The Biology of . . . Handedness

Studies of chimpanzees finally give southpaws a fair shake

By Jocelyn Selim|Tuesday, January 01, 2002
RELATED TAGS: PRIMATES

At the Yerkes Regional Primate Research Center in Atlanta, a large chimpanzee named Winston is taking part in an unusually pleasant experiment. Using his left hand, he reaches over and grabs a length of PVC pipe from primatologist Bill Hopkins, then he uses his right to scoop out some peanut butter smeared inside. "Winston's a righty," Hopkins says, offering another piece of pipe to a smaller chimp hovering nearby. This one grabs the pipe with his right hand and digs out the peanut butter with his left. "That's Winston's younger brother," Hopkins says.

Over the past 10 years, Hopkins's research has offered the first definitive proof that apes, like humans, have hand preferences: A third of the Yerkes chimpanzees are lefties and the rest are righties. But Winston and his brother point to an even more intriguing pattern: The younger the sibling, the more likely he or she is to be a lefty. And if handedness is clearly tied to birth order in chimps, it could throw a monkey wrench into theories of handedness in humans as well.

Granted, those theories already have a lot of explaining to do. Among humans, lefties are more likely than righties to suffer from dyslexia, schizophrenia, stuttering, and other disorders. But lefties are also more likely to be Mensa members, musicians, and U.S. presidents (Bill Clinton, George Bush Sr., and Ronald Reagan are all left-handed). And al-though left-handedness runs in families—notably in Queen Elizabeth II, Prince Charles, and Prince William—more than two-thirds of all lefties are born to right-handed couples. Even identical twins often have opposite hand preferences.

Handedness in people is so hard to figure out in part because it seems so deeply rooted in our brains—and in the very peculiarities that seem to make us human. In most primates and other animals, the hemispheres of the brain divide the processing of tasks somewhat equally. But in humans, the hemispheres tend to specialize: Nearly all righties process language in the left side of the brain, while many lefties process language on the right. Because handedness and language both seemed uniquely human traits, biologists long assumed that they were closely linked. One Oxford neurobiologist went so far as to argue that right-handedness could be traced back 200,000 years to a single mutation—a sort of genetic Big Bang that created hemispheric specialization, language, and higher cognitive functioning in one go. Right-handedness, to this way of thinking, is the most obvious mark of the genetic instructions that separate us from speechless, symmetric beasts.

Why, then, are there left-handers? In the 1980s, neurologists at Harvard thought they had the answer. Most lefties, they suggested, are just righties whose specialized brain centers have been reversed in the womb by abnormally high levels of testosterone. The concept has a faintly medieval ring: The word left is derived from the Old English lyft, meaning "weak" or "useless." In Latin, left translates as "sinister," and in the Bible it's tantamount to being "cursed into everlasting fire."

"The real question is why everyone wants left-handers to be defective," says Chris McManus, a neuropsychologist at University College London whose genetic model of handedness is one of the most widely accepted in the field. According to his model, lefties are, if not more evolved, at least more recent arrivals on the hominid scene—the product of a second mutation that occurred somewhere between 20,000 and 100,000 years ago, long after right-handedness became the norm. This mutation, rather than directly coding for left-handedness, simply cancels out the bias to the right, giving those who inherit it a 50-50 chance of ending up left-handed.

McManus's theory neatly explains how identical twins can have different hand preferences, as well as why people with a wide range of both gifts and disorders are more likely to be left-handed. "What inheriting a copy of the chance gene boils down to is more variability," he says. "Without the right bias, you get more unusual patterns of brain organization, which means more people at the top and bottom, and less in the middle."

All well and good. But if the genes for handedness evolved with humans, why are so many chimpanzees left-handed and in such predictable numbers? Until Hopkins's studies of captive chimps, that question never occurred to most primatologists. In the wild, chimpanzees seemed equally likely to be left- or right-handed. "If you're grabbing a branch with one hand and picking off fruit with the other," Hopkins says, "it matters less than when you have a task requiring fine motor skills that force you to subordinate one hand"—as the chimps at Yerkes do.

Nevertheless, Hopkins says, a close look at primate research since the 1920s shows that all primates have hand preferences, and those preferences follow a clear pattern: Lemurs and other prosimians tend to be left-handed; macaques and other old-world monkeys are evenly split between lefties and righties; among gorillas and chimpanzees, 35 percent are lefties, while in humans that percentage hovers around 10. In other words, the more primitive the primate, the more likely it is to be a lefty. Left-handedness, far from being a recent invention, seems to predate right-handedness.

Although humans are more likely than chimpanzees to be right-handed, infants of both species are equally likely to inherit their parents' hand preferences. "It's a little too coincidental to be explained by anything except that it's the same gene or genes across the board," Hopkins says. But unlike McManus and the Harvard group, he believes that there are two common but distinct ways to become left-handed. Either you lack the gene responsible for being right-handed, or you have the gene but something goes wrong during development that prevents the gene from being expressed.

First- and last-born chimps have the highest risk of developmental problems, so they are by far the most likely to be left-handed. "More than 50 percent of chimps that have at least five older siblings are left-handed," Hopkins says. "You know what the percentage is for second-borns? It's 12." At the same time, in chimps and humans born without the right-handed gene—and without any developmental problems—the trait could be linked to a set of genes that code for a set of special talents. "If we accept that there could be a genetic basis for both left- and right-handedness," Hopkins says, "what really matters is whether or not you're expressing your genes. If a left-hander is born to two right-handed parents, he's got reduced fitness. But if a right-hander is born to two left-handed parents, he may well be in the same boat."

As for the theory that handedness is the outward mark of a souped-up, speech-enabled, asymmetrical brain, that may be little more than modern-day phrenology. Brain scans suggest that handedness is more closely linked to motor control than to language. And although birds and frogs, like humans, process their vocalizations mostly on one side of the brain, they don't tend to show a marked preference for one limb over the other. "It's quite possible that what set humans apart was that speech began from gestures, which would explain an indirect association with handedness," says Michael Corballis, a handedness expert at the University of Auckland in New Zealand. "But it's one of those mysteries that refuses to resolve itself. Think of it this way: Primates do have very symmetrical brains, but then again, so did Einstein."

For more about Hopkins's work, see www.emory.edu/LIVING_LINKS/i/people/hopkins.html, or see "Genetic Influence on the Expression of Hand Preferences in Chimpanzees (Pan troglodytes): Evidence in Support of the Right-Shift Theory and Developmental Instability," Hopkins et al., Psychological Science, July 2001.

For a roundup of handedness heritability research in humans and other primates, see www.nurseminerva.co.uk/handedne.htm.

For more about the genetics and myths of handedness, see Lorin's Left-Handedness Site, www.duke.usask.ca/~elias/left/index.html.

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