A study of 13,536 individuals who participated in three national surveys conducted in the United States, revealed that those with a slender lower face are about 25% more likely to be left-handed.
This association may shed new light on the origins of left-handedness, as slender jaws have also been associated with susceptibility to tuberculosis, a disease that has shaped human evolution and which today affects 2 billion people.
The finding was published April 26 in the journal Laterality: Asymmetries of Body, Brain and Cognition. The author, Philippe Hujoel, is a professor at the University of Washington School of Dentistry and an adjunct professor of epidemiology at its School of Public Health.
"Almost 2,000 years ago a Greek physician was first to identify slender jaws as a marker for TB susceptibility, and he turned out to be right!" Hujoel said. "Twentieth-century studies confirmed his clinical observations, as slender facial features became recognized as one aspect of a slender physique of a TB-susceptible person. The low body weight of this slender physique is still today recognized by the Centers for Disease Control and Prevention as a marker for TB susceptibility."
He said the finding raises the hypothesis that the genetics that shape facial features and tuberculosis susceptibility also increase the likelihood for left-handedness.
Such a hypothesis may explain curious geographical coincidences. For example, the United Kingdom was described as the tuberculosis capital of Western Europe, and has a high prevalence of left-handedness and people with slender faces. Other populations, such as the Eskimos, were in the 19th century described as tuberculosis-resistant, having robust facial features, and typically depicted in art as showing right-hand dominance with tools and instruments.
Whether this is more than a coincidence needs further exploration, Hujoel said.
In the early 20th century, slender individuals were described as "ectomorphs" - a term that persists in popular culture as a reference to dieting and bodybuilding, Hujoel noted.
"In a world dominated by an obesity crisis and right-handers, ectomorphs can be different in their desires," he said. "Popular websites suggest they commonly express a desire to gain weight or muscle mass. Their slightly increased chance of being a 'leftie' is an additional feature that makes them different."