People who have a slender lower face are about 25 per cent more likely to be left-handed, a new study has found.
The finding may shed new light on the origins of left- handedness, as slender jaws have also been associated with susceptibility to tuberculosis (TB), researchers said.
Slender jaws are a common facial feature, affecting about one in five US adolescents. Past surveys measured the prevalence of this condition by evaluating how the upper and lower teeth come together.
People with slender jaws typically have a lower jaw which bites a bit backward, giving them a convex facial profile and what is commonly called an overbite.
"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!" said Philippe Hujoel, professor at the University of Washington.
"Twentieth-century studies confirmed his clinical observations, as slender facial features became recognised as one aspect of a slender physique of a TB-susceptible person," said Hujoel.
"The low body weight of this slender physique is still today recognised by the Centres for Disease Control and Prevention as a marker for TB susceptibility," he said.
The association may explain curious geographical coincidences, researchers said.
For example, the UK was described as the tuberculosis capital of Western Europe, and has a high prevalence of left- handedness and people with slender faces, they said.
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 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," he added.
The study was published in the journal Laterality: Asymmetries of Body, Brain and Cognition.