People who have slender lower face have higher odds of being left-handed. The same physical trait of having slender jaws has also been linked to susceptibility to tuberculosis.
Slender Jaw, Susceptibility To Tuberculosis And Left-Handedness
In the new study published in the journal Laterality: Asymmetries of Body, Brain and Cognition, Philippe Hujoel, from University of Washington School of Dentistry, used data from three national health surveys in the United States that involve more than 13,500 people.
He found that those with convex profile marked by narrow face and a slender jaw have 25 percent higher chances of being left-handed.
About one in five adolescents in the United States have slender jaw. Surveys have measured how common this condition is by looking at how a person's upper and lower teeth come together. Individuals with slender jaws tend to have a lower jaw that bites a bit backward, which gives a person the convex facial profile commonly called an overbite.
Hujoel said that the findings raise the idea that the genetics behind facial features and tuberculosis susceptibility increase odds for left-handedness.
"Bilateral retrognathism-a marker for a lower face phenotype characterized by a convex facial profile and slender jaws-was associated with a 25% increased odds for non-right-handedness," the researcher wrote in this study.
"This association between non-right-handedness and a convex facial profile may unexpectedly find its origin in the genetic polymorphisms which determine tuberculosis susceptibility."
Left-Handed And Right-Handed Populations
Certain populations appear to support this hypothesis. The United Kingdom, for instance, was described as Western Europe's tuberculosis capital. Left-handedness is prevalent here and the people are characterized by slender faces.
In the 19th century, the Eskimos, who are marked by robust facial features and typically depicted in art as having dominantly right-handed when handling tools and instruments, were described as tuberculosis-resistant.
Low body weight associated with slender physique is also recognized by the U.S Centers for Disease Control and Prevention as an indicator for TB susceptibility. As early as two millennia ago, a physician in Greece has also identified the link between slender jaw and TB susceptibility.
Left-Handedness In People With Psychotic Disorders
Besides susceptibility to tuberculosis, earlier research has also linked left-handedness to psychotic disorders. In a study conducted by Yale researchers, they found that 40 percent of people with schizophrenia or schizoaffective disorder are left-handed, a considerably high rate given that only 10 percent of the population is left-handed.
"In general, people with psychosis are those who have lost touch with reality in some way, through hallucinations, delusions, or false beliefs, and it is notable that this symptom constellation seems to correlate with being left-handed," said study researcher Jadon Webb. "Finding biomarkers such as this can hopefully enable us to identify and differentiate mental disorders earlier, and perhaps one day tailor treatment in more effective ways."
Left-Handed Presidents Of United States
Interestingly, a high percentage of recent presidents of the United states are left handed. Four of the last seven of America's commanders in chief, for instance, are left-handed and these are Barack Obama, Bill Clinton, George H.W Bush and Gerald Ford.