Pretty much any Internet page discussing left-handedness facts and fiction contains lots of interesting little anecdotes, such as the history of how left-handed people have been treated or forced to convert, and how the word sinister comes from the latin for left. But a deeper study reveals that there's much more to this unusual trait, and plenty that we still haven't been able to fully explain.
About 8-10% of people are left-handed. That's a moderately interesting little factoid, but beneath it is a glorious patchwork of data and research, showing the variations of that number through history and across the continents and various demographics, and changes in our understanding of what it means. Why are 8-10% of people left-handed? How did they get that way? Does it really mean anything? Has this 8-10% always been a part of the human story?
One interesting way of assessing handedness from prehistoric times is to look at negative hand paintings in Paleolithic caves. The oldest of these come from western Europe, ranging from 10,000 to 35,000 years ago. The artist would hold a reed or straw in one hand, and blow paint at the other hand pressed up against the wall. Among 507 such hand paintings, 79 out of 343 (just about one quarter) are unambiguously of the left hand. This would seem to suggest that about three times as many people were left-handed then than today, but it turns out this is not the case. Research in France in 2002 gave this same task to college students, and found exactly the same distribution. Those same students, when asked to throw a ball, were 9% left-handed; and when asked to write, were 8% left-handed. Almost all who held the blowing tube in their right hand to paint a negative image of their left were normally right-handed, but among those who held it in their left hand, only a third were normally left-handed. This is exactly the same distribution observed among the Paleolithic artists.
Records go back even farther, as much as 400,000 years, when Homo neanderthalensis held meat in his teeth, pulled it tight with his left hand, and gripped a sharpened tool in his right hand to cut it off. Sometimes he slipped, and left marks on his front teeth that showed the direction of his cut.
The 8% number is not at all consistent worldwide. In what some researchers have termed "formal" countries, where handedness is strictly enforced in schools, we find very few left-handers: 3.5% in China, 2.5% in Mexico, 0.7% in Japan. In "informal" countries where little such redirection is given we find the highest numbers: up to a high of 12.8% in Canada. This redirected handedness is called learned handedness, but the more significant factors that we'll discuss include natural handedness (genetic or inherited), and pathological handedness, in which some type of brain injury or other condition produces the left-handedness.
Another reason that 8% is not always the result obtained in studies (a lot of times you'll hear the rounder number of 10%) is that there's a marked difference among both genders and age groups. Universally, males tend to be left-handed slightly more often than females; but what's really intriguing is that left-handers are found less often in older age groups. About 15% of 10-year-old children are left-handed, and this percentage steadily declines as people grow older. By the age of 90, there are virtually no left-handers left in the population. Women tend to live longer than men anyway; but when you add handedness, the difference becomes truly startling. According to a famous article published in 1991 in Psychological Bulletin, right-handed women have a life expectancy of around 77, but left-handed men only live to about 62. Conversely, left-handed women and right-handed men have nearly identical life expectancies, of right around 72. Overall, right-handers live 9 years longer than left-handers!
Why? Theories abound, but proof has been hard to come by. A popular suggestion has been that a lot of left-handers gradually switch over and become right-handed, due to the daily need to deal with the prevalence of right-handed tools, controls, and implements of all types; or possibly due to pressure from parents or teachers to switch for reasons such as ease of writing from left-to-right. However statistical analysis of the data shows that if change of handedness has any effect at all, it's very small. It turns out that the observation is indeed best explained by reduced longevity among left-handers.
The cause of this left-handed mortality is very difficult to find, since virtually no records exist. Death certificates don't include handedness, so there's really no data to analyze. We have theories, like accidents caused by left-handed living in a right-handed world. We do have observational data that supports the accident theory: in all five categories of life-threatening accident types (sports, work, home, tools, and driving), left-handers are from 1.2 to 1.8 times as likely as right-handers to suffer fatal accidents.
Certain health problems are also correlated with left-handedness, which probably also contributes to increased mortality. A 1988 survey found that in 30 of 33 publications, infants who had undergone birth stress were significantly more likely to be left-handed. Lower Apgar scores — a measure of a baby's overall condition at birth — have been clearly associated with left-handedness. A 1987 study found that more than a third of 4-year-olds who had been born prematurely were left-handed. Another found that more than half of children born with extremely low birth weights — a full 54% — were left-handed. In total, left-handers are twice as likely as right-handers to have had a stressful birth. Such births often result in long-term neurological damage. Hypoxia (the lack of oxygen to the brain) may well be one of the culprits. It's also been shown that mothers who smoke during pregnancy, which causes hypoxia to the fetus, are more likely to produce left-handed offspring.
Another very interesting observation is that certain immune deficiencies are more prevalent among left-handers. In some of these cases, elevated testosterone levels in the fetus is known to be the cause. The salient point about testosterone is that it's always found in higher levels among male fetuses, so elevated levels are more dangerous for males than for females. Recall that more males than females are left-handed. Elevated testosterone appears to slow the development of neurons in the left hemisphere of the brain, causing the right hemisphere to develop better, resulting in dominance of the left side of the body.
So the prevailing theory for why left-handers die out of the population is that a combination of factors put them at higher risk of impaired longevity: accidents, neuropathology, immune deficiencies, and other causes.
But this is not at all to say that all left-handers are born with some physical deficiency. No evidence has ever been found showing that all left-handers have anything in common. Heritability is a major factor: left-handed children are more likely to have left-handed parents. A lot of research has been published showing possible genetic markers, but so far there is certainly nothing as specific as a "left-handedness gene".
