Left-Hand Man

Recently, I watched The King's Speech, the Academy Award-winning biopic starring Colin Firth and Geoffrey Rush. While I sat down to the film as much out of a vague interest in the subject matter as a feeling that I ought to after all its Oscar buzz, I got up two and some hours later with newfound curiosity in the link between stuttering and left-handedness. A neurobiologist at the core, I found myself positing the popularly contested question: Are handedness and language development linked? If so, does the prevalence of stuttering among lefties connote a common psychological or anatomical basis?

So from the outset, the debate is one complicated by a lack of standard, accepted definitions. Without such a framework, it is difficult to neatly categorize all the existing scientific literature. In spite of this, I have narrowed down the available information to a few bare facts.

• The ratio of lefties to righties has remained constant throughout history at 1:9.
• About 70% to 95% of people have left-hemisphere language specialization.
• About 1% of the human population stutters, more males than females.
• Many studies have both empirically and observationally demonstrated a link between left-handedness and stuttering.³

That's not too much to go on. And what there is to go on is convoluted, confusing, and hotly-debated. So what's a study that could potentially advance us toward a more definitive solution? Barring more invasive (and therefore more ethically questionable tests) I propose taking a large simple random sample of eighteen-month-old babies. (Handedness usually develops between eighteen and thirty-six months.⁴) By conducting a long-term observational study of these babies, charting both handedness and language development throughout the months, it should be possible to monitor what kinds of correlations exist between the progression of hand preference and the onset of stuttering. Moreover, by enlisting a large sample of babies and observing them throughout their childhood, it would additionally be possible to take psychological effects into consideration; that is, researchers could ensure parents and/or teachers aren't confounding the results by forcing the subjects to adopt a non-dominant hand and thereby potentially triggering a psychological state in which stuttering is a symptom.

Hypothetically speaking, according to causal hypotheses that state stuttering is a consequence of forced handedness, as fewer and fewer parents coerce their children into adopting a non-dominant hand as dominant, the percentage of stutterers should decrease. Yet no such decrease has been observed. This simple piece of evidence casts doubt on a common psychological basis for lefties and stutterers. Much more likely, in my opinion, is a shared neuroanatomical feature driving abnormal brain lateralization that occasionally, though not always, causes both left-handedness (or non-right-handedness) and stuttering. Clues confirming this possibility scattered themselves like bread crumbs throughout my research. For one, the brain hemispheres exhibit oppositional control (i.e., the left-hemisphere controls the right side of the body and vice versa). As both speech and handedness require fine motor control, it is possible that the human brain evolved the two together to avoid redundancy; thus, as language centers are generally found in the left-hemisphere, most people are righties. Yet about 25% of lefties process speech across the two hemispheres, and as I earlier described, not all humans have left-hemisphere language specialization. Take this a step further and it becomes feasible that for about 10% of the human population, fine motor control is somewhat developmentally "shifted" or "misplaced" from its usual position, resulting in left-handedness and, in extreme cases, stuttering.

Back in the day, the right-hand men of great leaders were also oftentimes the second-in-command: military or political assistants who provided invaluable services to the king or emperor. They were known as "right-hand men" because they were seen as symbolic of the leader's might, his sword hand, and his iron fist. Yet history is littered with a higher-than-proportional number of left-handed rulers. Alexander the Great, Joan of Arc, and Napoleon from days of old; Queen Victoria, Fidel Castro, and Barack Obama from days of new. In fact, 8 of the 44 US presidents have been left-handed, approximately 8.2% more than 10%; moreover, several psychologists have promoted the idea that lefties have personality traits particularly well-suited for political work, such as pushiness, dominance, and shrewdness.⁵ Given this odd statistic, it seems that lefties have more to say to the world than Morse code speech; whatever physiological differences are granted to them also seem to imbue at least some of them with a greater ability to lead. All that remains is for neurobiologists to elucidate exactly what hand brain lateralization has in the development of a panoply of characteristics, from handedness and language to leadership skills. In the meanwhile, maybe it's time to rethink the age-old term "right-hand man".

Image Credit: *_Abhi_* (via Flickr)

References:

Jones, R. K. (1966). Observations on stammering after localized cerebral injury. Journal of Neurology, Neurosurgery, and Psychiatry, 29, 192-195 PMID: 5937632

1. Barnsley, R. H., & Rabinovitch, M. S. (1970). Handedness: proficiency versus stated preference. Perceptual and Motor Skills, 30, 343-362 PMID: 5454044

2. Ross, E. (1984). Right hemisphere's role in language, affective behavior, and emotion Trends in Neurosciences, 7, 342-346 DOI: 10.1016/S0166-2236(84)80085-5

3. Bryngelson, B., & Clark, T. (1933). Left-handedness and stuttering Journal of Heredity, 24, 387-390

4. Santrock, J. W. A Topical Approach to Life-Span Development. Motor, Sensory, and Perceptual Development. 5th ed. Boston, MA: McGraw-Hill Higher Education, 2008: 172-205.

5. Niedowski, E. "The Curious Creation of Left-handed Leaders." The National. July 28, 2008.