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Left Handedness and Increased Mortality – Is there a link?
October 10, 2008
Handedness represents any given individual’s preference for one hand over the other in dextrosity and performing fine motor skills. The implication is that right handed and left handed people use different parts of their brain because of this, meaning biological differences between the two probably exist. Among homo sapiens, 10-13% are left handed, with slight variance between cultures (Raymond, Pontier, Dufour, Moller, 1996). Most people understand there are fewer left handed people than right handed people simply from observation of fewer fellow peers writing, throwing, or eating with their left hands. However, until the past couple decades, most people rarely questioned why there are fewer left handed individuals in our society. The most obvious explanation is that lefthanders die sooner than right handers. But is this possible? Why is this such a quiet issue if it affects more than ten percent of the population? Hence, the following question must be asked: Is there conclusive evidence that left handed individuals die sooner than their right handed counterparts? And if so, why?
The Case of the Dissapearing Southpaws
The realization that there must be an explanation for the decreased frequency of left handedness among humans led phychologist Stanley Coren to begin exploring the concept of handedness in society. He ammassed the results of older population studies in a meta analysis and concluded some startling theories (Coren, Halpern, 1991). Coren’s data showed a much higher mortality rate among left handed people than right handed by examing the age at which members of each groups died.
Coren then extrapolated data from an unselected sample of death records and concluded that statistically lefthanders die, on average, about 8 years earlier than do right handers. Naturally this statement shocked and angered many people, especially lefthanders. Following, Coren introduced a variety of possibilities for this discrepancy, including the “Right Hand World” theory, which states that the obstacles lefties experience in their day to day life in a right-hand-oriented world are what lead to accidents, stress, illness, and eventually increased mortality in lefties. In addition to this environmental theory, Coren proposed a complementary biological theory, which stated that lefties were inherently more prone to immune system problems and physiological or psychological conditions, all of which would lead to increased mortality rates. Because this issue is so controversial and potentially life-changing claim for millions of lefties, there have been numerous subsequent studies to try and discern the validity of Coren’s findings on both the environmental and biological fronts.
We Live in a Right Hand World
Obstacles to Left Hand Living
Although perhaps not apparent, there are countless numbers of digitally manipulated tools give preference to right hand usage. This, of course, is not any specific person’s fault, but rather a consequence of economic development and industrialization. If one was to make a design for, say, a pair of scissors, the inevitable result would be tailored to the right hand. This occurs because of the simple fact that the majority of the population is right handed. Hence, if making something preferenced for one hand to encourage ease of use and likeability, the chosen hand will almost certainly be the right one. There are innumerable examples of such items, including: school desks, power tools, sports equipment, writing tools, kitchen peelers, etc. Additionally, many items have been made to be right hand biased by society. These include watches (which are socially supposed to go on the left wrist, but when worn on a leftie’s left hand, encumber writing) and eating utensils, which polite society has tried to standardize as right handed throughout history. In addition to these obvious problems, there are many medical mixups where a left handed person is given a debilitating IV or injection into that arm, rendering him or her unable to use their preferred limb for extended periods of time (http://www.indiana.edu/~primate/lspeak4.html#medical). All of these factors could potentially cause health problems; for example, a lefty trying to use a right handed chain saw must manipulate his arms so much that they are at a much higher risk of injury. Or, consider the amount of strain put on a lefty’s back when twisting around to write on a right-handed lecture desk. This basic concept led Coren to conduct a study to determine the actual risk of health-influencing accidents that lefties incur in relation to righties. He performed a retrospective study on 1,896 college student in British Columbia, asking the subjects to report having an injury requiring medical attention in the last two years. Provacatively, Coren concluded that relative risk was highest for left-handed males when driving motor vehicles, seeming to corroborate the belief that lefties were more at risk for accidents than righties in the Right Hand World. He proposed that traffic laws, lights, gearshifts, etc. were so biased towards the right handed that they caused this statistical deviation in accident occurrence (Coren, 89). But of course, one single study cannot be the bias for a scientific discovery. After all, the cohort was limited to one age group and a narrow time frame, which does not directly account for increased mortality in the left handed. Additionally, the study was retrospective, meaning it was based on participant memories; is it not then possible lefties have better memories and remembered more of the injuries they had incurred? Although the study reported interesting findings, much more evidence was needed.
Is Coren’s Increased Accident Theory Correct?
Porac and Searleman decided to study an older cohort, because these people would be the ones to have their mortality directly increased by environmental accidents (Porac, Searleman, 2006). Their study analyzed the relationship between hand preference consistency and the prevalence of sleep problems, accidental injuries, falls, injuries from falls, and the prevalence of autoimmune and other illnesses in a sample of 1277 adults between the ages of 65 and 100 years. This study, it seems, would be much more relevant to the question at hand than Cohen’s, especially since it takes into account autoimmune problems that could potentially be caused by stress experienced from living in a Right Hand World. Porac and Searleman, sampling a relatively similar number of subjects (although still too small to be overtly conclusive), found that all comparisons showed no differences between the two hand preference types (although ambidextrous individuals were more likely to have recently had a fall). Clearly this data does not match with Coren’s data from over a decade earlier in terms of supporting the Right Hand World hypothesis. Because of this, it is clear any more such studies attempting to describe handedness and relation to negative environmental influence must include a wider range of age groups and a wider range of possible ills. Perhaps the accumulation of problems from younger years somehow stalls because of less physical activity on the part of the elderly. Regardless, the Right Hand World theory is still unresolved, and will be until greater quantities of more comprehensive studies are conducted and analyzed.
