Shots - Health News
2:17 pm
Mon December 2, 2013

Alleged Perils Of Left-Handedness Don't Always Hold Up

I recently stumbled upon a description of research out of Yale that suggested there was a link between left-handedness and psychotic disorders like schizophrenia.

Forty percent of those with psychotic disorders are lefties, one of the researchers said. That startled me. Only about 10 percent of people in the general population are left-handed. I'm one of them.

I've often read that I'm going to die earlier. Also, I'm bad with scissors. And now, it seemed, I'm at high risk for mental illness. Was my hand preference a lifelong curse?

The short answer: No.

After getting both my hands on the full study, I found that the work, while intriguing, falls far short of being conclusive.

The study, published in the journal SAGE Open, looked at the prevalence of left-handedness among people with mood disorders, like depression, and those with psychotic disorders.

Researchers collected data from 107 psychiatric patients in an outpatient mental health clinic for people with low incomes. Only a third of the people, or 35, had a psychotic disorder. All in all, that's not necessarily representative of the general psychiatric population — and it's a pretty small sample from which to draw a potentially big conclusion.

The researchers on this study acknowledge that its 40 percent finding is high compared to other research. Previous estimates on left-handedness in those with schizophrenia range from a low 7 percent — less than the prevalence in the general population — to 31 percent, the study says.

And there's one other thing. To determine if patients were left-handed, the researchers asked just one question: Which hand did people prefer to write with?

Psychiatrist Jadon Webb, the lead author and a clinical fellow at Yale School of Medicine, tells Shots that this simplicity helped the researchers get a higher participation rate. But other researchers say handedness is too ambiguous to determine reliably with that question alone.

Finding links between left-handedness and mental disorders is nothing new. In fact, lefties have gotten bad press for centuries.

"The words 'left' and 'left-hand' in almost all the world's languages have negative connotations — from 'sinister' in Latin and Italian to 'dishonest' in Mandarin," wrote Howard Kushner, a public health professor at Emory University who studies the medical and cultural implications of handedness.

In the early 20th century, an Italian criminologist connected left-handedness to feeble-mindedness and criminality. Since then, Kushner wrote, researchers have proposed links between left-handedness and learning disabilities, autoimmune disorders, autism and dyslexia, among other ailments.

"There's a 150-year history of trying to connect left-handedness to every pathology they can think of," Clare Porac, a psychology professor at Penn State Erie, tells Shots. "One of the issues ... is the following notion: Everyone should be right-handed, and if not, there's something wrong with the brain."

One claim that keeps popping up is that lefties die earlier, which two researchers in Canada posited in 1991. Forget that one. "That's a theory that has been discredited for 15 years in the scientific community," Porac says.

Other health claims, like the link between left-handedness and schizophrenia, continue to be debated in scientific research. One study finds a link, another doesn't — and when you look at it all, Porac says, it kind of evens out.

Why is it so difficult to come up with a scientific consensus?

First, lefties are remarkably inconsistent when it comes to hand use. Some write with their left and throw with their right, some use their right hand for everything but writing and some don't write with their left hand because they were trained not to. Definitions of what it means to be left-handed can vary among studies, making it hard to draw broad conclusions.

Second, handedness is often used as a proxy for brain lateralization, or which side of the brain does what. People often assume that left-handers are predominantly right-brained and right-handers are the opposite.

But that's not always true, Emory University's Kushner tells Shots. In fact, he says, language and speech are localized to the right side of the brain in only 18 percent of lefties. The more accurate way to tell where language functions reside is to do a brain scan.

And, finally, Kushner says many studies on schizophrenia link the disease to weak brain lateralization (or mixed-handedness, as some researchers call it), not even to left-handedness specifically.

In Porac's opinion, at least, there's no reason for lefties to worry. "I've interviewed hundreds of left-handers," she says, "and they're all pretty much OK."

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