What Makes Some of Us Left-Handed? Hint: Nobody Knows

All human beings contain a frightening tangle of primal impulses struggling for dominance, and that’s true not only for the chaotic psyche, but also the sober, dependable, symmetric old hands. Even the ordinary act of reaching for a fork or throwing a ball is the product of immensely complex genetic and neurologic negotiation.

BOOK REVIEW “On the Other Hand: Left Hand, Right Brain, Mental Disorder, and History,” by Howard I. Kushner (Johns Hopkins, 192 pages).

For all its sophistication, modern science is still unable to explain exactly why some of us prefer to do these tasks with the left hand and some with the right. Many theories are proposed and debated; studies yield data invariably suggestive but never conclusive. Moreover, as Howard Kushner systematically outlines in his short but meaty survey of the science and sociology of handedness, in some ways we are not that much further along than we were back in the days when our ancestors linked left-handedness to the sinister and the gauche, among many other undesirable traits, and just left it at that.

An emeritus professor at Emory University and San Diego State, as well as a visiting scholar at the University of California – San Diego, Kushner brings academic credentials in both neuroscience and the history of science to his overview. In addition, he himself is left-handed, as was his mother (who like many left-handed children of her era was forcibly retrained to use her right), and the residua of personal experience echo through the book.

Cultures all over the world have found left-handedness an easy way to distinguish the aberrant from the putatively normal, metaphor made conveniently visible. As a late-19th-century description of Maori worldviews put it, “a left hand that is too gifted and agile is a sign of a nature contrary to the right order, or a perverse or devilish disposition: Every left-handed person is a possible sorcerer, justly to be distrusted.”

Such conclusions were pervasive around the world, in developing and developed cultures alike, right up to the pronouncement of an American psychiatrist in 1946 that “sinistrality,” much like “contrariety in feeding and elimination,” was “nothing more than an expression of infantile negativism.” A few thinkers, Plato and Lao Tse among them, spoke up for left-handers, but such broadmindedness was uncommon: Not for nothing did Islamic scholars specify that both Allah’s hands are right hands.

To retrain a recalcitrant left-handed child at the turn of the 20th century, the Zulus immersed the hand in boiling water. Decades later, mainland China was re-educating its young so efficiently that by the 1980s, barely any left-handed Chinese adults could be found. The stammering King George VI of England was forced to eat and write with his right hand, but was allowed to play tennis with his left. Kushner’s mother was forced by Philadelphia’s public schools in the 1930s to both write and sew with her right hand. By the time her son came along, such re-education had become obsolete, at least in the enlightened New York public schools of the 1950s.

In many ways, Kushner suggests, the history of left-handedness parallels that of many disabilities. Damage from discrimination can be greater than any damage from the condition itself, while tolerance “serves as a barometer of wider cultural toleration and permissiveness.”

Meanwhile, explaining the science of handedness has long been stymied by definitions and methodology. Acceptable answers to the big questions require clear answers to the little ones, many of which remain perversely unanswerable.

How many left-handers are there? That depends on how you define handedness. (Is it the hand you write with? The one you sew or play tennis with?) When should the diagnosis be made — in utero, by observing which thumb the fetus sucks? Or after socialization by mother, teacher, peers? What about the ambidextrous, who may not care much which hand they use? Are they de facto left-handers, or just “non-right-handers”?

With these caveats, an estimated 10 percent of most human populations since the Stone Age have been considered left-handed, although the numbers in specific surveys have ranged from 0.06 percent to 25 percent, with the highest rates in the United States and Europe, and the lowest in China and Africa. Males always predominate.

Which side of the brain controls speech and language in left-handers? That’s a trick question: Usually, as in most (but not all) right-handers, the left side dominates. A study using functional brain imaging found that 18 percent of left-handers and 5 percent of right-handers were right-brained for language — although experts now caution that assuming one hemisphere controls all language tasks is overly simplistic.

Given this caution, does forcing a lefty to write like a righty really lead to language problems — stuttering, dyslexia, and the like — as has been widely assumed? The question is impossible to answer with today’s gold-standard randomized, controlled studies, and the voluminous observational data from the past is inconclusive. Even so, some case series have convincingly described stutterers who were cured when allowed to use their left hands.

The biological root cause of left-handedness remains unidentified. One theory popular in the 1980s posited that “stress” during pregnancy exposed the fetus to excessive testosterone, which then thwarted standard left-brain development and right-hand dominance. Most newer theories have struggled to incorporate the clear genetic components of handedness into a viable scheme. Some posit a gene with two alleles, one favoring right-handedness and the other noncommittal for handedness. Others doubt a single gene is involved. Do the “mirror neurons” of “monkey see, monkey do” behavior contribute to a comprehensive theory of handedness and language arising from gesture? Research has suggested that a gene that plays a crucial role in establishing body asymmetry in animals plays a role in human handedness as well.

Somewhat to Kushner’s chagrin, modern researchers are avidly pursuing associations between left-handedness and autism, homosexuality, and schizophrenia, with compelling theories but, in Kushner’s assessment, weak supporting data. Once again, he notes, the left hand is serving as a visible metaphor for behavioral difference as it did back in what we like to consider less enlightened times. This scientific landscape in perpetual, cyclical flux is well described by Kushner’s engaging, accessible panorama.

Abigail Zuger is a physician in New York City and a longtime contributor to the science section of The New York Times.