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Studies in the late 1980s by Stanley Coren and Diane Halpern were largely responsible for popularizing the myth that left-handers tend to die early. In one study, published in 1991, he claimed that left-handers may die up to nine years earlier than right-handers. However, the studies were seriously flawed and Coren�s conclusions erroneous. The findings were roundly refuted by more than a dozen studies in the 1990s, including studies by the National Institute on Aging, the United States Statistical Assessment Service and the American Academy of Actuaries, among others.
The Coren/Halpern study falls prey to a statistical anomaly known as the �death cohort� effect, whereby the conclusions drawn are skewed because the data is based on people at their date of death rather than their date of birth. Because of attitudes and social stigma against left-handedness earlier in the 20th Century, older people are statistically more likely to be (or to claim to be) right-handed than younger people. For example, in one recent study, only 3% of a sample of over-80s wrote with their left-hand, although 9% of them ate with their left hand, suggesting that they may have switched their writing style, or just covered up or denied their natural left-handedness. Thus, on average, left-handers were born later in the century than right-handers, giving the false impression that left-handers are statistically more likely to be younger than right-handers at death. Chris McManus uses the analogy that people who read Harry Potter books are typically younger than people who do not, which, in a survey of the recently deceased, would give the impression that Harry Potter readers die younger than non-Harry Potter readers.
Over the years, several studies have suggested that disorders related to brain development - like dyslexia, mood disorders, bipolar disorder, schizophrenia and attention deficit hyperactivity disorder (ADHD) - tend to be more common in left-handed people. It has been hypothesized that, in at least some cases, the connection may arise from the atypical lateralization of the brain. For instance, studies have shown that people with schizophrenia typically exhibit more symmetrical activation of their brain hemispheres (particularly in the areas of languange and phonology) than those without the disorder, as is the case with some, although by no means all, left-handers (see the section on Handedness and the Brain). The conclusions of these studies are not definitive, though, and the issue remains tantalizingly inconclusive and at times even contradictory.
To give a few examples of the kinds of findings which have come to light, about 20% of people with schizophrenia are left-handed, twice the incidence of left-handedness in the general population. A 1982 study by Norman Geschwind and Peter Behan reported that strong left-handers were 11 times as likely to suffer from dyslexia than strong right-handers, and that their rates of other learning disabilities and immune diseases were also relatively high. More recent studies in Sweden and Finland by Alina Rodrigue have found links between left-handed (and particularly mixed-handed) children and ADHD, indicating a doubled likelihood of language and scholastic difficulties at an early age, followed by ADHD as a teenager.
Several studies have suggested that sufferers from autism are significantly more likely to be left-handed than the general population (of the order of 60% more likely according to some studies, but over 5 times as likely according to others). Autists are also more likely to be mixed-handed or �ambivalent-handed�, and there is research which suggests that ambidextrous autists are generally lower-functioning than right- or left-handed autists. It has been pointed out in this context that the right hemisphere of the brain, often the dominant side for left-handers, is also usually the one responsible for the processing of emotional content, social cues, humour, etc, all areas in which autists often show deficits.
But once again, any connection is far from straightforward and far from easily explicable. For example, there is some evidence that those who suffer from autism tend to have a smaller corpus callosum, and real-life Rain Man Kim Peek (a mega-savant who actually suffered from FG syndrome rather than autism) was born with no corpus callosum at all. But most left-handers typically have thicker and more developed corpus callosums, possibly in order to facilitate the increased interhemispheral cross-talk required by their more balanced brain structure (see the section on Handedness and the Brain for more details).
Possible links between handedness and various other health problems have also been posited. For example, several different studies in 2005 showed that left-handers are more likely to suffer from schizophrenia, high blood pressure and irritable bowel syndrome, but that they have a lower incidence of arthritis and ulcers. What associations can justifiably be made between these apparently random findings is far from clear. Some other rather puzzling and seemingly illogical findings also arise from some of these studies. For example, a schizophrenic twin may statistically expect better health prospects if at least one of the twins (and not necessarily the schizophrenic one) is left-handed.
There is some evidence from a Dutch study carried out between 2005 and 2009 that left-handed women (particularly thin left-handed women, for some reason) are over twice as likely to contract premenopausal breast cancer than right-handed women, even taking into account variables like family history, smoking and socioeconomic status. This finding remains largely unexplained, although it has been conjectured that in utero exposure to steroid or estrogen hormone treatments may be responsible.
Such findings appear to be outliers, though. A detailed and comprehensive meta-analysis by Chris MacManus and Clare Porac among others has indicated that left-handers as a whole are in fact no more likely to suffer from auto-immune disorders, or indeed health problems in general, than right-handers.
As early as the 1920s, stuttering (or stammering) was associated with natural left-handers who had subsequently been trained to write with their right hand. The most high-profile case was that of the left-handed King George VI, whose stuttering began at about 7 or 8 years old (i.e. shortly after being taught to write with his right hand), although the link has never been definitively proven. A 1935 study even showed how, when allowed to revert to using their natural hand for writing, the stuttering of such individuals disappeared or was substantially ameliorated. However, studies since this time have muddied the waters somewhat, and most experts today do not claim to see a direct cause-and-effect relationship between hand-switching and stuttering.
There may, though, be some relationship between stuttering and the side of the brain used to process speech and language, which has in turn also been linked to handedness (although not to the extent initially reported - see once again the section on Handedness and the Brain). Some recent studies, for example, have suggested that stutterers are more likely to process speech and language on both sides of the brain, rather than in the more normal left hemisphere, and that, when treated with a drug that causes them to stop stuttering, this bilateral language processing also ceases, and reverts to single hemisphere processing.