Moreover, it's certain that no genetic marker will ever be found that indicates left-handedness in every case. One of the reasons we know this for a certainty is something we've learned from twin studies, which are a great way to see genetic traits. If there were a genetic cause for handedness, we would expect identical twins, who are genetic copies of each other, to have concordant handedness, meaning that both are the same, either left-handed or right-handed. We would expect fraternal twins, who are genetically as different from one another as any other siblings, to have the same distribution of handedness as the general population. But, strangely, this is not what we see at all. It turns out that handedness among identical twins, fraternal twins, and the distribution among the general population are all equal. Identical twins are discordant, meaning one is left-handed and the other is right-handed, just as often as are fraternal twins, and as often as all siblings. This strongly suggests that genetics are not the overwhelming driver of handedness. There's no definitive answer as to how identical twins could end up discordant, but one leading theory is that is has to do with physical positioning within the womb. Twin fetuses usually face each other, and we've often observed their limbs adopt mirror-image positions. This behavior may require no more exotic an explanation than a simple economy of physical space available in the womb. Handedness is often predicted by which hand is most often held closest to the fetus' mouth, and this may indeed be the totality of the explanation for why identical twins are so frequently discordant.
The question everyone wants to know about handedness is what special aptitudes left-handers have. Are they more creative? Are they more likely to be successful? The most surprising thing to me that I found in my own literature survey is that there seems to be weak evidence for just about any aptitude you can think of, except the most commonly believed ones. It seems clear that there is no good evidence that left-handers are especially creative or that they're less talented at analytical functions, both of which would seem to indicate right brain dominance. But tasks are distributed over both halves of the brain, and even though a left-hander may rely on his right brain to control his left hand, aptitudes and mental tasks appear not to rely so much on that same type of hemisphere dominance.
Instead, much published research has examined just about every other aptitude. College educated left-handers seem to earn more money. They are more likely to be homosexual or to have gender identity problems. They tend to be better at geometry and spatial analysis. They are able to think outside the box better. The list goes on and on and on, but I found that nearly all such conclusions were based on a small number of small studies, many of which were contradicted by results published by others. So my conclusion, based on my own survey, is that there are not really any widely accepted aptitudes possessed by left-handers. (I welcome any corrections to this finding.)
Sports and combat are a different matter. Most athletes and soldiers through history have been accustomed to going up against right-handed opponents, and a left-hander may bring his sword in or hit a ball from an unexpected angle. Today's top athletes do indeed have a larger distribution of left-handedness, especially in sports like baseball. But the statistics are clear that this is not due to any special talent above that of right-handers, but merely due to right-handers' greater physical difficulty in dealing with left-handed opponents.
So everybody hug a left-hander today. They don't have any special superpowers, but they are indeed at risk. It's the overriding issue of early mortality that is the focus of most of today's research, and rightfully so. And if you're in manufacturing, consider the importance of left-handed tools and controls, they're not just for convenience.
Cite this article:
Dunning, B. "Left Handed Myths and Facts." Skeptoid Podcast. Skeptoid Media,
15 May 2012. Web.
13 Feb 2016. <http://skeptoid.com/episodes/4310>
References & Further Reading
Coren, S., Halpern, D. "Left Handedness: A Marker for Decreased Survival Fitness." Psychological Bulletin. 1 Jan. 1991, 109: 90-106.
Dellatolas, G., Tubert, P., Castresana, A., Mesbah, M., Giallonardo, T., Lazaratou, H., Lellouch, J. "Age and Cohort Effects in Adult Handedness." Neuropsychologia. 1 Jun. 1991, Volume 29, Issue 3: 255-261.
Hirsch, S. "Law and Mental Disorder." Lancet. 7 Apr. 1979, Volume 313, Issue 8119: 759–761.
Lord, T. "A Look at Hand Preference in Homo Sapiens." The American Biology Teacher. 1 Nov. 1986, Volume 48, Number 8: 460-464.
Medland, S., Perelle, I., De Monte, V., Ehrman, L. "Effects of Culture, Sex, and Age on the Distribution of Handedness: An Evaluation of the Sensitivity of Three Measures of Handedness." Laterality. 1 Jul. 2004, Volume 9, Number 3: 287-297.
O'Callaghan, M., Tudehope, D., Dugdale, A, Mohay, H., Burns, Y., Cook, F. "Handedness in Children with Birthweights Below 1000 g." Lancet. 16 May 1987, Volume 1, Number 8542: 1155.
Ross, G., Lipper, E., Auld, P. "Hand Preference of Four-Year-Old Children: Its Relationship to Premature Birth and Neurodevelopmental Outcome." Developmental Medicine and Child Neurology. 1 Oct. 1987, Volume 29, Number 5: 615-622.
Searleman, A., Porac, C., Coren, S. "Relationship Between Birth Order, Birth Stress, and Lateral Preferences." Psychological Bulletin. 1 May 1989, Volume 105, Number 3: 397.
Van Agtmael, T., Forrest, S., Williamson, R. "Genes for Left-Handedness: How to Search for the Needle in the Haystack?" Laterality. 1 Jan. 2001, Volume 6, Issue 2: 149-164.
Waldfogel, J. "Sinister and Rich: The Evidence that Lefties Earn More." Slate. The Slate Group, 16 Aug. 2006. Web. 12 May. 2012. <http://www.slate.com/articles/business/the_dismal_science/2006/08/sinister_and_rich.html>