Last week I was failing miserably at darts, a game that transforms an unfortunate limb into a repetitive launching machine that advertises human inaccuracy and weakness. Somewhere after 11pm, long past my middle-aged bedtime, I gave up and started throwing with my left hand. Between gasps, the points started racking up. By the end of the night, I was considering a new career.
Maybe that’s a stretch, but the issue of handedness has intrigued me since I worked on left-handed cursors in the Stone Age. While I’m not a true lefty, I am goofy-footed with a lefty son who gets puzzled by our asymmetrical world. How is handedness determined? Does it link to health patterns in the brain or body? Why haven’t we evolved to eliminate left-handedness? Similarly, why haven’t animals (except parrots and kangaroos) evolved to express handedness? Let’s explore the roots of right, left, and the odd space in between.
Believing in right or left
In Latin, the word for “right” is “dexter,” and the English derivative “dexterous” means skillful. The Latin word for left is “sinistra,” the precursor to “sinister” which means malicious or evil. If we examine superstitions over cultures and centuries, we see that the left side has taken the brunt of our fear and mistrust. Myths surrounding the ominous left have ranged from throwing salt to ward off evil spirits to bird flights of bad luck. The political meaning of left is traced back to King Louis XVI’s radical opposition who sat on his left. Chinese references to left imply impropriety, while the Swedish term for left originates from a term describing adultery. The Biblical references to strength and preference for the right hand are notably abundant. Most significantly, many cultures in Africa, Middle East, and Asia have delegated the left hand for unclean hygienic patterns alongside religious beliefs in a more honorable right hand.
Perhaps the etymology and myths were exaggerating subconscious wariness toward a persistent minority population. In fact, left-handedness and mixed handedness scientifically link to certain brain and body conditions. There are actually four types of handedness: left-handedness, right-handedness, mixed-handedness (cross-dominance), and ambidexterity. Around 90% of the population is right-handed and 10% left-handed, with men holding a slightly larger percentage of likelihood than women. About 30% of the population is capable of changing hand preference between tasks (mixed-handedness), and true ambidexterity is extremely rare. Left-handed figures have trended upward as younger generations have not been pressured to conform as much as older ones.
Our most contemporary lopsided myth is the concept of right-brained and left-brained individuals. One of the largest, most impressive brain studies to date was unable to find associations between activity in specific brain regions and handedness. The paper explains that we should reconsider our perspective:
…there will be different forms of left-handedness which may manifest differently…
– Front Psych, 2014
As another study pointed out, the only patterns in brain activity appear in cerebral lobes, not hemispheres. While it doesn’t mean there aren’t personality types based on brain activity, the underlying mechanisms are not as simple as brain hemispheres.
Hormones go hand in hand
Handedness develops in the womb, and we start to recognize it by toddlerhood. We inherited much of our handedness, but studies of the prenatal environment are revealing other controllable factors such as estrogen and testosterone exposure. As many as 29% of female left-handers may owe their predisposition to a high risk pregnancy and older maternal age. Left-handedness shows an alarming correlation with prenatal synthetic estrogen exposure. A doubled risk of breast cancer in premenopausal left-handers may also reflect this exposure. Estrogen is believed to elevate autoantibodies and contribute to autoimmune diseases – chronic conditions that are more common in left-handers and their immediate families.
Higher testosterone levels influence the microbiome in order to protect mice from autoimmunity, The male fetus appears more sensitive to low testosterone which can fluctuate with seasons and sun exposure. This phenomenon, seasonal anisotropy, is proposed to explain higher rates of left-handedness in men born during winter months. While vitamin D is positively associated with testosterone levels and reproductive functions, there is no research that examines prenatal vitamin D levels and handedness.
Since prenatal hormone exposure also influences homosexuality, it is no surprise that homosexual subjects show much greater odds of being left-handed. Specifically, homosexual men had 82% greater odds than heterosexual men, and homosexual women had 22% greater odds than heterosexual women. Interestingly, there is evidence for some gay men to express extreme right-handedness. Keep in mind, there is strong scientific support for epigenetic (environment and genetic) factors in homosexuality.
