The hypotheses about lefties are right: They are prone to psychosis. According to research at Yale University, led by Jadon Webb, people who are left-handed are far more likely than the general population to develop a psychotic disorder. The researchers reviewed the data of 107 patients who visited an outpatient psychiatric clinic in a low-income, urban community. Those who were left-handed were significantly more likely to have psychotic disorders – 40% more likely, in fact. In contrast, patients with mood disorders, such as depression or bipolar disorder, were not more likely to be lefties.
Psychosis and the Brain
The left and right hemispheres of the brain are asymmetrical, and each seems to mediate different cognitive functions. Scientists have long suspected that the same regions of the brain that control language also control psychosis. The asymmetry of the brain hemispheres appears to support the development of the process of language acquisition, and disparities in the development of brain asymmetry may be at the core of schizophrenia.
Speaking and comprehending language involves many functions, and different, specific functions of language are in each hemisphere. Therefore, in order to communicate, both hemispheres must work in conjunction. As such, one hemisphere acts dominantly, and the non-dominant hemisphere must contribute in a responsive manner.
People who are psychotic have disrupted communication abilities, which is likely from disruptions in the function of their brain hemispheres. The symptoms of psychosis, involve seeing and hearing things that are not reality-based, which means they are communication-based.
The interactions between the two hemispheres of the brain are also likely related to handedness. Paul Broca, a French anthropologist, suggested 150 years ago that lefties use their right hemispheres, and righties use their left hemispheres. Researchers have not authoritatively proven this over the years: A 2013 study by researchers at the University of Munster, Germany, found only 27% of lefties use a dominant right hemisphere for language function. These researchers concluded that left-handedness increases the likelihood of one’s language functions being dominated by the right hemisphere, but that being a lefty is not a precondition or an inevitable consequence of right-hemisphere language dominance. Scientists are certain, though, that there is a connection among handedness, language, and brain hemisphere asymmetry.
Handedness: Neurological, not Social
Left-handedness is correlated with other other conditions, as well. Researchers at the University of Louisville, Kentucky, studied the relationship between anxiety and handedness. They concluded that left-handedness is associated with greater anxiety than right-handedness, but only when the lefties are inconsistent, i.e., they usually use their left hands to write, but sometimes use their right hands. This conclusion points to the idea that handedness consistency is a pertinent factor in personality.
Click to Read Page Two: Australia Research on Handedness