Genesis of Hand Preference

Survival of the Fittest

The reason the majority of people are right handed is still unknown. There are several theories. Darwin's theory of evolution states that the most fit survive, but why would right-handed people be more likely to survive than left-handed people? One theory, called the "sword and shield hypothesis," suggests that right-handed people would fight holding the sword in their right hand and their shield in their left hand. In contrast, left-handed people would hold their swords in their left hand and the shield in their right. When right- and left-handed people were in a battle, the right-handed people would be more likely to survive because, independent of hand preference, the heart is on the left side of the chest and hence more vulnerable when the shield is held in the right hand. The problem with this theory is that there is evidence that even prior to the advent of swords the majority of people had a right-hand preference, and this theory would not help explain why creativity might be linked to handedness.

Fetal Position

Another theory suggests that the fetus in its mother's womb is more likely to be in a position where its left arm and hand are toward the mother's spine and thus have less freedom to move than does the right arm. This gives the right hand-left brain an advantage that increases as the child matures. The problem with this theory is that it cannot account for the observation that the position of the fetus in the womb is not random and again this theory will have little explanatory value for the relationship between hand preference and creativity.


Many cultures discourage the use of the left hand. Even in the United States and England, countries that are noted for their tolerance of diversity, the scientific term for left-handedness is sinistral, which means evil. In many other languages the terms used for left-handedness also have negative connotations. In the United States, children who write with their left hand are no longer trained to switch hands and to write with their nonpreferred right hand, but only a few generations ago this was a widespread practice. Despite the relaxation of cultural pressure in the United States, the percentage of people who are right handed has remained about 90%. Thus, although cultural factors might influence the overall prevalence of hand preference, cultural influences do not appear to play a critical role.

Brain Asymmetries

Although almost all people who study hand preference would agree that hand preference is related to some form of brain asymmetry, the reason why a person prefers one hand or the other is still not entirely understood. I briefly discuss three of the major theories: (a) language dominance, (b) praxis laterality, and (c) deftness.

Language Laterality

Many people think that the hemisphere that is dominant for mediating language determines handedness. When patients get strokes they usually damage either the right or left half of their brain (hemispheres). As I mentioned in chapter 2, more than 130 years ago Paul Broca, a French physician and anthropologist, reported eight patients who were all right handed and all had an impairment of speech-language (aphasia) from a left-hemisphere injury. On the basis of these observations, he proposed that if a person prefers his or her right hand then the left hemisphere mediates speech-language. Subsequent studies have replicated his findings, and there is now overwhelming evidence that more than 95% of right-handed people have their speech-language mediated by their left hemisphere.

Each hemisphere controls the opposite hand, and one of the most important skilled activities we perform with our hands is writing. Writing requires both language and motor skills. Because in right-handed people it is the left hemisphere that mediates language and controls the right hand, people who have language in their left hemisphere might prefer to write with their right hand because this hand has direct access to the systems that mediate language. If a person who has language mediated by the left hemisphere attempts to write with the left hand, he or she must first transfer linguistic information from the left to the right hemisphere, which controls the left hand. The interhemispheric transfer of information is less efficient than direct access. Thus, language laterality has been posited to be an important factor in hand preference.

One of the problems with the language-writing theory of hand preference is that children show evidence for hand preference even before they write. In addition, even more than 90% of people who were never taught to read and write are right handed.

About 10 years after Broca's report, Karl Wernicke, a 19th-century German neurologist, demonstrated that the left hemisphere of right-handed people is also important for the comprehension of language. Many of the actions that we perform with our hands and arms are in response to language commands, both internal and external. Because the same hemisphere that comprehends language controls actions of the right hand, the use of the right hand might be more efficient than the use of the left hand that is controlled by the nonlanguage-dominant right hemisphere.

As I mentioned, the major band of fibers that connects the two hemispheres is called the corpus callosum. Occasionally, strokes injure the corpus callosum and sometimes the corpus callosum is cut by neurosurgeons to prevent the spread of epileptic seizures in patients who cannot be adequately controlled by medications. Right-handed patients who have had callosal injury often find that their left hand acts in a bizarre fashion. For example, one patient I had the privilege of examining told me that once, when she was getting dressed, she wanted to wear a blue dress and thus took a blue dress out of the closet with her right hand. When she went to get the matching blue shoes with her left hand, her left hand picked up red shoes. The type of observation that this patient related to me has led some people to call this left hand an "alien" hand because it appears to be under control by some foreign force.

Stories such as these suggest that our conscious intentions, or goal-directed actions, are mediated by the verbal left hemisphere. People with callosal disconnection, can not communicate these intentions to their nonverbal right hemisphere, which might have different intentions. Because conscious intentions might be mediated by "inner speech," the right hand would have privileged access to these intentions.

It takes several years after a child is born before its corpus callosum becomes entirely myelinated, and unmyelinated nerve fibers inadequately transmit information between the hemispheres. Thus, during the early stages of development, the child's right hand and arm has privileged access to verbal intentions, and this asymmetrical access in children is even greater than that found in normal adults.

