Ambidextrous children are at a greater risk of slipping behind at school, new research by Bristol University has found.
The researchers recommend testing pupils for "mixed-handedness", as those children that had failed to become left or right-handed are more likely to progress slowly up to the age of 14.
About 10 per cent of the UK population is left-handed. Brad Pitt and Angelina Jolie are among celebrity left-handers.
Using information gathered on 12,000 UK children born in 1991, the researchers analysed children's development between the ages of three-and- a-half and 14.
The research team - Paul Gregg, Katharina Janke and Carol Propper, from the Centre for Market and Public Organisation - concluded that left-handed boys did less well before age five, but then performed as well as right- handers. Left-handed girls did slightly less well at ages 11 and 14. But the most noticeable difference was among those who are ambidextrous in early life. This group had lower development which was still present at age 14.
The researchers said although the effect was not large - about half the size of that associated with being male - mixed-handedness could be an early indication of a child who will struggle later on in school.
Theories on mixed-handedness generally associate it with the brain being adversely affected, possibly during pregnancy, the report said.
There is thought to be no single cause for left-handedness. One theory is that some people become left-handed due to damage as the brain develops. Others may have brains which are natural mirror images of right-handed people.
A third cause could be `learned' left-handedness - children become adept with their left hand while playing as toddlers and then decide to use it to write with.
A recent study in Australia found that left-handed children, and those who use both hands to write, are more likely perform badly in tests.