A Biological Perspective on Left Handedness
It’s All in the Genes
Regardless of the validity of the Right Hand World hypothesis, it is still a real possibility that lefties have increased mortality because of a biological bias. Many studies have tried to determine this, and scientists are finally coming closer to an agreement. The core question necessary to address when utilizing this approach is whether or not being left handed is a genetically heritable trait. Medland and colleagues designed a study to find an answer. Their study involved a meta-analysis of 54,270 twins and siblings from 25,273 families, and determined handedness by a writing/drawing assessment or self reporting. By comparing and contrasting the handedness of monozygotic twins, dizygotic twins, and siblings using a complex statistical algorithm, Medland was able to calculate the heritability of handedness. Medland found that, similar to previous meta-analyses, additive genetic effects accounted for 24.64% of the variance with the remainder accounted for by non-shared environmental influences (Medland et al., 2008). This means handedness is indeed a heritable trait and therefore theoretically at the whims of natural selection. Why is this important? Because if being left-handed causes increased biological mortality, shouldn’t the genotype have been selected against, resulting in decreased lefty frequency over time? First, it is important to analyze any studies that have correlated left handedness with increased rates of biological disease.
The Lefty’s Lament
Ramadhani and colleagues studied the association between innate handedness and total mortality, and cause-specific mortality, using 12,178 middle-aged Dutch women who were followed for about thirteen years. The cause-specific mortality categories included total cancer, breast cancer, colorectal cancer, circulatory system, and cerebrovascular issues. Also, such potential covariates as socioeconomic status, age, and BMI were adjusted for. Overall, the study was very thorough, and included a large representation of people, albeit all female. And, importantly, the study found a statistically significant increase in mortality in these women, and an increase in disease frequency (Ramadhani et al., 2007). There are more studies that conclude increased disease and disability in left-handed individuals. One group of biologists claims to have isolated the gene that causes left handedness, the gene LRRTM1. And, this gene is associated with schizophrenia (Francks et al., 2007). Another study examined 270 school-aged children and showed that right handed students statistically outperformed left handed students in terms of language ability (Natsopoulos, Kiosseoglou, Xeromeritou Alevriadoc, 1998). Finally, there has been shown to be a link between left handedness and increased PTSD symptoms; one study randomly sampled 596 individuals, diagnosed 51 of them as having PTSD symptoms, and found there were significantly more left handers among this subgroup (Choudhary, O’Carroll, 2007). Although these studies sampled a small number of individuals and require conformational evidence from more cohesive studies in the future, there appears to be a general trend that being left handed either causes or is a serious factor in numerous health problems. So, back to the evolutionary standpoint, one might expect the frequency of lefties to have been decreasing over time.
Fight Advantage Hypothesis
One particular study directly addresses this question and provides some potential historical insight. Apparently, the frequency of lefties in society has not changed since Neolithic times. This conclusion is based on archaeological evidence that a right-hand bias in tool making or tool use is observable in several Hominine taxa from up to 1.9 million years ago. In this study, Raymond specifically tries to address the persistence of left handedness even when other studies have shown it to be a trait associated with decreased evolutionary survival. First, Raymond surveyed the percentage of lefties found in student athletes and professional athletes. Both had a greater frequency of lefties than the general population in interactive sports; that is, ones where two or more participants either physically contact each other or pass a ball of some kind, as in tennis. Stemming from this contemporary observation and the idea that sporting performance is likely to be a good indicator of fighting abilities, Raymond conducted archaeological literature studies in which it was found that, similar to sports today, lefties have had a greater advantage in hand-to-hand combat throughout history. This is because, since lefties are the minority, right handers cannot not cope as well with them when they are encountered in combat. Following this theory, Raymond concluded that the stability of the left handed phenotype results from a balance between advantage in combat and a disadvantage from increased mortality due to environmental and biological factors (Raymond, 1996). Although this theory is uniquely interesting and theoretically possible, there is realistically no way to conclusively validate the claims simply because of a lack of accurate statistical evolutionary information regarding humanoid history and origins. However, this study does bring up the interesting concept of handedness as an evolutionary phenomenon, which is an idea that could surely benefit from more research
Conclusions and Contradictions
Compare and Contrast
To date, there have been numerous studies that call into question the original claims made by Cohen regarding increased mortality for left handed individuals. Another study focusing exclusively on women collected data about the mean age of death in correlation to handedness, similar to Cohen’s methodology. This study found no difference in the mean age at death of left-handed, right-handed, and ambidextrous women (Cerhan, Folsom, Potter, Prineas, 1994). Along these same lines, a longitudinal study that was conducted with participants at least 50 years old found no correlation between left-handedness and more pronounced age-related cognitive decline (Van der Elst, Van Boxtel, Van Breukelen, Jolles, 2008). Finally, a neurological study explored the health outcomes of patients’ health mortality after sudden cardiac arrest and if this was of any relation to forebrain lateralization, which is equated to handedness. This study concluded that left-handers may actually have a lower risk of sudden death from cardiac arrest than right-handers (Ozdemir, Hachinski, 2008). So what does this all mean?
