Left-handers have been the subject of curiosity, stigma and even fear over the centuries. Researchers now, however, are recognizing the scientific importance of understanding why people use one hand or the other to write, eat or toss a ball.
Handedness, as the dominance of one hand over the other is called, provides a window into the way our brains are wired, experts say. And it may help shed light on disorders related to brain development, like dyslexia, schizophrenia and attention deficit hyperactivity disorder, or ADHD, which are more common in left-handed people.
Other recent research suggests that mixed-handedness—using different hands for daily tasks and not having a dominant one—may be even more strongly linked than left-handedness to ADHD and possibly other conditions.
About 10% of people are left-handed, according to expert estimates. Another 1% of the population is mixed-handed. What causes people not to favor their right hand is only partly due to genetics—even identical twins, who have 100% of the same genes, don't always share handedness.
More important, researchers say, are environmental factors—especially stress—in the womb. Babies born to older mothers or at a lower birth weight are more likely to be lefties, for example. And mothers who were exposed to unusually high levels of stress during pregnancy are more likely to give birth to a left-handed child. A review of research, published in 2009 in the journal Neuropsychologia, estimated that about 25% of the variability in handedness is due to genetics.
On average there is no significant difference in IQ between righties and lefties, studies show, belying popular perceptions. There is some evidence that lefties are better at divergent thinking, or starting from existing knowledge to develop new concepts, which is considered an element of creativity. And left-handed people have salaries that on average are about 10% lower than righties, according to recent research performed at Harvard University that analyzed large income data bases, although findings of some earlier studies were mixed.
Left-handedness appears to be associated with a greater risk for a number of psychiatric and developmental disorders. While lefties make up about 10% of the overall population, about 20% of people with schizophrenia are lefties, for example. Links between left-handedness and dyslexia, ADHD and some mood disorders have also been reported in research studies.
The reasons for this aren't clear. Scientists speculate it could be related to a concept known as brain lateralization. The brain has two halves. Each performs primarily separate, specialized functions, such as language processing, which mainly takes place in the left hemisphere. There is lots of communication between the hemispheres.
Typically in right-handers, the brain's left side is dominant. But this tendency doesn't hold up with lefties, as scientists previously believed. Some 70% of lefties rely on the left hemisphere for their language centers, a key brain function, says Metten Somers, a psychiatrist and researcher who studies brain lateralization at Utrecht University Medical Center in the Netherlands. This doesn't appear to present problems, scientists say.
The other 30% of lefties appear to exhibit either a right-dominant or distributed pattern, Dr. Somers says. They may be more prone to impaired learning or functioning, and at greater risk for brain disorders, he says.
Hemisphere dominance is typical and more efficient. Symmetry, in which neither side is dominant, is believed linked to disorders, researchers say. People with schizophrenia, for instance, exhibit more symmetrical activation of their brain hemispheres than those without the disorder, studies show.
In a 2008 study, Alina Rodriguez, a psychology professor at Mid Sweden University in Östersund who studies handedness, brain development and ADHD, found that left- or mixed-handedness in children was linked to a greater risk of difficulty with language as well as ADHD symptoms. In another study published last year in Pediatrics, involving nearly 8,000 Finnish children, Dr. Rodriguez found that mixed-handedness rather than left-handedness was linked to ADHD symptoms.
And knowing that a child was mixed-handed and had ADHD symptoms at age 8 helped predict much more accurately than just knowing they had symptoms at that age whether the child would continue to have symptoms at age 16. (What happens when people are forced to switch from writing with their dominant hand to the other isn't well known, experts say.)
One reason that not more is known about lefties is that many studies of how the brain works prohibit left-handers from participating because their brain wiring is known to be different, says Robin Nusslock, a psychology professor at Northwestern University in Evanston, Ill., who uses neuroimaging to study mood disorders.
Lefties have an advantage in sports such as tennis, fencing and baseball, when up against a righthanded competitor, but not in noninteractive sports such as gymnastics.
.A potential pathway between prenatal stress and brain wiring could be cortisol, the body's main stress hormone, which can interfere with brain development, says Carsten Obel, a professor at the public-health department at Aarhus University in Denmark who has conducted research on the prenatal environment and risk of disease. Cortisol is able to pass over the placenta barrier to influence the baby.
Several studies show that stressful life events, such as the death of a loved one or job loss, during pregnancy increase the risk of having non-right-handed children. In one study of 834 Danish mothers and their 3-year-old children, Dr. Obel and his colleagues found that mothers who reported multiple stressful events during their third trimester of pregnancy and experienced distress were more than three times as likely to have a mixed-handed child, 17% compared with 5%, according to the 2003 paper published in Developmental Medicine & Child Neurology.
Another large study followed 1,700 Swedish mothers and children until the kids were 5 years old. It found that mothers with depressive symptoms or who underwent stressful life events while pregnant were more likely to have left- or mixed-handed children. The work was published by Dr. Rodriguez and her colleagues in 2008 in the Journal of Child Psychology and Psychiatry.
Experts suggest that left- and mixed-handedness could be used as a risk factor for possible psychiatric or developmental conditions, along with behavioral difficulties, such as having a hard time in school. The presence of such risk factors could prompt early evaluation for those conditions, they say.
—Christina Tsuei contributed to this article.
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