Research Suggests Some Mental Health Treatment Isn't As Effective For The Left-Handed — Here's Why


Obviously we know now that being right-handed or left-handed doesn’t determine whether or not you’re “evil” or “unlucky”; in this day and age, we’ve moved beyond those superstitions. However, it turns out that being right-handed or left-handed does affect how we process emotions — which, in turn, might have some major implications for some mental health treatments. In a new study published by researchers Geoffrey Brookshire and Daniel Casasanto in the journal Philosophical Transactions of the Royal Society B: Biological Sciences, emotional lateralization, as it’s called, might be more complex than we’ve hitherto though — and, in fact, it might be connected to which hand is your dominant one.

If you’re familiar with the idea of being “left-brained” or “right-brained,” emotional lateralization is sort of similar: It posits that the left and right hemispheres of the brain are responsible for processing different emotions. According to an overview of the history of emotional lateralization published in 2008, the big breakthrough for the idea occurred in 1978, when Richard Davidson presented a paper showing evidence that the left prefrontal cortex processes positive emotions, while the right prefrontal cortex processes negative ones. What made this theory so momentous was the fact that it ran counter to what was previously believed — that the right hemisphere processed emotion exclusively, while the left hemisphere processed “intelligence.”

Trouble is, most of the research that’s been conducted on emotional lateralization has focused primarily on right-handed people — which means that both of those previous theories might be completely incorrect. It certainly hasn’t done the 10 percent of the population who are left-handed any favors; indeed, what we’re now discovering is that the way emotional lateralization works for righties may not be the same as the way it works for lefties at all. For lefties, things might be reversed.

This is where something called the “sword and shield” hypothesis comes into play. First proposed and tested by Brookshire and Casasanto in a 2012 paper published in the journal PLoS One, the hypothesis focuses on motor control, stemming from the understanding that the actions we take are closely linked with our emotions. The sword and shield hypothesis describes actions as either “approach” or “avoidance,” noting that approach actions are the sort usually performed with someone’s dominant hand, while avoidance actions are usually performed with the nondominant hand — a phenomenon we can see in combat techniques from bygone eras: “In centuries past,” wrote the researchers, “sword fighters wielded the sword in the dominant hand when approaching an enemy, and raised the shield with the nondominant hand to avoid attack.”

What Brookshire and Casasanto found at the time was that “approach motivation was lateralized to the same hemisphere that controls the dominant hand” — that is, if you’re right-handed and you take a swing with your sword at an enemy, the motion is connected to the left hemisphere of your brain, while if you’re left-handed, the action is connected to the right hemisphere of the brain. Replace “take a swing with a sword at an enemy” with any other kind of “approach” action — picking up something you want to eat, writing a note to someone you need to communicate with, etc. — and, well… you can see how that works.

From there, the question became about how the sword and shield hypothesis might affect emotional lateralization — again, with the understanding that our emotions and our actions are very closely linked. As Casasanto explained it in a press release for the current study, “You would wield the sword in your dominant hand to make approach-related actions like stabbing your enemy, and use the shield in your nondominant hand to fend off attack” — that is, “your dominant hand gets the thing you want and your nondominant hand pushes away the thing you don’t.” The researchers hypothesized that, if the sword and shield hypothesis was in fact correct, then three other things should also be true: According to Casasanto, “Approach motivation should be mediated by the left hemisphere in strong right-handers… it should completely reverse in strong left-handers… [and] for everyone in the middle of the handedness spectrum, approach emotions should depend on both hemispheres.”

And, in the current study, the hypothesis does appear to check out: In left-handed people, things operate in the opposite way they do for righties.

For the study, a sample of 25 adults whose handedness ranged from strong right-handedness to strong left-handedness were exposed to transcranial direct current stimulation — that is, they had the left and right hemispheres of their brains stimulated by an electrical current (don’t worry; it was a painless current) — for about 20 minutes a day for five days. They also reported how strongly they felt certain approach-related emotions like pride, determination, and enthusiasm both before and after the five days of transcranial direct current stimulation.

What Brookshire and Casasanto found is that “the strength and direction of participants’ handedness predicted whether electrical stimulation to frontal cortex caused an increase or decrease in their experience of approach-related emotions”— that is, as the press release puts it, “strong righties who were zapped in the left hemisphere experienced a boost in positive emotions,” and vice versa for lefties. However, when the participants were stimulated in the hemisphere of the brain that didn’t control their dominant hand, they either experienced no change — or they experienced a detriment.

These findings totally change our understanding of emotional lateralization. Said Casasanto, "The big theoretical shift is, we’re saying emotion in the brain isn’t its own system. Emotion in the cerebral cortex is built upon neural systems for motor action.”

Here's why all this matters for mental health: A type of neural therapy for treating depression and anxiety that has been approved by the Food and Drug Administration (FDA), transcranial magnetic stimulation (TMS), functions quite similarly to the transcranial direct current stimulation used in Brookshire and Casasanto’s study. As the American Psychological Association describes it, TMS operates via a device that sends “short but intense magnetic pulses into the brain, where they generate an electrical current.” These pulses are centered on the left prefrontal cortex and are intended to encourage approach-related emotions. But according to Brookshire and Casasanto’s study, if TMS is applied to the left prefrontal cortex, it will only encourage approach-related emotions for right-handed people. For left-handed people, at best, it will do nothing — and at worst, it will make them feel even worse than they already do.

Neural therapy is somewhat controversial — typically it involves not the application of magnetic or electric currents, but the injection of an anesthetic, which, as Quackwatch points out, is generally not recognized as an effective treatment by the medical community. But for those who seek it out — especially those who seek out TMS — the risk of it not only not working for lefties, but actually causing more problems could very well be an issue.

More research on the sword and shield hypothesis needs to be done, of course — but we also might need to rethink how we apply some sorts of treatments. Suggested Casasanto in a press release, “[The research] suggests strong righties should get the normal treatment… strong lefties should get the opposite treatment, and people in the middle shouldn’t get the treatment at all.”

If you’ve been thinking about trying TMS or something like it, it’s worth considering whether you’re right- or left-handed — and move forward or pull back accordingly.