When ancient Roman seers conducted their magical rituals, they would face north. East--to the right--represented luck and positive omens. The west--to the left--was the dark realm of the dead. The Bible tells of the good sheep who, on Judgment Day, will find comfort at the Savior's right hand, while the sinners at his left will be condemned to eternal damnation.

The division between right and left, between good and evil, persists in today's idiomatic expressions: a left-handed compliment is really no compliment at all; someone with two left feet is clumsy on the dance floor. Yet someone's right-hand man is always there for him. We vilify the left, thanks to centuries of derision by the right-handed majority. Across cultures, righties outnumber lefties nine to one. What causes such a glaring disproportion? The answer lies between the hemispheres of the brain.

Genes, Perhaps

As with many fundamental human traits, scientists are immediately drawn to genes for explanations. The probability that two right-handed people would have a left-handed child is only about 9.5 percent. The chance rises to 19.5 percent if one parent is a lefty and 26 percent if both parents are left-handed. The preference, however, could also stem from an infant's imitation of his parents. To test genetic influence, starting in the 1970s British biologist Marian Annett of the University of Leicester hypothesized that no single gene determines handedness. Rather, during fetal development, a certain molecular factor helps to strengthen the brain's left hemisphere, which increases the probability that the right hand will be dominant, because the left side of the brain controls the right side of the body, and vice versa. Among the minority of people who lack this factor, handedness develops entirely by chance.

Research conducted on twins complicates the theory, however. One in five sets of identical twins involves one right-handed and one left-handed person, despite the fact that their genetic material is the same. Genes, therefore, are not solely responsible for handedness.

Genetic theory is also undermined by results from Peter Hepper and his team at Queen's University in Belfast, Ireland. In 2004 the psychologists used ultrasound to show that by the 15th week of pregnancy, fetuses already have a preference as to which thumb they suck. In most cases, the preference continued after birth. At 15 weeks, though, the brain does not yet have control over the body's limbs. Hepper speculates that fetuses tend to prefer whichever side of the body is developing quicker and that their movements, in turn, influence the brain's development. Whether this early preference is temporary or holds up throughout development and infancy is unknown.

Brain Damage--Not

Genetic predetermination is also contradicted by the widespread observation that children do not settle on either their right or left hand until they are two or three years old. This fact dismisses, finally, the socially disparaging theory that lefties are lefties as a result of some kind of brain damage--a position that some scientists maintained as recently as two decades ago. In the 1920s researchers indicated that there was a rather high incidence of left-handedness among children who had undergone difficult births. They also found many more lefties who had learning disorders and epilepsy. Common neurological ailments were more prevalent among twins than singles, and twins had a higher incidence of left-handedness.

In 1982 renowned Boston neurologist Norman Geschwind floated a theory that handedness was driven by the body's own immune system. The idea occurred to him while attending a conference on reading and writing disorders; people with such difficulties reported an unusually high occurrence of immune system problems or migraine headaches in their families. With further research, Geschwind discovered that left-handed people are two and a half times more likely to suffer from allergies, reading and writing handicaps, skeletal abnormalities, stuttering, and thyroid illnesses--a constellation of effects dubbed the Geschwind syndrome. Other researchers conducting similar studies, however, have failed to find such links, casting doubt on Geschwind's theory.

One-Sided Argument

But even if these correlations were true, they did not explain what actually causes left-handedness. Furthermore, specialization on either side of the body is common among animals. Cats will favor one paw over another when fishing toys out from under the couch. Horses stomp more frequently with one hoof than the other. Certain crabs motion predominantly with the left or right claw. In evolutionary terms, focusing power and dexterity in one limb is more efficient than having to train two, four or even eight limbs equally. Yet for most animals, the preference for one side or the other is seemingly random. The overwhelming dominance of the right hand is associated only with humans.

That fact directs attention toward the brain's two hemispheres and perhaps toward language, which in most people is clearly seated in one hemisphere or the other. Perhaps left- and right-handedness is unbalanced because the brain's functions are divided asymmetrically. In our daily lives this unequal distribution is hardly recognizable, yet each hemisphere possesses particular strengths and weaknesses.

Interest in hemispheres dates back to at least 1836. That year, at a medical conference in Montpellier, France, physician Marc Dax reported on an unusual commonality among his patients. During his many years as a country doctor, Dax had encountered more than 40 men and women for whom speech was difficult, the result of some kind of brain damage. What was unique was that every individual suffered damage to the left side of the brain. At the conference, Dax elaborated on his theory, stating that each half of the brain was responsible for certain functions and that the left hemisphere controlled speech. Other experts showed little interest in the Frenchman's ideas.