Difficulties in Data Interpretation
Obviously the question of handedness and mortality is an extremely complicated one with many interacting variables and dynamic theories. There is no doubt there are fewer lefties than righties among humans, but the answer to why is far from being conclusive. There is some evidence that shows increased risk for accidents among lefties, and there certainly are mechanical preferences for right hands by everyday objects, but are accidents and health problems stemming from these devices actually responsible for increasing the mortality of lefties? Or are there other factors, such as different thought patterns in lefties, which might somehow account for increased accidents lefties supposedly experience? Also, there is not consensus that lefties really do incur more illnesses and health problems than righties, as discussed above. In terms of the biological consequences of being left handed, even more conflicting data exist. There are many studies regarding health outcomes related to handedness, but there seems to be just as much data in support of increased left handed diseases and illness as there is against them. Another interesting point is that of sex related to handedness and mortality. It is understood that females live longer than males, but there are statistically many more left handed males than females. Could increased left handed mortality in males contribute to the overall trend of greater female longevity? There are many unanswered questions regarding left handedness and health.
The fundamental problem of all the above studies and lack of consistency is their experimental designs; they only test specific health related outcomes in hopes of finding a relationship between this and handedness. Also, their cohorts are mostly limited to one age group and sex, with most only utilizing a small population base. A better analytical approach would be to design experiments that actually test the psychological stress status of left versus right handers. Because it has been shown that stress causes an increase in a variety of different illnesses, and because lefties supposedly live in the stressful Right Hand World, a study linking both these biological and environmental approaches would be very informative. If indeed lefties incur more stress than righties, it would make sense they would develop more physical and psychological diseases later down the line. This method might be best executed as a meta analysis, and would probably present the strongest argument of all the studies mentioned for or against a link between handedness and mortality.
In the end, there is no thoroughly conclusive evidence that left handedness is a risk factor for increased mortality. After Cohen’s original study sparked a wave of interest in the subject, multiple more recent studies have found no corroborating data to support his claims. However, with the discovery of the left-handed gene, and its relation to schizophrenia, there does seem to be at least some biological significance of being left handed. The effect it has on lefties, though, is arguable, and data is extremely inconsistent. In the future, more cause-focused and biologically specific studies may help unwind some of the mystery between lefties and longevity. In the mean time, it appears lefties are here to stay.
Cerhan, J.R., Folsom, A.R., Potter, J.D., Prineas, R.J. (1994). Handedness and mortality risk in older women. American Journal of Epidemiology, 140(4), 368-74.
Choudhary, J.R., O’Carroll, R.E. (2007). Left hand preference is related to posttraumatic stress disorder. Journal of Traumatic Stress, 20(3), 365-9.
Coren, S. (1989). Left-handedness and accident-related injury risk. American Journal of Public Health, 79, 1040-1041.
Coren, S., Halpern, D.F. (1991). Left-handedness: a marker for decreased survival fitness. Psychological Bulletin, 109(1), 90-106.
Francks, C., Maegawa, S., Lauren, J., Abrahams, B.S., Velayos-Baeza, A. (2007). LRRTM1 on chromosome 2p12 is a maternally suppressed gene that is associated paternally with handedness and schizophrenia. Molecular Psychiatry, 12(12), 1129-39.
Medland, S.E., Duffy, D.L., Wright, M.J., Geffen, G.M., Hay, D.A., Levy, F., et al. (2008). Genetic influences on handedness: Data from 25,732 Australian and Dutch twin families. Neuropsychologia, 9, 1000-1016.
Natsopoulos, D., Kiosseoglou, G., Xeromeritou, A. (1998). Do the hands talk on mind's behalf? Differences in language ability between left- and right-handed children. Brain and Language, 64(2), 182-214.
Ozdemir, O., Hachinski, V. (2008). Brain lateralization and sudden death: its role in the neurogenic heart syndrome. Jounal of the Neurological Sciences, 268(1-2), 6-11.
Porac, C., Searleman, A. (2006). The relationship between hand preference consistency, health, and accidents in a sample of adults over the age of 65 years. Laterality, 11(5), 405-14.
Ramadhani, M.K., Elias, S.G., van Noord, P.A., Grobbee, D.E., Peeters, P.H., Uiterwaal, C.S. (2007). Innate handedness and disease-specific mortality in women. Epidemiology, 18(2), 208-12.
Raymond, M., Pontier, D., Dufour, A.B., Moller, A.P. Frequency-dependent maintenance of left handedness in humans. (1996). The Royal Society, 263(1377), 1627-33.
Van der Elst, W., Van Boxtel, M.P., Van Breukelen, G.J., Jolles, J. (2008). Is left-handedness associated with a more pronounced age-related cognitive decline? Laterality, 13(3), 234-54.
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