Lawrence University in Canton, N. 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. 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. 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 the current study, Halpern and Coren collected all the death certificates recorded in the two counties over a period of several months--about 2, 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 individuals. 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 All other subjects left-handers and mixed-handers were assigned to a non-right-handed group. When we turn to the effect of handedness on life span, the results are striking in their magnitude.
Critics of these studies have noted that their methodologies were flawed, as they assumed a static proportion of left and right handed people throughout time, despite the fact that many people who were born in early s were likely pressured to become right-handed at an early age and would not identify as left-handed at death.
Historical records support this argument, presented in the book Language Lateralization and Psychosis :. This meant that using a single cohort of individuals who died in the year would be biased by the fact that people who identified as left-handers were, statistically speaking, a younger group of people. A good explanation of the result of such a bias was provided in a series of letters to the editor regarding the NEJM study, notably the following from epidemiologist Kenneth Rothman:.
Comparing mean ages at death is a classic fallacy, as it involves comparing only the numerators of rates, rather than the rates themselves.
Using the same approach as Halpern and Coren, one would conclude that nursery school is more dangerous than paratrooper training, since the mean age at death of children in nursery school is much lower than that of paratrooper trainees. Not only would left-handed people have been encouraged not to be during this period, life was also pretty difficult for them and they quickly became very conspicuous.
The result of all of this was that left-handers became stigmatised - regarded as cack-handed, stupid. So, some of the people who had died on those Californian lists may well have been born left-handed, but spent most of their lives acting and identifying as right-handers. Their families would have described them as such, when the researchers came knocking.
To see why, imagine an exaggerated scenario where there were no left handers at all born before - 40 years ago. If we now look at death records for and ask who, among the dead, was left-handed we would see that all of them died at or before the age of That would be much younger than the average of age at death of right-handers.
Nothing like this exaggerated scenario ever occurred in reality - but the number of people identifying as left-handed did grow dramatically during the 20th Century. So the idea that left-handers die nine years earlier than right-handers is a myth.
What about non-fatal injuries? Should we be worried about people like Claire Allen struggling in the kitchen with knives designed for the right-handed?
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