Flashback Diabetes Friday: Taking the Upper Hand: Is There Really a Connection Between Type 1 Diabetes and Left-Handedness?

For decades, scientists have postulated that left-handedness is associated with autoimmune diseases such as type 1 diabetes. This theory was recently brought to the attention of Diabetes Health by Joan Hoover, our patient advocate adviser, who found that many of the children with diabetes she came across in her work were left-handed. Studies, however, have yielded conflicting results, rendering the validity of this theory controversial.

Diabetes Health decided to examine the evidence about the connection between left-handedness and autoimmune disorders—from well-founded scientific research to the results from our own internal “readers’ poll”—to find an answer. Our results: people with diabetes are three times more likely to be left-handed than the general population.

Drawing a Comparison

Defining the number of left-handers among the general population is an inexact science in itself. According to most sources, including the Columbia Electronic Encyclopedia, left-handed people compose approximately 10 percent of the general population. So how does that compare to the number of left-handers among people with diabetes?

Some Researchers Find a Connection

Notably, the phenomenon that more left-handed people have diabetes is restricted to type 1 diabetes, according to published scientific data.

Norman Geschwind, PhD, is perhaps the most prominent scientist to establish the connection between handedness—or cerebral lateralization, as he called it—and autoimmune diseases, including type 1 diabetes. An accomplished neurologist from Harvard Medical School in Boston, Massachusetts, who specialized in behavioral disorders, Dr. Geschwind and a colleague conducted a study of left- and right-handers in Great Britain, published in the August 1982 issue of the Proceedings of the National Academy of Sciences.

Using a questionnaire, the researchers asked 506 people (253 left-handers and 253 right-handers) in London, England, and 494 people (247 left-handers and 247 right-handers) in Glasgow, Scotland, about their history and their relatives’ history of autoimmune disease, migraines and developmental disorders.

They found “an elevated frequency” of all three conditions in the left-handed groups compared to the right-handed groups. “The frequency of autoimmune disease reported by the left-handed subjects” was slightly more than two times greater in both geographic groups of left-handers, according to the study. The researchers also found a higher likelihood of an autoimmune disease in the first- and second-degree relatives of the left-handed subjects compared to those of the right-handed subjects.

These findings are “in conformity with the hypothesis that autoimmune disorders would be present more often in left-handers and their families than in right-handers,” the researchers safely concluded.

So There’s a Connection—But What’s the Likelihood?

Stanley Coren, PhD, of the University of British Columbia, reviewed several studies establishing the connection between handedness and autoimmune diseases in his book “The Left-Hander Syndrome: The Causes and Consequences of Left-Handedness” (Free Press, 1992). Based on the findings of Camilla P. Benbow, EdD, of Vanderbilt University in Nashville, Tennessee, Coren asserts in his book that “left-handers are twice as likely as right-handers to suffer from [type 1] diabetes.”

“In other words,” Coren writes ruefully, “the gloomy news is that left-handers seem to be particularly susceptible to autoimmune diseases and problems.”

Coren’s “twice as likely” figure is consistent with Dr. Geschwind’s findings, although Geschwind’s figure included a handful of autoimmune diseases and was not exclusive to type 1 diabetes.

Other Researchers Are Not So Sure

Other researchers do not unequivocally assert that left-handedness is definitively linked to type 1 diabetes.

In his study published in the August 1993 issue of the Medical Review of Chile, Dr. Clavero, of the University of Chile in Santiago, states that such a theory “is not supported” by his findings. Dr. Clavero examined the connection between handedness and autoimmune and speech disorders in 260 subjects (65 with type 1 diabetes, 65 with type 2 diabetes and 130 control subjects). “No differences were found for strong left-handedness,” he writes.

Alan Searleman, PhD, of St. Lawrence University in Canton, New York, did not discover significant results linking left-handedness and diabetes in his research either. Publishing their results in the February 1987 issue of Neuropsychologia, Dr. Searleman and a colleague considered the theory that left-handedness is associated with autoimmune disorders by comparing the handedness of people with type 1 or type 2 diabetes, Crohn’s disease or ulcerative colitis against a healthy control group. They did find a significant connection between left-handedness and Crohn’s disease or ulcerative colitis, but no connection with diabetes. They did note, however, that males with type 1 diabetes were slightly more likely to be left-handed than males with type 2.

“The evidence that type 1 males were more likely than type 2 males to be left-handed was fairly weak,” Dr. Searleman told Diabetes Health, “but this result is consistent [with previous research].”

Joan Hoover’s Survey

A survey conducted by Joan Hoover generated results curiously similar to those of Drs. Geschwind and Coren. After noticing that many of the children she met who had type 1 diabetes were left-handed, she decided to investigate further. Hoover talked to researchers and searched for literature on the subject, to no avail. Then she found herself face to face with several hundred children with type 1 diabetes while visiting several diabetes camps. During each visit, she went to the dining hall and asked the children who were left-handed to raise their hands. Approximately one-third of the children at each camp (out of approximately 300 children altogether) turned out to be left-handed.

“That was well above the incidence in the general population,” Hoover told Diabetes Health. “I felt that I had proven my thesis.” Since then, Hoover has brought up the topic at lectures and in articles but reports that she has yet to find a researcher who thinks the topic is worth further investigation.

Results From the Diabetes Health Readers’ Poll

Brimming with curiosity, Diabetes Health decided to conduct its own “handedness” poll, posing the question “Are you right- or left-handed?” to our newslist subscribers. Of the 559 people with diabetes who responded, a staggering 32 percent (178 people) are left-handed and 68 percent (381 people) are right-handed. The percentage of left-handed people represents a figure more than three times higher than the percentage of left-handers in the general population (10 percent), and it is consistent with the results of Hoover’s poll and some scientific research. The accompanying chart breaks down our results by type 1 and type 2 separately.

DIABETES HEALTH READER SURVEY

Are you right- or left-handed?

Type 1

73.3%  right-handed
26.7%  left-handed

Type 2

61%  right-handed
39%  left-handed

(General population: 10% left-handed)