Why are only 10 percent of people left-handed?
Faye Flam
Issue date: 11/16/07 Section: Science And Technology
For years, many psychologists assumed that lefties' brains were reversed, with language capacity concentrated in the right side of the organ. Subsequent work shows that is sometimes the case - but not always.
A large body of research shows the majority of righthanders follow the typical pattern, using the left hemisphere for language. Lefthanders' brains appear less predictable: About half have language abilities concentrated in the left, 10 percent in the right, and 40 percent make use of various regions on both sides.
Many animals are right- or left-pawed, or -footed or -flippered. Mice, for example, will consistently use either the right or left paw to press a lever. Unlike humans, however, most species are divided 50-50.
"Years ago geneticists tried to breed left- and righthanded mice," says Chris Walsh, a neurologist at Harvard Medical School and the Howard Hughes Medical Institute. The offspring were still evenly divided.
In humans, handedness runs in families, though not in an easily predictable way. Lefthanders are about twice as likely as righties to produce lefthanded children, but most of their offspring will still be righthanded.
A few years ago, UCLA's Geschwind scanned the brains of identical twins, hoping to understand the connection between handedness, heredity and brain structure. He found that pairs of righthanded twins tended to share a more asymmetrical brain structure than did lefthanded pairs or mixed sets.
The finding backed the idea that genes either drive the developing brain toward righthandedness or leave it to chance.
No single righthandedness gene has turned up despite many efforts to find it. Three months ago, however, a team led by Oxford's Francks discovered one that may at least play a role. They found that lefthanders tend to share a variant of the gene they named LRRTM1, but it appears to influence handedness only if it is inherited from the father. (Genes whose dominance is contingent upon which parent contributes them make up about 1 percent of the total in humans.)
A large body of research shows the majority of righthanders follow the typical pattern, using the left hemisphere for language. Lefthanders' brains appear less predictable: About half have language abilities concentrated in the left, 10 percent in the right, and 40 percent make use of various regions on both sides.
Many animals are right- or left-pawed, or -footed or -flippered. Mice, for example, will consistently use either the right or left paw to press a lever. Unlike humans, however, most species are divided 50-50.
"Years ago geneticists tried to breed left- and righthanded mice," says Chris Walsh, a neurologist at Harvard Medical School and the Howard Hughes Medical Institute. The offspring were still evenly divided.
In humans, handedness runs in families, though not in an easily predictable way. Lefthanders are about twice as likely as righties to produce lefthanded children, but most of their offspring will still be righthanded.
A few years ago, UCLA's Geschwind scanned the brains of identical twins, hoping to understand the connection between handedness, heredity and brain structure. He found that pairs of righthanded twins tended to share a more asymmetrical brain structure than did lefthanded pairs or mixed sets.
The finding backed the idea that genes either drive the developing brain toward righthandedness or leave it to chance.
No single righthandedness gene has turned up despite many efforts to find it. Three months ago, however, a team led by Oxford's Francks discovered one that may at least play a role. They found that lefthanders tend to share a variant of the gene they named LRRTM1, but it appears to influence handedness only if it is inherited from the father. (Genes whose dominance is contingent upon which parent contributes them make up about 1 percent of the total in humans.)
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Thomas Anderson
posted 8/16/09 @ 12:11 AM EST
Left-handedness appears to average around 20 percent among autistics, and is also very high among children with dyslexia and other learning problems. Some researchers have linked these problems to perinatal anoxia which suggests that left-handedness too might be a result of "brain damage" at the time of birth. (Continued…)
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