Although language laterality might be one of the important factors that determine hand preference, there is one important observation that suggests that it cannot be the entire explanation. Some patients who have epilepsy have an abnormality of the cerebral cortex that becomes electrically irritable and is the source of the epileptic seizures. If medications cannot control these patients' seizures, the surgeon might elect to remove this abnormal part of the cortex. The surgeon, however, does not want to remove any part of the cortex that is important for language. Thus, before a patient undergoes surgery, the neurologist injects a barbiturate into the right and then the left carotid artery (the major artery that feeds blood to each hemisphere). This barbiturate puts the hemisphere that receives this sedative asleep. If when the left hemisphere is put to sleep a person cannot speak, it is the patient's left hemisphere that is dominant for language and vice versa. Using this selective hemispheric anesthesia procedure, investigators (Milner, 1974) have learned that 70% of people who prefer their left hand have left-hemisphere dominance for language, and thus these people who prefer to use their left hand have language dominance that is similar to people who prefer their right hand. Fifteen percent of left-handers, however, do have their language mediated by their right hemisphere, and the final 15% have both hemispheres mediating language. The observation that the majority of left-handed people are left-hemisphere dominant for speech and language provides evidence that language laterality cannot solely account for hand preference. Thus, there might be other factors that determine hand preference.

Praxis Laterality

Throughout our lives we learn to perform skilled acts such as using tools, writing, and drawing. Many of the acts are important for creative endeavors. Watson and Heilman (1981) examined a right-handed patient who had a stroke that damaged her corpus callosum. When we asked her to pantomime using a tool, such as a hammer, she performed flawlessly with her right hand and arm but performed poorly when using her left arm. Because this woman was right handed, her left hemisphere probably mediated language. When we gave her the command to use a hammer, it was the left hemisphere that decoded this linguistic message; perhaps she could not pantomime the use of a hammer with her left hand because her callosal damage prevented the verbal message from reaching the right hemisphere, which controls the left hand. We, therefore, made a hammering pantomime and asked her to copy or imitate this pantomime. Again, she performed flawlessly with her right hand but poorly with her left hand. When we also gave her an actual hammer to use with her right hand then her left hand, she also showed this same hand asymmetry. Imitation and the actual use of tools and objects can be performed in the absence of language. Thus, this woman's failure to correctly use tools and objects with her left hand suggests that some other form of knowledge is stored in her left hemisphere and with callosal disconnection could not be accessed by her right hemisphere.

Liepmann (1920) suggested that in people who prefer their right hand, their left hemisphere not only mediates language but also contains the memories or representation of how to perform skilled acts. He provided support for this hypothesis by demonstrating that in right-handed people the loss of the ability to perform skilled movements (apraxia) was associated with left- but not right-hemisphere strokes. He also suggested that it was the laterality of these learned movement (praxis) representations that accounted for hand preference.

In general there has been strong support for this postulate, but there have been reports of right-handed people who developed apraxia from right-hemisphere injuries. One of these patients whom we examined also was aphasic (Rapcsak, Gonzalez-Rothi, & Heilman, 1987). Thus, this patient's hand preference or handedness could not be entirely explained by either language or praxis laterality, suggesting that another asymmetry might be important in determining hand preference.


Deftness is defined as the ability to make fine, precise, and independent finger movements. Although the term dexterity is also used for these abilities, this word derives from the word dextral, which pertains to the right hand or right side, and many people are more deft with their left hand than with their right. Nonhuman primates, such as old world are very deft, and Lawrence and Kuypers (1968) found that in monkeys, injury to the corticospinal system, which sends movement messages from the cerebral cortex to the motor neurons in the spinal cord, impaired these monkeys' ability to make precise independent finger movements.

Magnetic stimulation can produce electrical currents through the skull and thereby activate the corticospinal motor neurons in the motor cortex. Triggs and coworkers (1994) found in right-handed people that the threshold for activating the motor neurons in the left hemisphere was lower than it was for the right hemisphere, suggesting that right-handed people have larger motor representations in their left hemisphere than in their right hemisphere. Nudo, Milliken, Jenkins, and Merzenich (1996), however, found that the cortical representation of hands could change with practice and therefore it was unclear if the asymmetries of the motor cortex were induced by nurture (practice) or nature. Foundas et al. (1998), using structural magnetic resonance imaging, measured the size of the hand area in the brains of right-handed people and found that the motor cortex on the left side was larger than that on the right. Although the physiological configuration of cortex might change with practice, it is unlikely that the anatomic configuration would change, and the findings of Foundas et al. suggest that there are anatomic asymmetries of the motor system, and these asymmetries might in part account the functional asymmetries found by Triggs et al. as well as account for hand preference.

All right-handed people, however, are not more deft with their right hand than left hand, and although most people have greater arm strength with their preferred arm, there are people who have a non-preferred hand that is stronger that their preferred hand. Thus, deftness, like language and praxis laterality, cannot alone explain hand preference, and hand preference appears to be related to multiple factors. In addition, none of the factors that have been described previously can explain why hand preference would influence creativity. Thus, perhaps handedness might be linked to another factor or factors that influence creativity.

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