People are probably left-handed for the same reason snail shells grow one way or the other - it's in their genes.
Scientists in the UK have linked four gene variants to left-handedness, which affects about 10 percent of the population. Three of them are involved in building the "scaffolding" inside cells, known as the cytoskeleton, particularly in the brain.
"Many animals show left-right asymmetry in their development, such as snail shells coiling to the left or right, and this is driven by genes for cell scaffolding," said Professor Gwenaëlle Douaud of the University of Oxford.
"For the first time in humans, we have been able to establish that these handedness-associated cytoskeletal differences are actually visible in the brain.
"We know from other animals, such as snails and frogs, that these effects are caused by very early genetically-guided events, so this raises the tantalising possibility that the hallmarks of the future development of handedness start appearing in the brain in the womb."
Previous research had suggested about 25 percent of variation in handedness was down to genetic factors.
The scientists looked at the genomes of more than 38,000 left-handers, 10,000 of which also had their brains scanned. The same genes linked with left-handedness were responsible for better connections between language-related parts of the brain's white matter.
"The language areas of the left and right sides of the brain communicate with each other in a more coordinated way," said Oxford analyst Akira Wiberg.
"This raises the intriguing possibility for future research that left-handers might have an advantage when it comes to performing verbal tasks."
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On the other hand - no pun intended - the scientists also found a correlation between left-handedness and increased risk of schizophrenia. They said it was only a "very small" link however, and there's no indication whether one causes the other.
Previous research has had mixed results on finding links between left-handedness and artistic skill.
"Throughout history, left-handedness has been considered unlucky, or even malicious," said Dominic Furniss, joint senior author on the study.
"Indeed, this is reflected in the words for left and right in many languages. For example, in English 'right' also means correct or proper; in French 'gauche' means both left and clumsy.
"Here we have demonstrated that left-handedness is a consequence of the developmental biology of the brain, in part driven by the complex interplay of many genes. It is part of the rich tapestry of what makes us human."
The research was published in journal Brain.