Whether we are left of right-handed is probably determined before we are born with ultrasound studies showing a preference for left or right thumb sucking in the womb at 13 weeks. Although scientists don't know why people are right or left-handed, research out this week has unveiled the first genetic instructions in our DNA linked to handedness and they clearly influence the structure of our brain.
Ninety per cent of humans are right-handed and aren't the only members of the animal kingdom that show handedness. About 75 per cent of chimpanzees and gorillas are right-handed while 66 per cent of orangutans are lefties. Even animals that don't technically have hands can show a natural preference for a side - half of mice are left-pawed and some species of tree frog preferentially jump away from predators in only one direction.
The 10 per cent of humans who are naturally left-handed have had to deal with words that define them negatively. For example, the word "sinister" derives from "left hand", and historically those who showed this hand preference were labelled as witches. In French the word for left is "gauche", which can also mean clumsy – whereas in English the word "right" can also mean correct.
There have been a few theories about why some of us are left-handed, including a popular but incorrect myth that it is related to how stressed a mother is during her pregnancy.
Although science is still a long way off understanding the specifics behind what controls handedness, studies, including those on twins, estimate that about 25 per cent of handedness is controlled by genetics while 75 per cent is based on environmental factors.
A meta-analysis of 50 studies found a clear relationship between neurodevelopment disorders and handedness, and a new study in the journal Brain this week brings us one step closer to understanding where this link comes from.
The study looked at brain images and genetic data from more than 7000 UK biobank volunteers, 721 of which were left-handed. Detailed analysis of their DNA found four genome locations associated with left-handedness, three of which were also associated with brain development in areas related to language. The correlating brain scans found that in those volunteers who were left-handed, the regions associated with language on the left and the right sides of the brain communicated with each other in a more coordinated way. This suggests that left-handed individuals might have a natural advantage when it comes to verbal skills.
Interestingly, the brain development regions that were found to be different for left-handed volunteers were also related to an increased likelihood of having schizophrenia and anorexia nervosa, while decreasing likelihood of developing Parkinson's disease.
These differences could be explained by the DNA differences of the left-handed volunteers being specifically located in the areas that create the instructions for building the cytoskeleton. The cytoskeleton is the scaffolding that helps to organise the inside of the body's cells, including the white matter - or connecting part - of the brain. This study was the first to show visible cytoskeleton differences that resulted in structural changes in the white matter in the brain.
While this study didn't discover a single gene for handedness, it only used data from UK volunteers and a more diverse study could potentially bring us closer to finding out if there is a handedness gene. What it did find, however, is that being left-handed does influence the structure and function of our brain – something that could help to explain the exceptional talents of left-handers Mozart, Marie Curie, Barack Obama and Bill Gates.