Over time, however, scientists found more and more evidence of people experiencing speech difficulties following injury to the left brain. Patients with damage to the right hemisphere most often displayed disruptions in perception or concentration. Major advancements in understanding the brain's asymmetry were made in the 1960s as a result of so-called split-brain surgery, developed to help patients with epilepsy. During this operation, doctors severed the corpus callosum--the nerve bundle that connects the two hemispheres. Unhinging the halves prevented an uncontrolled neurological firing in one part of the brain--the start of an epileptic episode--from exploding into a brainwide storm that caused terrible seizures. But the surgical cut also stopped almost all normal communication between the two hemispheres, which offered researchers the opportunity to investigate each side's activity.

After many experiments on split-brain patients, neuroscientists determined that both hemispheres can perceive, learn and remember independently, although the type of processing and the level of performance vary. The left side was particularly strong in analytical functions such as language processing, whereas the right side was better equipped to handle spatial and musical tasks. The left hemisphere processed sensations and thoughts as discrete elements, whereas the right hemisphere preferred to interpret such inputs as a single whole.

As understanding grew, it became clear that differences between the hemispheres mean little for healthy people. Every bit of information that enters one half of the brain is available to the other half via the corpus callosum. For higher functions such as learning, in particular, both halves work in concert. The primary exception is language.

Language--Half the Story

In 1949 neurosurgeon Juhn Wada devised the first test to provide access to the brain's functional organization of language. By injecting an anesthetic into the right or left carotid artery, Wada temporarily paralyzed one side of a healthy brain, enabling him to more closely study the other side's capabilities. Based on this approach, Brenda Milner and the late Theodore Rasmussen of the Montreal Neurological Institute published a major study in 1975 that confirmed the theory that country doctor Dax had formulated nearly 140 years earlier: in 96 percent of right-handed people, language is processed much more intensely in the left hemisphere. The correlation is not as clear in lefties, however. For two thirds of them, the left hemisphere is still the most active language processor. But for the remaining third, either the right side is dominant or both sides work equally, controlling different language functions.

That last statistic has slowed acceptance of the notion that the predominance of right-handedness is driven by left-hemisphere dominance in language processing. It is not at all clear why language control should somehow have dragged the control of body movement with it. Some experts think one reason the left hemisphere reigns over language is because the organs of speech processing--the larynx and tongue--are positioned on the body's symmetry axis. Because these structures were centered, it may have been unclear, in evolutionary terms, which side of the brain should control them, and it seems unlikely that shared operation would result in smooth motor activity. Small physiological details could have tipped the scales in favor of the left hemisphere, such that an impulse from the left brain would reach the right vocal cord faster, giving the left brain an advantage in producing language.

Language and handedness could have developed preferentially for very different reasons as well. For example, some researchers, including evolutionary psychologist Michael C. Corballis of the University of Auckland in New Zealand, think that the origin of human speech lies in gestures. Gestures predated words and helped language emerge. If the left hemisphere began to dominate speech, it would have dominated gestures, too, and because the left brain controls the right side of the body, the right hand developed more strongly.

Creativity in Hand

Perhaps we will know more soon. In the meantime, we can revel in what, if any, differences handedness brings to our human talents. Popular wisdom says right-handed, left-brained people excel at logical, analytical thinking. Left-handed, right-brained individuals are thought to possess more creative skills and may be better at combining the functional features emergent in both sides of the brain. Yet some neuroscientists see such claims as pure speculation.

Leonardo da Vinci is perhaps the most famous lefty of all. Researchers disagree on how much his handedness drove his creativity. Proponents note that he had a propensity to paint holistic and symmetrical settings. He grouped people in centered, pyramidal structures. In his famous painting The Last Supper, he situated the apostles around the table in a way that makes an observer view the group as an integrated whole. Their body postures focus the eye toward the middle, where Jesus sits. Right-handed artists such as Rembrandt, in contrast, seem to prefer more asymmetrical relationships--for example, grouping trees in the lower left-hand corner of a canvas while extending a green mountain landscape out to the right.

Some of these observations may be little more than wild speculation. Leading the way is the story of Alan M. Turing, the mathematician who is widely credited with founding modern computer theory before his death in 1954. Some analysts see Turing as a poster boy for psychologist Geschwind's view that left-handedness, and resulting mathematical creativity, stems from immune system dysfunction. Turing was extremely allergic to pollen, wearing a gas mask to protect himself on his daily bicycle ride to the University of Cambridge.

Fewer scientists are ready to claim that left-handedness means greater creative potential. Yet lefties are prevalent among artists, composers and the generally acknowledged great political thinkers. Possibly if these individuals are among the lefties whose language abilities are evenly distributed between hemispheres, the intense interplay required could lead to unusual mental capabilities.

Or perhaps some lefties become highly creative simply because they must be more clever to get by in our right-handed world. This battle, which begins during the very early stages of childhood, may lay the groundwork for exceptional achievements. Then again, lefties with bilateral language processing could represent a new evolutionary advance for Homo sapiens, in which creative constellations between the hemispheres are being tested. Lefties might like to raise that notion the next time a right-hander puts them down.