Kevin Denny, an economics professor at University College Dublin in Ireland, is not left-handed. Many years ago, though, he was staying with a friend in Kansas City, Missouri, who is left-handed. "He had a book on the subject. I got fascinated," Denny says. "I eventually realized I could publish papers on [it]. Handedness has lots of interesting aspects – historical, scientific, cultural. So it was a nice, albeit challenging, way of working in another discipline that I have no background in." One of the aspects Denny focused on was mental health.
There has been a fair amount of research that associates left-handedness with the likelihood of psychotic disorders like schizophrenia. But Denny wanted to see if handedness was linked to affective disorders, particularly major depression. Looking at large population survey data from 12 European countries, he found that left-handers are significantly more likely to have depression symptoms than right-handers. "For example, left-handers are about 5 percent more likely to have reported having ever experienced symptoms of depression," he writes in the paper, published in the journal Laterality in 2009.
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A more recent paper, published in Philosophical Transactions of the Royal Society B: Biological Sciences, adds a more disturbing fact about lefties. According to lead author Daniel Casasanto, associate professor of human development and psychology at Cornell University, a common treatment used in cases of major depression may not only be ineffective, it could actually be detrimental to anyone who is not predominantly right-handed, which comprises about 50 percent of the population.
Those with major depression or anxiety who do not respond to medication or psychotherapy are sometimes treated with electrical or magnetic stimulation. Mild electrical current is applied to the left side of the brain because studies have shown that this is the region that controls so-called approach-related emotions. Numerous studies have suggested that emotions linked to approaching and engaging with the world, such as happiness, pride and anger, are controlled in the left side of the brain, while emotions associated with protection or avoidance, like fear, reside in the right side.
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However, Casasanto realized that this idea is based on studies that were conducted almost exclusively on right-handed people. Because handedness is cross-wired with the brain – the right hemisphere controls the left side, and vice versa – he wondered if emotions were also housed in different sides. "We predicted this reversal because we believe core aspects of emotion are based on a new theory of how basic dimensions of emotion are organized in the human cerebral cortex," Casasanto says.
The Sword and the Shield
The new theory, called the sword and shield hypothesis, links how we use our hands with the way emotions are organized in our brains. In days of yore, knights held their swords in their dominant hand to attack, an approach action, and their shields with their nondominant to protect themselves, an avoidance action. Knights of today do pretty much the same. "If I ask you to throw a ball, you use your dominant hand. But if I throw a ball at your face, you defend with your nondominant hand," Casasanto says. "You pick an apple off a tree with your dominant hand, and brush branches away with your nondominant hand. We see this over and over in studies."
Using studies based on a electroencephalogram, or EEG, Casasanto found that approach emotions are created in the hemisphere of the brain that controls the dominant "sword" hand, and avoidance emotions in the hemisphere that controls the nondominant "shield" hand. Thus, to be effective, electrical or magnetic stimulation should be applied to the right hemisphere of the brain for lefties, not the left hemisphere. "This study suggests for a lefty with depression, this therapy for right-handers is exactly wrong for you," he says. "This means we have a ton of textbooks to correct."
Living in a Right-Handed World
As with gender, handedness is rarely all or nothing; most people fall somewhere on a spectrum. Casasanto, himself a righty, says about 50 percent of the population is strong right-handed, about 10 percent is predominantly left-handed and the remaining 40 percent are somewhere in the middle. "So the standard treatment only fits about half the population," he says. "The closer you are to strong righty, the more you benefit from the standard treatment. The more lefty you are, the more you need the opposite treatment. Lefties will actually get worse [from standard treatment] and those in middle shouldn't get this at all." He notes that this study was done on healthy people, so it remains to be seen if these effects will be seen in a clinical setting.
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Why would left-handedness increase the risk for mental illness in the first place? Denny's paper offers a couple of possibilities. There is evidence that some chronic illnesses are more common in left-handers, and this might contribute to higher rates of depression. There may also be common genetic factors to depression and atypical handedness, as there appear to be with psychosis.
Casasanto has another hypothesis: As any lefty knows, the world is made for right-handers. Water fountains, scissors, soup ladles and school desks are just a few of the many daily products that are designed to be used by righties and give lefties fits. "In our right-handed world, lefties use their nondominant hand much more often than righties do," he says. "It is possible that interacting with a world created mostly by righties for rights, which forces lefties to use their shield hand more often, increases activity in the brain hemisphere that is responsible for emotions like fear and sadness."
Casasanto is currently piloting a study to see if motor actions can cause changes in people's experience of approach or avoidance emotions. Surely, lefties around the world are anxious (figuratively) to learn the results.
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