Scientists have discovered the first gene which appears to increase the odds of being left-handed.
Left-handers' brains are set up differently
The Oxford University-led team believe carrying the gene may also slightly raise the risk of developing psychotic mental illness such as schizophrenia.
The gene, LRRTM1, appears to play a key role in controlling which parts of the brain take control of specific functions, such as speech and emotion.
The study appears in the journal Molecular Psychiatry.
The brain is set up in an asymmetrical way.
In right-handed people the left side of the brain usually controls speech and language, and the right side controls emotions.
However, in left-handed people the opposite is often true, and the researchers believe the LRRTM1 gene is responsible for this flip.
They also believe people with the LRRTM1 gene may have a raised risk of schizophrenia, a condition often linked to unusual balances of brain function.
Lead researcher Dr Clyde Francks, from Oxford University's Wellcome Trust Centre for Human Genetics, said the next step would be to probe the impact on the development of the brain further.
He said: "We hope this study's findings will help us understand the development of asymmetry in the brain.
"Asymmetry is a fundamental feature of the human brain that is disrupted in many psychiatric conditions."
However, Dr Francks said left-handed people should not be worried by the links between handedness and schizophrenia.
He said: "There are many factors which make individuals more likely to develop schizophrenia and the vast majority of left-handers will never develop a problem.
"We don't yet know the precise role of this gene."
About 10% of people are left-handed.
There is evidence to suggest there are some significant differences between left and right-handed people.
Australian research published last year found left-handed people can think quicker when carrying out tasks such as playing computer games or playing sport.
And French researchers concluded that being left-handed could be an advantage in hand-to-hand combat.
However, being left-handed has also been linked to a greater risk of some diseases, and to having an accident.
Dr Fred Kavalier, a consultant geneticist at London's Guy's Hospital, said: "I don't think left-handed people should be alarmed.
"Undoubtedly there are many, many other factors that contribute to schizophrenia. This may be a tiny little element in the big jigsaw."
Marjorie Wallace, of the mental health charity SANE, said scientists working in its research centre in Oxford were also looking at the link between brain asymmetry and schizophrenia.
She said: "We desperately need research into the origins of psychosis to better understand why some people are more vulnerable than others.
"Then the treatment could be more targeted and carry the potential to prevent this devastating condition which affects one in 100 people worldwide."
Jane Harris, of the mental health charity Rethink, said: "No-one really understands what causes schizophrenia yet.
"It is probably a combination of factors, including genetics, problems in childbirth, viral infections, drug use, poverty and urbanisation."