What Makes a Lefty: Myths and Mysteries Persis
Can openers, scissors and spiral-bound notebooks discriminate against lefties. Despite such challenges, 10 to 12 percent of the human population has historically preferred the left hand.
Why doesn't the number ever waiver? Nobody knows for sure, but new research supports a body of evidence that suggests genetics have a hand in it all.
In the meantime, the myth remains that lefties are more artistic. And the idea that left-handed fighters have an advantage persists on scant evidence, supported by Scottish lore and Rocky Balboa's heroics in the ring.
Look, Mom: Both hands!
Like many traits, handedness is probably determined by a complex interaction between genes and the environment, experts figure.
Left-handers are more likely to have a left-handed relative. But researchers have yet to find the gene or set of genes that pick one hand over the other.
Most scientists agree that handedness exists on a continuum. The idea helps explain why some people bowl with their left but hold a spoon in their right. Truly ambidextrous people, who have indifferent preference for either hand, are extremely rare.
In a new study, researchers measured the width of elbows in living people and in skeletons from a medieval British farming community.
The researchers assumed the 9-to-1 ratio of handedness would match the ratio of bigger right to left elbows. The prediction held true in the modern-day group, but not for the medieval bones.
Most of the ancient farmers' left and right elbows were the same size.
"It's obvious that they were using both hands equally," said anthropologist Amanda Blackburn from the University of Manitoba. "It's not fair to say they were ambidextrous in the true sense of the word, but they may have had a tendency to use both hands equally. It's a behavior they may have learned rather than just being born like that."
The findings will be published in the April issue of the journal Current Anthropology.
Oppressing the left
Lefties have long suffered. In India and Indonesia, eating with the left hand is considered impolite. Chinese characters prove extremely difficult to write with the left hand. Not so long ago, teachers slapped the wrists of left-handed American elementary students.
Humans have shown the ability to learn to use their non-preferred hand after injuries, when required to perform manual labor, or in the face of cultural pressure.
Yet preference for handedness appears to take root in the womb, or even earlier.
One genetic model, called the right shift theory and developed by psychologist Marian Annett at the University of Leicester, suggests that a single gene increases the likelihood of being right-handed.
"The essence of my right shift theory is that there is a gene that helps to develop speech in the left hemisphere of the brain and increases the probability of right-handedness," Annett told LiveScience.
Whatever evolutionary jog made humans left-brain dominant for speech also made us right-side dominant, Annett argues. Since our closest relatives—chimpanzees—can't talk, the gene must have arisen in recent evolutionary history. One study found most chimps prefer to fish for termites with their left hand. But other recent research shows most chimpanzees favor their right hand when throwing overhand.
"The prevailing genetic model seems to be pretty strong. There are only a few weak points that are yet to be addressed. Not only can they not pinpoint a gene, there's conflicting data out there too," said David Wolman, author of "A Left Hand Turn Around the World" (Da Capo Press, 2005).
In a twist on the genetic model, the gene for hand preference might also be the gene for hair whorl direction, the way a person's hair turns on the top of their head. Half of people with counterclockwise whorls prefer their left hand, according to research by Amar Klar at the National Cancer Institute.
The same system that patterns hair and handedness could also play a role in the asymmetrical organization of the brain. "It is clear that the same genetics control both traits, along with the side of the brain where language is processed," said Klar.
The artistic myth
The answer to left-handedness is likely in the brain, and probably has to do with that organ's asymmetry, scientists generally believe. Somewhere in our lopsided brains is something, probably a gene or two that determines which hand prefers to throw a ball and which hand likes to write.
Unfortunately, scientists can't open up someone's brain and see a sign for hand preference Wolman said.
For anyone to move their left hand, or anything on their left side, instructions come from the right side of the brain. Motor centers of the brain control the hands; lefties have more dominant motor centers on the right side of their brain.
But just because the directions come from the side of the brain associated with artistic function, doesn't mean a lefty's more likely to compose a Shakespearean sonnet.
"The big myth is that the right side of the brain is somehow a creativity bull's-eye. That's not the case, and doesn't have anything to do with handedness. You need resources from both sides of your brain to be creative. All people use both sides of the brain," Wolman told LiveScience.
Lefties have had the upper hand in hand-to-hand combat since the Bronze Age, and even today, in the boxing ring. Left-handedness could be beneficial in times of violence, and genetically passed from one generation of fighters to the next, as shown by Charlotte Faurie and Michel Raymond of the University of Montpellier II in France.
While a righty fought with a sword in his right hand and a shield in his left, a left-handed swordsman could make strong surprise attack on the opponent's unprotected right side. Recall Rocky Balboa's last-minute switch to his southpaw.
The Kerr family of Scotland, known for sinister swordsmanship, went so far as to build Ferniehirst Castle with an unusual staircase that spiraled counterclockwise. The architecture provided left-handed fighters more freedom to swing their sword.
Today, the common Scottish terms Kerr-handed, kerry-fisted and corry-fisted mean left-handed.
The concept of lefties advantageously killing off all the righties doesn't hold strong, however. The 9-to-1 ratio of right- to left-handedness existed long before the advent of sword and shield warfare and continues to this day.
Some researchers suggest prenatal levels of testosterone determine hand preference. Brain damage from trauma in the delivery room is another explanation. "Proud lefties cringe at the thought of it," said the left-handed Wolman.
"The genetic model has wider support among the laterality community than brain damage at birth or levels of hormones in the womb," Wolman said. "At the end of the day, everyone seems to go back to the gene."