Left-handers tend to die, on average, about nine years earlier than right-handers, according to a surprising and controversial new study published today by California and Canadian researchers.
In analyzing data on nearly 1,000 deaths in San Bernardino and Riverside counties over a period of several months, two psychologists found that male and female left-handers were much more likely to die as a result of industrial and vehicular accidents, and somewhat more likely to die from other causes.
The researchers attributed much of the dramatic increase in accidental deaths to the fact that most machines are designed for right-handers. Certain neurological and immunological defects often associated with left-handedness were also thought to play a role in the shortened life spans.
The study is the first to suggest that the previously documented susceptibility of left-handers to accidents is associated with increased mortality. It is also the first to suggest that health problems linked to high rates of birth trauma in left-handers may be potentially life-threatening.
Both the authors and other researchers were surprised at the large difference shown in the study, published in the New England Journal of Medicine.
"It's really hard to fathom a difference that large not having been noticed before," said psychologist Alan M. Searleman of St. Lawrence University in Canton, N.Y. But Diane F. Halpern of Cal State San Bernardino, one of the co-authors, countered that no one had ever looked for such differences until now.
"The effect was so large it is unlikely to have happened by chance," she said, adding that there is no reason to believe the study was flawed. "We had very stable data. There is nothing peculiar about this study."
Searleman said the study could have significant practical ramifications, including the possibility that life insurance companies might charge a higher rate for left-handers or that auto insurers might charge more for left-handed drivers. "I wouldn't want to see that (happen) unless the data are replicated and reliable," he said.
In the population at large, about 9% of women and 13% of men are left-handed, but previous studies have shown a peculiar age distribution. At the age of 10, 15% of the population is left-handed. At 20, 13%. By 50, the figure drops to 5%. And by 80, it is less than 1%. "That's what initially got us interested in studying this," Halpern said.
Some left-handedness, perhaps the bulk, is genetically based, but much of it is also associated with some disturbance of brain development. Among the factors implicated in previous studies are high levels of male hormones in the mother during pregnancy and trauma during birth, including Rh incompatibility, prolonged labor, breech birth, prematurity and low birth weight.
Perhaps as a result of such factors, researchers have suggested, lefties are more prone to a variety of problems, including neuroticism, allergies, insomnia, learning disorders, migraines, autism and disorders of the immune system. They also have higher rates of alcoholism and smoking.
In a previous four-year study of 1,896 students, one of the current study's authors, Stanley Coren of the University of British Columbia in Vancouver, found that left-handed people were 89% more likely to suffer serious accidents. Left-handed men in that study were at the greatest risk when behind the wheel of a car: They were 135% more likely than right-handers to have an automobile accident.
In the current study, Halpern and Coren collected all the death certificates recorded in the two counties over a period of several months--about 2,000 in total. Excluding individuals under the age of 6 and those who died from homicides or suicides, they wrote the families of the deceased and asked for information about the cause of death and whether the victim wrote, drew and threw a ball with the right or the left hand. They obtained usable data on 987 individuals.
"The fact that we collected all the death certificates in order made this a truly random sampling" characteristic of the entire population, Halpern said.
Overall, they found that the mean age of death for right-handers was 75 years, compared with 66 years for left-handers and those who used both hands. For women, the corresponding figures were 77.7 for right-handers and 72.8 for left-handers. For men, they were 72.3 for right-handers and 62.3 for lefties. The figures are in accord with previous studies showing that women tend to live about six years longer than men.
Halpern and Coren also found that about 7.9% of the left-handers died in accidents, compared with 1.5% of right-handers. For automobile accidents, the corresponding figures were 5.3% for left-handers and 1.4% for right-handers.