By Bob Grant
With almost complete certainty, I can predict that you, dear reader, are right-handed. If I were a betting man, I’d put money on it. I’d make the same bet if you were reading this in India or Iowa, Kansas or Kathmandu. And a hundred years from now, I’d make the same bet again.
I can be so sure of myself not because I am some prodigious prognosticator, but because about 90 percent of humans are right-handed. That phenotypic ratio—nine right-handed people for every lefty—is relatively stable, not just across cultures and geographic regions, but perhaps across the span of human evolution. The archaeological record suggests that hominins were predominantly right-handed as far back as 2 million years ago,1 and a 2010 study of the wear patterns on 32,000-year-old Neanderthal teeth found that this extinct cousin of Homo sapiens was likely about 88 percent right-handed.2 It’s a ratio that has baffled scientists for more than a century, despite the fact that handedness is immediately familiar to everyone who uses their limbs. “We understand handedness, because it’s easy to say which hand you use for writing,” says Silvia Paracchini, a geneticist at the University of St. Andrews in the U.K. who is searching for genes that underpin human handedness. “But the reality is that we know very little about it. We cannot explain it with data.” Scientists are unsure how or why humans are so strongly biased toward right-handedness. What are the biological underpinnings of the behavioral asymmetry? Is this overwhelming dominance of righties reinforced by culture, or is it strictly a product of genetics?
Recently, some of the most interesting findings have come not from studying handedness in humans, but from observing the behaviors and brains of other animals. Once considered to be a uniquely human trait, handedness may exist on the individual level in other species, and may be common in primates. At the population level, some primate groups even appear to show biases toward one side or the other. A 2011 study involving nearly 800 great apes conducted by Emory University neuroscientist Bill Hopkins and colleagues found that for a particular bimanual task—which necessitates the use of both hands, one dominant and one supportive—gorillas, chimps, and bonobos tend to be right-handed, while orangutans showed a population-wide preference for using the left hand.3 And in 2012, Hopkins and collaborators in China found that a population of wild Sichuan snub-nosed monkeys (Rhinopithecus roxellana) is predominantly left-handed (about 70/30) for the same bimanual task.4
By far the most biased species is Homo sapiens, and some investigators hold firm that a consistent and overwhelming handedness bias to the right is a uniquely human attribute, much like language and other higher cognitive functions. Handedness, they say, likely stems from consistently reinforced cultural norms, and any reported biases toward one side of the body in nonhuman animals are primarily the result of experimental or observational artifacts or statistical error.
But in the past couple of decades, Hopkins and his collaborators have provided strong evidence that chimps do favor one side of their bodies over the other. The captive chimp colonies that Hopkins studies are 60 percent to 70 percent right-handed, regardless of the proportion of individuals in each colony that were human-reared,5 and recent scans of the chimps’ brains reveal corresponding brain asymmetries—hallmarks linked to hand preferences in the highly complex human brain. “I think the chimps offer an interesting comparison, because—biologically, genetically, cognitively, and anatomically—they show a lot of homologies to humans,” says Hopkins.