What Does Left Hand Know That Right Hand Doesn't?
Joan Arehart-Treichel
Many accomplished people have been, or are, left-handed. But left-handedness also has another side, psychiatric research suggests.
On January 20, Barack Obama was sworn in as president of the United States. It was a triumphant day not just for Democrats, African Americans, and Americans who wanted a leadership change, but for left-handed Americans. The reason? Only 10 percent of the American population is left-handed, and Obama is one of this minority group.
In fact, four of the past six presidents—Gerald Ford, Ronald Reagan, George H.W. Bush, and Bill Clinton—were also left-handed, demonstrating that in spite of the age-old prejudice and discrimination against left-handed individuals, "lefties may have the right cerebral stuff." Moreover, many other famous people have been, or are, left-handed—the Roman general Julius Caesar, the Renaissance painter Michelangelo, American novelist Mark Twain, Beatle Paul McCartney, television mogul Oprah Winfrey, and Supreme Court Justice Ruth Bader Ginsberg.
Some scientific studies also attest to the positive mental aspects of being left-handed or ambidextrous.
Two Pakistani researchers explored the effect of handedness on the intelligence level of students. The sample included an equal number of left-handed and right-handed students drawn from various universities in Pakistan, altogether 150 subjects. Subjects were assessed for both handedness and intelligence. The researchers found no significant difference in intelligence between subjects from various educational levels, but they did find that left-handed subjects were significantly more intelligent than right-handed subjects. Results were published in the January 2007 Journal of the Indian Academy of Applied Psychology.
In a study of 250 healthy undergraduate students, ambidextrous individuals were found to engage in more magical ideation than either left-handed or right-handed persons were. Such ideation, the researchers proposed, may reflect heightened creativity. Study results appeared in the January 2002 Laterality: Asymmetries of Body, Brain, and Cognition.
Then Johns Hopkins economists determined, in a nationally representative sample of 5,000 men and women, that left-handed college-educated men earned 15 percent more than right-handed college-educated men did, even after possibly confounding factors such as age, race, IQ, level of education, and marital status were considered.
"Our findings are quite contrary to expectations," they acknowledged in their study report, which was published by the National Bureau of Economic Research in 2006. However, they did not find the same wage differential for women in the sample.
Nonetheless, some studies reported in the psychiatric research literature suggest that being left-handed or ambidextrous has another side—that left-handed or ambidextrous persons are more likely to have certain mental disorders than right-handed persons are. Moreover, the number of these studies, as well as their quality, intimate that the links they have made between left-handedness or ambidexterity and mental disorders are not simply related to chance.
During the 1970s and 1980s, for instance, some studies noted what appeared to be an unusually large percentage of left-handed or ambidextrous people among those with autism, dyslexia, stuttering, or neurodevelopmental disorders. Much more recently, DSM-IV developmental coordinational disorder (DCD), which can impair both large and fine motor skills and make everyday activities such as getting dressed, writing by hand, and participating in sports difficult, has also been coupled with left-handedness. In the February 2008 Journal of Child Neurology, two Israeli scientists reported that out of 98 children with DCD, 31 percent were left-handed, and 13 percent were ambidextrous. In the October 2008 Canadian Journal of Psychiatry, Canadian researchers reported that out of 19 children with DCD, 37 percent were left-handed.
An unusually large number of persons with schizophrenia also appear to be left-handed. In one study, some 400 subjects with schizophrenia, major depression with psychosis, bipolar psychosis, or no psychiatric illness were evaluated on handedness. The schizophrenia subjects were left-handed significantly more often than the other groups were, the researchers reported in the May 1994 Journal of Abnormal Psychology.
Tyrone Cannon, Ph.D., a professor of psychiatry at the University of California at Los Angeles, and coworkers have since discovered, while comparing the childhood neurocognitive test results of individuals with schizophrenia with those of their siblings or controls—altogether 258 subjects—that 32 percent of those with schizophrenia were left-handed, compared with only 12 percent of their siblings and only 9 percent of the controls. These results were published in the November 2003 American Journal of Psychiatry.
Further, Clyde Francks, Ph.D., of the University of Oxford in England, and his team reported something intriguing in the July 31, 2007, Molecular Psychiatry—that they had found a gene that increases the odds of being left-handed. The gene, dubbed LRRTMI, appears to be the first gene discovered that has an effect on handedness. They also found that the gene might slightly increase the risk of developing schizophrenia.
