How you judge everything from job candidates to consumer products to politicians may be influenced by your handedness. (Illustration by James Steinberg)

Left-hand man

By studying handedness, psychologist Daniel Casasanto hopes to understand the relationship between body and mind.

Left-handers make great baseball pitchers, but they’re generally considered lousy subjects for studies of the brain. “Every good neuroscientist knows you don’t test lefties,” explains Daniel Casasanto, assistant professor in psychology. “They mess up your data.”

Perhaps it’s justice. After years of being relegated to uncomfortable right-handed desks (to say nothing of scissors or spiral notebooks) lefties exact their revenge in MRIs. Among righties, brain lateralization—the control of functions and behaviors by particular brain hemispheres—is fairly consistent. Among lefties, however, it’s messier and much less predictable, complicating results.

But rather than avoiding southpaws, Casasanto has devoted years to studying them, hoping to learn how and why their brains diverge from those of right-handers. It’s part of a larger effort to understand the relationship between our bodies and our minds. Does experiencing the world in different bodies cause us to develop correspondingly different brains? Casasanto thinks so.

Handedness offers especially powerful insights into the links between cognition and bodily experience. It’s “a model system where human bodies differ in clear and measurable ways, and in consequential ways—because our hands are a point of interface between the mind and the world,” he explains.

It’s still not known why human handedness varies. While there appears to be a strong genetic component, environment plays a role too: identical twins share the same genome but not always the same handedness.

Casasanto’s research has shown that, where handedness is concerned, experience matters more than genetics. Handedness is something you do rather than something you are. Lefties whose handedness was “corrected” in childhood behave consistently like righties in his studies.

Casasanto started by exploring whether handedness influences perceptions of abstract concepts, like good and bad. In one study, subjects were presented with a drawing of a cartoon character, viewed from above, with an empty box on either side. They were told the character loved pandas but hated zebras (or vice versa) and then asked to draw a panda in the box that best represents good things, and a zebra in the box that best represents bad things. He found the subject’s choice of the “good” side was strongly associated with their handedness.

The same study asked subjects to look at two columns of Fribbles, alien cartoon characters with various arm- and trunk-like appendages. Subjects were asked to assign positive or negative characteristics such as attractiveness or sadness to the Fribbles. Again, the results varied by handedness, with lefties assigning positive characteristics to the Fribbles on the left side of the page. The outcome was the same whether subjects responded orally or used their hands to indicate their choices—and the pattern extends far beyond the realm of the fictional Fribbles. The study showed identical effects for people picking among consumer products and between hypothetical job candidates whose names and qualifications were displayed in two columns.

Handedness might even influence voting behavior. Lefties, Casasanto revealed in a 2015 paper, were 15 percentage points more likely than righties to vote for the candidate they saw on the left side of the ballot in a simulated election. Some states present opposing candidates’ names in two columns—a ballot design that may have unwittingly influenced election outcomes for years.

These findings surprised Casasanto, given how strongly Americans associate “right” with “good.” This mapping is reinforced in idioms like “my right-hand man” or “two left feet,” and in customs like raising your right hand as you swear to tell the truth in court. “This body-based pattern goes against deeply entrenched patterns in our language and culture,” he says.

The old myth that lefties are “right-brained” and therefore creative and artistic is just that—a myth, discredited by research. But Casasanto has found that lefties’ and righties’ brains do vary in how they organize a basic dimension of emotion—approach and avoidance motivation.

Emotions are either approach related, like happiness or anger, or avoidance related, like fear. Generally speaking, we perform approach-related activities with our dominant hands and avoidance-related activities with our nondominant hands. Casasanto calls this the “sword and shield” pattern: if you were a knight, you’d hold your sword in your dominant hand and your shield in the weaker hand.

Studies dating back as far as 1972 have shown that the left frontal lobe, which controls the right hand, is activated for approach-motivated emotions. But most of these studies didn’t take lefties into account. In a 2012 paper, Casasanto found that approach motivation is on the right frontal lobe in lefties. (Among the ambidextrous, approach motivation is “smeared across both hemispheres,” he says.) The finding bolstered Casasanto’s hypothesis that the lopsided ways in which all of us interact with the world cause our brains to develop in predictably lopsided ways too.

Of course, bodies differ in many ways other than handedness, and Casasanto thinks those dissimilarities might also influence cognition, just as being left- or right-handed does. For instance, he discovered that your own eye color influences your perception of the eye color of others.

The research stemmed from an argument between Casasanto and his wife about James Bond’s eye color in a recent film. His blue-eyed wife insisted Bond’s eyes were blue, “and I,” brown-eyed Casasanto says, “was expressing skepticism about whether you could have a blue-eyed James Bond.”

The marital disagreement—and Casasanto’s ensuing study—yielded an interesting discovery: after seeing a photo of a celebrity, you’re more likely to remember them as brown-eyed if you’re brown-eyed yourself. “This is part of a broader tendency to use our own body as an index of what is likely to be true about other people’s,” he explains. Mostly it’s “a pretty good heuristic: what’s true of my body is likely true of yours in many cases, but of course ... it only works on average.”

For the record, the most recent James Bond, actor Daniel Craig, is blue-eyed. And left-handed.