Left-handed in a right-handed world

Press, 1 July 1983, Page 13

Left-handed in a right-handed world

By

JANE E. BRODY

“New York Times”

Left-handed people, already burdened by minor inconveniences in a right-handed world and by a language full of such ego-deflating figures of speech as two left feet and left-handed compliment, also seem to face an increased risk of certain disorders that can further impair the quality of their lives. A series of new studies shows that left-handed people are more likely to suffer from learning disabilities, stuttering, migraine headaches and, according to the latest findings, auto-immune diseases, like ulcerative colitis, myasthenia gravis, and celiac disease, in which the body attacks its own tissues.

The researchers emphasise, however, that their findings do not mean that left-handed people are at a biological disadvantage. Rather, they consider left-handed-ness as an example of “biological diversity” that may be disadvantageous in some ways and beneficial in others.

“Until you look at the full range of conditions and over-all life expectancy among left-handed people, you cannot say whether it’s a disadvantage,” says Dr Norman Geschwind, a leading researcher in the field. “It could be that lefthanded people have only half the rate of lung cancer that righthanded people do.” The studies by Dr Geschwind and others suggests that the seemingly disparate array of conditions overrepresented among left-handed people may all stem from variation in foetal brain development that is related to male sex, possibly the male sex hormone testosterone. ’ The result is that part of the left

side of the brain grows more slowly than the right, leading to a change in the usual dominance patterns of the brain. At the same time, testosterone has been shown to interfere with the development of the immune system before and after birth. The job of this system is to recognise and protect the “self’ against invasion by foreign organisms and tissues. “I think left-handedness is only a marker of what I call anomalous brain dominance,” Dr Geschwind says. Previous studies of left-handers have revealed such traits as better spatial relations, more emotional expressivity on the left side of the face, greater recovery of speech function after a brain injury, as well as a greater tendency toward cigarette smoking and alcohol abuse. Left-handers have a decided advantage in certain sports activities, especially tennis, baseball, and softball.

Although left-handedness had previously been associated with learning disorders and certain other problems, the possible biological significance of the connections had been unappreciated. In fact, the high incidence of the learning disability, dyslexia, among left-handed people had often been attributed to attempts by parents and teachers to force them to use their right hands. The late Nelson Rockefeller, a dislexic who tended to read and write words backwards, was a lefty whose father had tried unrelentingly but unsuccessfully to convert him to a righty. The new finding of a greatly increased risfci of auto-immune

disease among those who are strongly left-handed grew out of a chance observation by Dr Geschwin, a neurologist at Harvard Medical School and Beth Israel Hospital in Boston. At a meeting of dyslexia researchers, he noticed that when people described their family histories, there appeared to be a disproportionate number of patients with auto-immune diseases among those with dyslexia and in their families.

However, Dr Geschwind is quick to note that his finding was not merely serendipitous. Quoting the nineteenth-century scientist Louis Pasteur, he says: “In the fields of observation, chance favours the prepared mind.” Dr Geschwind’s was prepared to look at the full spectrum of autoimmune disorders among lefthanded people and among dislexics on the theory that a genetic predisposition to a condition may be expressed differently in different people. Instead of studying the incidence of left-handedness among dyslexics, as most previous researchers had done, Dr Geschwind decided to study left-handed people to determine what problems existed among them and their families. With the help of Peter Behan, a neurologist at the University of Glasgow, he devised a questionnaire that would select those people who were most strongly left-handed and compared them with strongly right-handed people. .■ On average, Dr Geschwind be-

lieves, about 70 per cent of the population has standard brain dominance, with the left side of the brain controlling language and handedness, making most of those individuals right-handed. The remaining 30 per cent of people have more symmetrical brain dominance, and of these about a third are strongly left-handed. In the first study, involving 253 left-handers (gleaned from patrons of a shop for left-handers in London) and an equal number of righthanders, the left-handed group was found to have 12 times more learning disabilities and nearly three times more auto-immune diseases. Their relatives also had higher rates of these problems. In the second study, the subjects were gathered from among the general population of Glasgow. Here too, the incidence of autoimmune disease was two and a half times higher among the lefthanders.

A third study compared the frequency of left-handedness among patients with migraine headaches or immune disorders. A higher percentage of left-handers was found among the headache patients and those with myasthenia gravis, an auto-immune disease. Dr Geschwind believes that some cases of migraine may result from an immunological disorder. In studies not yet published, researchers found “a distinct increase in childhood allergies among strongly left-handed people.” Other researched have

reported a high rate of allergies among stutterers, an increased frequency of food allergy among hyperactive children, and high rate of immune disorders among children with autism. “In all these conditions there is an elevated rate of left-handed-ness,” Dr Geschwind reported to a conference this month, and that suggests the possibility of a common origin.

The origin he and Dr Behan propose involves a region of the brain called the planum temporale, an area involved in speech. Scientists had long thought both sides of the brain were of equal size.

Then in 1968, Dr Geschwind and Dr Walter Levitsky discovered a difference that was even visible to the naked eye: in most brains, the planum temporale was much larger in the left hemisphere than in the right. This was in accord with previous knowledge that the left hemisphere controlled language.

Dr Geschwind said that in brains, the planum temporale is larger on the left 65 per cent of the time; in 24 per cent the two sides are equal, and in 11 per cent the planum is larger on the right. In strongly left-handed people, the Boston scientist suggests, the usual assymetry may be disturbed, resulting in equal or greater development of the planum temporale on the right side of the brain. Left-handedness, learning disabilities, autism, and stuttering are all found more frequently among boys than girls, Dr Geschwind noted. This suggests that a sexrelated factor is involved. « According to the scientist, ani-

mal studies have shown an effect of testosterone may also be the determining factor in delaying growth of the left planum. Testosterone is produced in large quantities by the male foetus, but the female foetus is exposed to only small amounts of this hormone. “Testosterone affects the development of the structure of the immune system,” Dr Geschwind said. One effect it may have is on the thymus gland, which helps to distinguish self from foreign. Furthermore, a gene that favours the development of dyslexia had been shown to be located on the same chromosome that contains a gene important to immune function. It has also been suggested that this same gene is essential for the formation of the testes.

One possible glitch in the testosterone theory stems from the fact that auto-immune diseases are far more common in women. However, Dr Geschwind cites evidence that in adult life testosterone protects against immune derangements, so that even if men are more susceptible to such disorders, their susceptibility is not expressed until late in life, when testosterone levels decline.

“Certain auto-immune diseases primarily affect young women and old men,” he noted. He added that men with Klinefelter’s syndrome, a genetic abnormality that results in very low levels of testosterone, have a high indicence of autoimmune disease.

Many further studies are needed to clarify the apparent relationship between testosterone, left-handed-ness, and the disorders linked to it.