an easy time. Up until relatively recently, many left-handers were punished for favouring their left hand, and even today, left-handed people live in a world largely designed for right-handers. As to why we have a hand preference at all is a question that has stumped biologists for decades.
Up to roughly 10 percent of the population of the US, UK, and Australia is left-handed, but the exact figure differs in each country. We do know that this preference starts to show even before we're born, with more small movements in one arm over the other.
Other theories have suggested differences in the hemispheres of our brain, epigenetic factors, and exposure to prenatal hormones may contribute to handedness.
This new study found that some of the genetic differences were in microtubule genes and brain morphology – meaning that the early developmental research might be onto something.
"With respect to handedness, microtubule proteins play important roles during the development and migration of neurons, plasticity and neurodegenerative processes," the researchers write in their paper.
"The association between handedness and variation in microtubule genes also provides insights into differences in the prevalence of various neuropsychiatric disorders and left-handedness observed in some epidemiological studies."
So, although this new research isn't the final say on why people are left-handed, right-handed, or ambidextrous, it's an exciting step forward.
The research has been published in Nature Human Behaviour.