It's not correlation-causation but a new study has found that, among those with mental illnesses, left-handers are far more likely to suffer from psychotic disorders such as schizophrenia. 

Scientists and psychologists have long been interested in handedness because the brain develops asymmetrically and some cognitive processes develop from the left or right side. Since hand dominance is a convenient measure it has been a focus for decades, with some research finding a great prevalence of psychosis in left-handed people.

The authors of the new paper examined 107 individuals from a public psychiatric clinic seeking treatment in an urban, low-income community and determined the frequency of left-handedness within the group of patients identified with different types of mental disorders. They found that 11% of those diagnosed with mood disorders such as depression and bipolar disorder were left-handed, which about the same rate in the general population, however, 40% of those with schizophrenia or schizoaffective disorder were left-handed.

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After multivariate adjustment, the odds ratio for left-handedness in psychotic illness was 7.9 (p < .001) compared with mood disorder and left-handedness appeared to be similarly high for both schizoaffective disorder and schizophrenia. Left-handedness was not different according to age or gender, but was found to be less common among Blacks compared with Caucasians after adjustment.

The authors also discuss additional factors that might be tied to the connection between schizophrenia and left-handedness such the variation of brain lateralization, scholastic achievement or race.

"Our results show a strikingly higher prevalence of left-handedness among patients presenting with psychotic disorders such as schizophrenia and schizoaffective disorder, compared to patients presenting for mood symptoms such as depression or bipolar disorder," wrote the authors. "Our own data showed that whites with psychotic illness were more likely to be left-handed than black patients. Even after controlling for this, however, a large difference between psychotic and mood disorder patients remained."

Citation: Jadon R. Webb, Mary I. Schroeder, Christopher Chee, Deanna Dial, Rebecca Hana, Hussam Jefee, Jacob Mays, and Patrick Molitor, Left-Handedness Among a Community Sample of Psychiatric Outpatients Suffering From Mood and Psychotic Disorders', SAGE Open October-December 2013 3: 2158244013503166, October 30, 2013 doi:10.1177/2158244013503166