Lefties make up about 10 percent of the general population. But researchers have found that in populations with certain mental disorders, that rate goes up. Previous studies have found that people with psychosis had a 20 percent likelihood of being left-handed, though a small study in the journal SAGE found the rate of psychotic lefties may be even higher. Researchers at Yale University in New Haven, Connecticut, and the University of Texas Southwestern Medical Center in Dallas assessed 107 patients at outpatient psychiatric clinics. For those with mood disorders such as depression or bipolar disorder, the rate of left-handedness was close to average, at 11 percent. But in people with psychosis, such as those with schizophrenia or schizoaffective disorder, the rate of left-handedness was 40 percent, well above average. Researchers theorize brain laterality plays a role.