PTSD: A View From the Left
Posttraumatic stress disorder (PTSD), too, has been coupled with left-handedness. Two Scottish researchers—Carolyn Choudhary, Ph.D., and Ronan O'Carroll, Ph.D., of the University of Stirling—screened a general population sample of some 600 people for handedness and PTSD. They found that 11 percent of the sample was left-handed, that 9 percent of the sample met all DSM-IV criteria for PTSD, and that significantly more left-handers than right-handers comprised the PTSD group. Results were reported in the June 2007 Journal of Traumatic Stress.
PTSD has likewise been associated with ambidexterity. A study of some 2,500 U.S. Army veterans found that veterans who were extremely ambidextrous (about 3 percent of the study sample) were twice as likely to have developed PTSD after combat as were veterans who were not extremely ambidextrous. Veterans who were extremely ambidextrous and who experienced especially high combat exposure were nearly five times as likely to have developed PTSD after combat as were those who were not extremely ambidextrous and who experienced especially high combat exposure. Study results, published in the May 17, 2007, Psychosomatic Medicine, held firm even when some possibly confounding factors such as age, race, intelligence, and age at entry into the Army were considered.
But perhaps the greatest surprise is that left-handedness has been linked with a mental disorder considered more psychosocial than biological in origin—pedophilia. James Cantor, Ph.D., an assistant professor of psychology at the University of Toronto in Canada, and colleagues found, in a study of some 400 sexual offenders, that the odds of being left-handed were about two times greater among pedophiles than among sexual offenders targeting adults as their victims. In fact, more than 30 percent of the pedophiles were left-handed, that is, three times the rate in the general population. Results were published in the August 2005 Archives of Sexual Behavior.
Researchers Offer Theories
So, since left-handedness and ambidexterity have been linked with certain neurodevelopmental disorders, schizophrenia, PTSD, and pedophilia, what might that link be?
Left-handedness and ambidexterity may share some underlying biological origins with them, some researchers contend.
For example, Choudhary and O'Carroll do not believe that the reason why left-handed persons are more likely to develop PTSD is because they are more at risk of trauma in a right-handed world than are right-handed people. The reason why is because they found no significant difference between the number of traumas experienced by left-handed subjects and by right-handed ones. However, the researchers do suspect that the reason that left-handed individuals may be more likely to develop PTSD is because their brains are responding to, or processing, emotional events differently from the brains of right-handers.
Francks suspects that the gene LRRTMI might set the stage for both left-handedness and schizophrenia by influencing the development of brain asymmetry. Asymmetry is an important feature of the human brain, with the left side usually controlling speech and the right side controlling emotion. In left-handers, this pattern is usually reversed. Iris Sommer, M.D., Ph.D., of the University of Utrecht in the Netherlands, agreed with Francks. In fact, she has found that, just as speech often originates in the right side of the brain in left-handed persons, it is also the case for persons with schizophrenia.
And as for why left-handed persons may be disproportionately represented among known pedophiles, Cantor believes that the answer may lie in brain characteristics common to both conditions.
In any event, even if left-handedness and ambidexterity share common biological pathways with some mental disorders, left-handed individuals should not conclude that they will necessarily develop these mental disorders, authorities on the subject stress.
For example, regarding the finding that Francks and his colleagues have made—that the gene that predisposes to left-handedness may also increase the risk of schizophrenia—"People really should not be concerned by this result," he said. "There are many factors which make individuals more likely to develop schizophrenia, and the vast majority of left-handers will never develop the problem."
As for Choudary and O'Carroll's finding that left-handers are significantly more at risk of PTSD, Choudhary said, "The development of PTSD is multi-factorially determined and incompletely understood. In this context, left-handedness seems to be an important part of the jigsaw and one previously neglected, but though apparently important, is only one determinant of who might develop PTSD following trauma."
All in all, Choudhary stressed, "The majority of left-handers are not going to develop mental health problems because of their left-handedness."
Cantor agreed: "The association between left-handedness and disease is very complex.... Being left-handed does not make a person appreciably more likely to have any given disease."
So being left-handed or ambidextrous, like virtually everything in life, has both its pluses and minuses. And if you are left-handed or ambidextrous, why not try to capitalize on your mental strengths—say, exceptional intelligence, economic shrewdness, or creativity? Who knows, you might even become president! 
Footnotes
Joan Arehart-Treichel is left-handed.
Psychiatr News March 20, 2009
Volume 44, Number 6, page 22
© 2009 American Psychiatric Association