Results from Casey and Nuttall (1990)…found that non-right-handed women and right-handed women with non-right-handed relatives rated themselves as less feminine and had more masculine sex role identification than right-handed women with right-handed relatives.
–Psych Bull, 2000
Further investigation hints that hormones intermingle with stress and environment to shape handedness during pregnancy. Maternal smoking during pregnancy was associated with non-right-handedness in offspring. Even maternal anxiety during pregnancy (18 weeks) was associated with mixed-handedness in the child.
Left with links to disease
Unfortunately, left-handedness has been linked to more negative health conditions than positive ones. Left-handedness has been associated with alcoholism in women and risk of fractures in men, and another study found left-handed children with 70% increased odds of a psychiatric diagnosis. Yet there is consistent need for clarification. For example, left-handedness is more prevalent in autism, but new research suggests that these children show more variable handedness, possibly explaining their developmental difficulties with motor skills. It appears that autistic individuals may suffer from some type of asymmetry that affects certain regions of the brain and presents itself as variable handedness. One study describes that the volume of the left hippocampus is larger in both autistic children and their parents. Interestingly, young adult left-handers show greater volume in the left hippocampus.
Similarly, variable or mixed handedness – is associated with greater age-related decline. Other neurodevelopmental disorders like dyslexia and schizophrenia are also strongly associated with non-right-handedness, sharing genes and possible environmental factors. Weakness of handedness in both right and left-handers was associated with higher risk of depression. Not surprisingly, inconsistent handedness was associated with anxiety as well, but only in left-handers. Considering the range of findings on left or variable handedness, future studies should always consider degree of handedness when evaluating conditions.
On the bright side
For left-handers and other non-right-handers, there is some good news. Left-handedness has been widely associated with creative thinking, and new research clarifies that creativity and magical thinking were negatively correlated with degree of hand preference. While magical thinking presents many flaws in science – including the problem of hastily rushing to conclusions without sufficient evidence – it also correlates with an enhanced ability to update one’s beliefs. Mixed-handers show this type of conceptual flexibility, less likely than consistent-handers to side with the status quo when given suitable alternatives. This behavior is a result of random cerebral variation according to expert researcher Chris McManus, where left-handers show a range of activation patterns and spread of neural activity.
One economic study found that left-handed males earn 4% more than right-handed counterparts, but left-handed females earn 8% less than right-handed counterparts. Furthermore, left-handed earnings are better in manual labor fields for both genders, defying predictions that lefties might struggle in physical occupations. Indeed, a recent test describes left-handers as more capable of training their nondominant hand than right-handers.
The left-handed protection for certain physical conditions is even more interesting. A large study shows that left-handedness correlates with lower risk of ulcers and arthritis. Another paper discovers left-handers with reduced risk of developing herpes zoster, the virus that causes chicken pox and shingles. One investigation shows left-handedness much lower in COPD and pneumonia and somewhat higher in asthma. Furthermore, the risk of brain tumors is reduced in patients described as left-handed or ambidextrous. It is notable that brain tumors are less likely in people affected by allergies or autoimmune diseases, including asthma and diabetes. These findings illuminate possible connections between the origins of handedness and autoimmunity, particularly as they may relate to cancer.
An emerging theory of protection
It is speculated that the less lateralized (and more broadly distributed) brain activity in lefties may help protect them from brain aging later in life. There is no consensus on the genetic underpinnings, particularly certain APOE genotypes (ε2) that may favor both left-handedness and reduced incidence of Alzheimer’s disease. We know that certain APOE genotypes (ε4) that might affliate with right-handedness show increased risk of Alzheimer’s disease. In any case, there are proportionally fewer left-handed patients with Alzheimer’s disease, and subjects with the condition show slower rate of decline than their right-handed counterparts. Researchers postulate:
“Perhaps the more widely distributed nature of some left-handed patients’ cognitive abilities places them at greater risk for manifesting deficits in the first place, but helps to mitigate the subsequent rate of deterioration of these abilities.”
