The problem of being a left-hander

A new study, “Approach motivation in human cerebral cortex”, in the
Philosophical Transactions of the
Royal Society B: Biological Sciences
highlights a new kind of bias in mental health: left-handedness. While door knobs and car ignitions drive home the point that the world revolves around right-handers, Daniel Casasanto, associate professor at Cornell University, argues that a left-handed bias in mental health treatment may actually be “detrimental”.

The textbook wisdom is that each hemisphere of the brain is home to a specific type of emotion. Emotions linked to negotiating and engaging with the world (like happiness, pride and anger) live in the left side of the brain, while emotions associated with avoidance (like disgust and fear) are housed in the right. However, these studies were conducted almost exclusively on right-handers. In left-handers, emotions like alertness and determination are housed in the right side of their brains.

Casasanto’s theory, called the “sword and shield hypothesis”, challenges this dogma and argues that the way we perform actions with our hands determines how emotions are organised in our brains. Emotion isn’t always located in specific halves and can be spread out across the hemisphere. To explain that, he speaks of medieval sword fighters who would wield their swords in their dominant hand to attack the enemy (approach action) and raise their shields with their non-dominant hand to fend off attack (avoidance action). Thus dominant hands — and not left or right — determined the location of the emotion of engagement and fear.

The work has implications for a current treatment for recalcitrant anxiety and depression called neural therapy. Similar to the technique used in the study and approved by the U.S. Food and Drug Administration, it involves a mild electrical stimulation or a magnetic stimulation to the left side of the brain, to encourage emotions related to approach actions.

Based on experiments, Casasanto claims that the treatment could be damaging for left-handed patients. Stimulation on the left would decrease life-affirming approach emotions. Giving left-handers the standard treatment will probably make them worse, he says. “And because many people are neither strongly right- nor left-handed, the stimulation won’t make any difference for them, because their approach emotions are distributed across both hemispheres.”

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