A new Yale-led study revealed that left-handed people are more likely to develop psychotic disorders that alter thinking and perception, like schizophrenia, but have no greater chance of developing mood disorders like bipolar disorder or depression.
While about one in 10 Americans is left-handed, the study found that roughly 40 percent of individuals with schizophrenia were left-handed. In contrast, only 11 percent of those with mood disorders were found to be left-handed. The study did not investigate the mechanism behind this pattern, but study authors speculate that handedness may be an indicator for individual differences in brain structure that contribute to the development of psychiatric diseases.
“What we found that was somewhat startling to us was how many more people were left handed in the psychotic category than in the mood disorder category,” said Jadon Webb, lead author and psychiatrist at the Yale Child Studies Center.
The study was conducted among 107 patients in a low-income, urban psychiatric clinic that treated patients with both psychotic and mood disorders. Patients were asked directly which hand they wrote with and were not presented with extensive questionnaires, brain tests or lab tests, as are typical of more traditional psychiatric research, a study design choice which Webb said helped attract as many participants as possible.
In the finding, study authors stated that abnormalities in development of relative cognitive and physical function of the two hemispheres of the brain are thought to be integral to schizophrenia and other psychotic disorders. Hand dominance is useful in studying brain abnormalities because it has been shown to correlate to with language expression, memory skills and visual and emotional processing, which are impaired in the presence of some psychiatric illness.
Correlation between left-handedness and schizophrenia was first proposed in a 2005 study, and correlation between left-handedness and bipolar disorder was first suggested in a 2008 study, according to study authors. In both of these previous studies, patients with psychiatric illness were compared to individuals who showed no mental abnormalities. In contrast, this research is the first to directly compare handedness in these two clinical groups, according to the study.
Since schizophrenia is such a complex condition, this research provides another clue about the complex neurobiology of the condition, said David Valle, a researcher at the Johns Hopkins Conte Center for Schizophrenia Research.
Webb said that future research may make use of this study’s results by comparing early-life events that cause schizophrenia with early-life events that cause left-handedness to see whether there is overlap between them. Webb said that discovery of such an overlap could enable doctors to determine which courses of treatment would be most effective for their schizophrenic patients.
“It may also be that if you’re left-handed and psychotic that that could tell you something about your treatment course or maybe even possibly tell you about what treatments are more or less likely to be effective,” he said.
Study author and professor at the University of Iowa College of Pharmacy Mary Schroeder said that she is optimistic that left-handedness and other potential biomarkers will be useful for further research and may even enable doctors to develop more fast-acting and cost-effective treatments.
According to the National Institute of Mental Health, approximately 2.4 million Americans have schizophrenia.