Research on Left-Handedness
Awareness of handedness goes back millenniums. Handedness appears to have remained about 93% right-handed over 5,000 years if interpretations of evidence from surveys of art works can be trusted. Even with this long history of recognition of right- and left-handedness, many unanswered questions remain as to why this pattern of dominance exists.
Theories and studies of lateralization and brain dominance have abounded for decades, and educators have built entire teaching methodologies on pet theorems, only to have them come into question after more serious evaluation. And even with all the sophisticated research of recent years made possible by new technologies, folklore and pet theories often continue to influence the way in which we attempt to explain and react to left-handedness.
The subject becomes even more intriguing when we consider the fact that the right-sided tendency is species-specific to humans. While some animals are right- or left-pawed in relatively equal numbers, only humans show this large percentage with right-hand preference.
Statistical studies reveal all sorts of correlations between left-handedness and other variables, such as various diseases or cognitive factors. For example, among dozens and dozens of groups with elevated prevalence of left-handedness are individuals with allergies, alcoholism, epilepsy, and eczema (to name only a few such correlations); analysis of career choices with a higher than expected rate of left-handers includes professional baseball players, architects, lawyers, musicians—and the list goes on. Often these correlations are spurious, and attempts to explain why humans prefer one hand to the other are more conjecture than good science. Theories and research studies about handedness may be loosely grouped under various categories. For this brief discussion we will comment only on research related to the brain and to genetics.
Neuroscience has come of age within the last two decades. Brain imaging technologies using magnetic resonance imaging (MRI), nuclear magnetic resonance imagery (NMRI), positron emission tomography (PET), electroencephalograms (EEG), and other devices for the collection and analysis of electrochemical data are shedding new light on the operation of the brain. The potential for research from neuroscience to help in understanding handedness and how this relates to learning holds great promise. In spite of all we are unsure about, there are things that we can say with some certainty from our knowledge of brain studies.
The human brain has two cerebral hemispheres, the left and the right. The left hemisphere processes things more in parts and sequentially, and is the center for language, science, mathematics, and logic. This left side of the brain is usually dominant, and because the left side of the brain controls the right side of the body, these individuals are right-handed.
The right side of the brain synthesizes and is a source of dreams, fantasies, art, music, and feeling. In left-handed individuals the right side of the brain is dominant (and thus they are left-handed). Almost half of left-handers use their right hemisphere for language. In some left-handed people though, writing may be controlled completely and independently from the right hemisphere of the brain.
There is a tendency to label left-handers as more feeling oriented and talented in the arts than right-handers and one can point to examples of many individuals where this is the case. However, one cannot necessarily jump to the conclusion that left-handed individuals are “right-brained” in the sense that they are more talented in the arts or other so-called right-brain attributes.
Some studies show that the abilities of left-handers are as diverse as those of right-handers. In other words, human behavior is more complex than can be explained by handedness alone. Intelligence, talent, and environmental factors interact with an individual’s pattern of hemispheric dominance and language localization.
Researchers have explored the idea that left-handedness is produced by a brain abnormality, such as a neural defect, perhaps caused by a diminished blood supply to the left hemisphere during fetal growth. Early brain damage in the left hemisphere can result in a shift of language and dominant limb function to the right hemisphere. There does seem to be a correlation between premature birth, prolonged labor, breech births, and the tendency toward left-handedness.
Another theory, as yet unsubstantiated, suggests that the male hormone testosterone might be responsible for handedness. This hormone slows left hemisphere development in the male fetus and could account for left-handedness.
Is left-handedness inherited? Left-handed parents are more likely to have left-handed children. It has been demonstrated that handedness runs in families, but of course environmental pressure must also be considered, for parents may purposefully or incidentally teach their children to be right- or left-handed. Adoption studies suggest that handedness is under genetic control as the results indicate the handedness of adopted children is more likely to follow that of their birth parents than their adopted parents.
Some researchers have hypothesized the existence of a single gene which confers right-handedness; individuals lacking this gene display random handedness with about one-half being right-handed and the other half being left-handed. One problem with this single-gene theory is that other studies show that when both parents are left-handed they have a 30% to 40% chance of having a left-handed child. If left-handedness were a recessive trait and both parents are left-handed, then all of their children should be left-handed. Perhaps more than one gene is involved, or other more complex factors come into play. When a full gene map has been developed, the genetic factors will hopefully become more clear.
In the final analysis, we still have much to learn about handedness, its causes, and the manner in which it interacts with human behavior. The probability of major breakthroughs because of advances in neuroscience and genetic studies appear to be high. We can look forward to a better understanding of lefties and “why they are the way they are” in the not-too-distant future.
Jensen, Eric. Teaching with the Brain in Mind. Alexandria, Virginia: Association for Supervision & Curriculum Development, 1998.
Kelly, Evelyn B. Left-Handed Students: A Forgotten Minority. Bloomington, Indiana: Phi Delta Kappa, 1996. —wdl
Permission to copy, but not for commercial use.