– Arch Neurol., 1999
The right chicken or the left egg?
As with many other issues in science, it remains unclear if genetics and environment first influence the asymmetry of handedness and brain development, or if the asymmetry of handedness influences genetic expression and brain development.
…”there is strong evidence indicating that motor asymmetry of the arms and hands is initiated very early during human embryonic development, possibly even before the cerebral cortex exerts significant influence (Hepper, 2013).
– Front Psych, 2014
We have only just begun to examine how neurological conditions are programmed in the womb at a cellular level. Calcium signaling, or spontaneous calcium ion activity, drives early brain development. Exposure to sex hormones impacts this delicate calcium activity in the first week of life, influencing brain development and an enormous range of physical and psychological behaviors later in life. In fact, problems with calcium signaling are largely responsible for the development of Alzheimer’s disease, bipolar disorder, and schizophrenia.
Hunting for handedness
The history and geography of left-handedness suggest important environmental and genetic influences that are rarely discussed. An impressive paper by McManus explores patterns in the United States and Europe.
Except for Florida and Nevada, states with higher rates of self-reported left-handers generally sit farther north. Maine, Connecticut, Delaware, and Vermont show left-handers at over 13%. Gathered data hints at a slightly similar trend in Europe, with 12-13% rates in Britain, the Netherlands, and Belgium. This might support a role for vitamin D through its effect on prenatal testosterone levels, particularly as vitamin D deficiency associates with an increased risk of autism, schizophrenia, and depression – conditions affiliated with left-handers.
Genetics, ethnicity, and even immigration may also affect these curious statistics. Clearly, the northern territory of Finland does not follow the Northern European pattern, but the lower rates of neighboring Russia and related immigration may have influenced values over time. As McManus describes, a survey of German soldiers in the early 1900s showed that lefties were more populous in southern regions nearest Switzerland and Austria with higher rates of left-handedness. And when looking at US ethnic distribution, New England ancestry shows more French, English, and Irish populations – with natively high lefties – than almost any other region of the country. New England also has a striking absence of Mexican immigrants who, with lower native rates of left-handedness, would theoretically lower their averages. A comparison of immigration dates, ancestry, and degree and direction of handedness would be highly useful data, especially considering that schizophrenia is more prevalent in countries with high immigration rates – particularly recent immigration.
Additional evidence for ethnic or cultural forces is shown in the above chart. Hispanics and Asians have lower rates of left-handedness in their native countries (10.5% and 5-6%, respectively) and show lower rates of change in left-handedness over time. We cannot untangle the strong and invisible effect of cultural pressure that discourages left-handedness around the globe. Stigmatization may be even more pronounced until generations recognize and adjust to more tolerant cultures following migration.
The right, left, and stigma
When humans don’t understand the nature behind our biological inconsistencies, we are capable of constructing and spreading harmful, contagious beliefs.
“…for two-thirds of the world’s population, being born left-handed exposes one to discrimination and stigma.”
– Endeavor, 2013
Furthermore, repression of left-handedness leads to a range of psychological and social challenges. We are lacking information on these invisible victims in the world, many who have been reshaped from early ages. Studies of brain blood flow reveals that converted left-handers – even after writing right-handed for decades – continue to show left-handed brain activity. In fact, scientists are predicting that forced switching to right-handedness may even strengthen the left-handed sensorimotor areas of the brain. It is also impossible to study left-handedness outside of a right-handed world, and we desperately need a grasp on the long-term effects of left-handed adaptation to devices and tools.
What might be more disturbing is our tendency to prefer one of two perceived sides, even when they are not proven to be cut so clear. Even scientists have typically omitted left-handers in studies to reduce variance, preventing us from understanding more about asymmetry and brain development. As we move toward a more accurate understanding and definition of handedness, we can start to see and support the brain and function of the nondominant arm. Perhaps we can evaluate changes in our degree of handedness to predict and prevent psychological disturbances or even health conditions. Most importantly, we can move away from arcaic beliefs of left-oriented inferiority or negativity and embrace how our physical differences reflect necessary diversity in humanity.
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