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See Inside April 2006

Taking Sides

Is being right-handed all for the greater good?
EVOLUTIONARY PRESSURE



RANDY FARIS Corbis
Ask why most people are right-handed, and the answer might fall along the same lines as why fish school. Two neuroscientists suggest that social pressures drive individuals to coordinate their behaviors so that everyone in the group gets an evolutionary edge.

Approximately 85 percent of people prefer their right hand, which is controlled by the left hemisphere of the brain. One theorized benefit of locating a particular function in one hemisphere is that it frees the other to deal with different tasks. But that idea does not explain why population-wide trends for handedness exist in the first place. Moreover, evidence gleaned in recent years has overturned the long-held belief that human handedness is a unique by-product of brain specialization attributable to language. A suite of studies has revealed brain lateralization in species from fish to primates. Last August, for instance, scientists discovered that in the wild, chimpanzees show hand preferences.

The presence of lateralization throughout the animal kingdom suggests some benefit from it, contend neuroscientists Giorgio Vallortigara of the University of Trieste and Lesley Rogers of the University of New England in Australia. Also, last August, in the journal Behavioral and Brain Sciences, the two presented evidence to support their idea that social constraints force individuals toward asymmetry in the same direction. They noted, for example, that baby chickens attack more readily when a threat appears on their left. And Rogers has found that chicks with more asymmetrical brains form more stable social groups: perhaps by approaching each other on the right, she hypothesizes, the chicks fight one another less and are more likely to notice predators.

Lateralization seems to confer an advantage for some fish as well. In certain species, the majority tend to swim left when a predator attacks, whereas other species head right. The potential benefits of such patterns may not seem intuitive: a predator could learn that attacking a fish on one particular side is more effective. But Vallortigara and Rogers's idea fits with the conventional explanation of why fish school at all. When threatened, fish turning in the same direction have a greater chance of survival than if they scatter to become a darting swarm of head-butting fish.

Nevertheless, the bird and fish data do not explain human handedness. "The issue then becomes: maybe this lateralization long predates the rise of the mammals," speculates Robin Dunbar, an evolutionary psychologist at the University of Liverpool in England. "And mammals are lateralized as they are in the brain simply because their ancestors were, going back to the origins of the fish."

Research on chimpanzees supports that idea. Elizabeth V. Lonsdorf of the Lincoln Park Zoo in Chicago and William D. Hopkins of the Yerkes National Primate Research Center at Emory University recently published data showing that wild chimpanzees display heritable population-wide hand preferences in certain tool-aided tasks--for instance, two thirds of the chimpanzees they observed in the wild preferred holding a stick in the left hand to dig termites out of a hole. Previously, primates had exhibited right-hand preferences in captivity but no handedness in the wild, leading scientists to speculate that they became lateralized through interacting with humans.

The chimpanzee findings fill in what had been a troublesome missing link between lower vertebrates and humans. "Getting away from the human uniqueness argument has really been good," Hopkins says. "It has made people really rethink some of their ideas."

Other explanations for lateralization exist--for instance, that it was passed down as part of a larger genetic package that improved its possessor's chance of survival in a way unrelated to a specific lateralization. "I would take exception to the idea that there's one definitive approach, because it is such a slippery question and one that requires inference," says neuroscientist Jeffrey Hutsler of the University of Michigan at Ann Arbor.

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