Another study has shown that people with schizophrenia are more likely to be left-handed. Is there a deeper connection?

A new study examined 107 psychiatric patients at a public clinic in an urban community. Researchers found that among patients with mood disorders like depression and bipolar disorder, 11 percent were left-handed, about the same rate as in the general population.

But when they examined patients with schizophrenia and schizoaffective disorder, they found that 40 percent were left-handed.

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Dr. Jadon Webb, M.D., Ph.D., and a fellow at Yale University, said he chose left-handedness as a convenient way to measure lateralization, or how symmetrical the two sides of a person’s brain are. In many people, one side of the brain is the dominant, or stronger, half for certain functions, such as writing by hand.

Webb asked patients which hand they preferred to write with, getting a 97 percent response rate.

However, expert Dr. Milan Dragovic questions whether left-handedness is a useful way to determine which half of the brain is dominant.

“Asking someone which hand is used for writing can hardly be considered as a test,” said Dragovic, an associate professor at the School of Psychiatry and Clinical Neurosciences at the University of Western Australia. “Social tolerance towards left-handedness can influence prevalence rates. For example, prevalence of left-handedness is significantly higher in western countries than in eastern European countries or China.”

Hand preference also isn’t the only marker of dominance, he adds. “In humans, there are many other lateralities. For example, a person may write with their right hand, but prefer their left foot for kicking a ball, and vice versa.”

Dragovic says he is skeptical of Webb’s claim that there’s a special link between left-handedness and psychiatric disorders. “Left-handedness is also related to many other conditions, such as Rett syndrome, strabismus, breast cancer, maternal smoking, epilepsy, stuttering, alcoholism, and some forms of mental retardation,” he said.

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Might there be a common cause of both left-handedness and schizophrenia? “We don’t know for certain that there is a causative mechanism here, we have only really measured associations,” Webb cautioned. “Perhaps some of the left handedness-psychosis association might be due to a common early environmental insult…such as flu, trauma, or malnutrition.”

Dr. Timothy Crow doesn’t accept Webb’s explanation. “The closer you look at environmental factors, the less impressive the evidence becomes,” said Crow, Honorary Director at the Prince of Wales Centre for Research on Schizophrenia and Depression at Warneford Hospital in Oxford.

His team examined a wealth of data in the U.K. National Child Development (NCDS) database and didn’t find any such connection. Instead, Crow thinks that the origin of psychosis may have to do with the way parents’ X and Y chromosomes recombine to form a child’s DNA sequence.

In fact, researchers aren’t certain what causes left-handedness in the first place. “There is a lot of evidence that left-handedness is also associated with some special skills,” Dragovic said. “You will more commonly find left-handed people in subpopulations, such as architects, mathematicians, painters, tennis players, etc. My personal speculation is that left-handers, as a subpopulation, display much broader variation than the right-handers.”

This means that left-handers might be both more likely to excel in creative fields and more likely to suffer from psychiatric disorders, the two extreme ends of a spectrum.

The one thing the three researchers can agree on is that schizophrenia is a complicated and variable condition. It is best thought of as a syndrome, a cluster of symptoms that may or may not have related causes, rather than a single disease.

“To me, the diagnosis of schizophrenia is like the diagnosis of high blood pressure,” said Webb. “It describes a general condition but says nothing of the underlying causes. We know that the causes of high blood pressure are many, and this is almost certainly true of schizophrenia as well.”