Children who are ambidextrous, using either hand with the same ease, may be more likely to have mental health, language and academic problems than their peers, according to a new study.
The researchers say the findings may help teachers and health professionals identify children who are particularly at risk of developing these problems.
The researchers aren't sure what is behind this link, though they suggest differences in the brain between ambidextrous individuals and those who have a dominant hand may play a role. In fact, scientists aren't sure why some people can use both hands equally well (with no dominant hand), a skill also known as mixed-handedness.
They also warn that since the phenomenon is rare, with just one out of every 100 people being mixed-handed, the study only focused on a small group of these individuals.
"But our results are statistically and clinically significant," said lead researcher Alina Rodriguez of the Imperial College London. "That said, our results should not be taken to mean that all children who are mixed-handed will have problems at school or develop ADHD. We found that mixed-handed children and adolescents were at a higher risk of having certain problems, but we'd like to stress that most of the mixed-handed children we followed didn't have any of these difficulties."
The study is detailed in the most recent issue of the journal Pediatrics.
Handedness and language
The study involved nearly 8,000 children in Northern Finland, 87 of whom were mixed-handed, who completed questionnaires at ages 7 and 8, and again at 15 and 16. Since 90 percent of the population is right-handed, they compared both ambidextrous and left-handed individuals to this norm – right-handedness.
Parents and teachers also completed questionnaires regarding the 8-year-olds' language abilities, scholastic performance and behavior. Teachers specifically reported whether children had problems with reading, writing or mathematics, and rated academic performance as below average, average or above average.
For all variables except math, left-handed individuals showed no problems compared with their right-handed peers. For math, at ages 7 and 8, left-handers were 30 percent more likely to have problems compared with right-handers. And ambidextrous children were nearly 90 percent more likely than right-handers to have these math problems.
Results also showed the ambidextrous 7 and 8-olds were twice as likely as their right-handed peers to have language difficulties and to perform poorly in school.
By age 15 or 16, mixed-handed adolescents were also at twice the risk of having symptoms of attention deficit/hyperactivity disorder (ADHD). And those ambidextrous teens with ADHD had more severe symptoms than their right-handed counterparts.
The mixed-handed teens reported having greater difficulties with language than left- or right-handed peers.
Brain hemisphere in charge
Here's how the link might work: Scientists know handedness is linked to the hemispheres of the brain. For instance, studies have shown that in right-handed individuals, the brain's left hemisphere is more dominant.
"Handedness is seen as a proxy for how the brain is functioning, and it's not a perfect measurement. A more accurate method would be to use fMRI [magnetic resonance imaging] scanning, but this isn't possible in a large-scale study," Rodriguez told LiveScience.
"All we can say from this is that they have an atypical brain lateralization; that just means the brain circuitry and function is likely to differ from the normal pattern," seen in right-handed individuals, she said.
And the right hemisphere, for instance, might not function in the same way for ambidextrous individuals as it does for right-handed individuals, Rodriguez said. That could explain the association with ADHD, as one study has shown that those with ADHD have difficulties processing information normally processed in the right hemisphere of the brain.
The results would likely extend beyond Finland, the researchers say. "There is no reason to believe that American children would behave in any other way as the children in this study," Rodriguez said.
The research was funded by the Academy of Finland, the Sigrid Juselius Foundation, Thule Institute, University of Oulu, and the National Institute of Mental Health. Rodriguez received support from VINNMER.