Handedness And Longevity

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The data suggest that a significant number of lefthanders arrived at their left-handedness through some form of pathological event during their fetal development or associated with their birth, and that this same pathology also resulted in other physical or psychological problems. From Table I, it should be clear that a number of these problems, especially those associated with reduced immune system efficiency, might represent major health risk factors.

Since the late 1970s a number of studies have examined handedness as a function of demographic factors, such as age, sex, and race. The studies that examined handedness over the life span found something unexpected—namely, that there is a diminishing percentage of left-handers in older age groups. Although the rate of decline varies across studies, it is usually large, from about 15% left-handedness in teenagers to less than 1% in 80-year-olds. Figure 5 shows the age proportion of left-handers for various age groups based on the average values taken from seven different studies that examined this issue.

Researchers first thought that this decrease in the number of left-handers might be due to social factors since older people, educated earlier in the 20th century, are more likely to tell stories about how they were forced to use their right hands by teachers and parents than are people born later in the century. Forced switching of left-handers to right-handedness in older age groups could explain this age-related decline in the number of left-handers. However, as discussed earlier, handedness is relatively difficult to change. In addition, several analyses of the literature suggest that the proportion of adult left-handers in North American populations has not changed since the early 1900s.

Figure 5 The percentage of left-handers diminishes with increasing age.

Figure 5 The percentage of left-handers diminishes with increasing age.

The alternative explanation for failing to find older left-handers might be that they are simply no longer in the population. That is, left-handers have a shorter life span, and the reason that we fail to find many older left-handers is because a large proportion of them have died early. When this hypothesis was first suggested, it evoked a lot of media attention and controversy. Recently, evidence supporting this conclusion has been obtained from several sources. For instance, a number of studies have examined the life spans of professional baseball and cricket players since records for these sports include not only birth and death dates but also information on the hand that was used in throwing. These have confirmed that left-handers seem to die younger. In other studies, researchers examined randomly selected death records and then contacted surviving next of kin to obtain handedness information. Again, the results suggest that the longevity of left-handers is less than that of right-handers. Although not all of the research that has been done on this issue has found the expected differences, whenever there is a significant relationship between handedness and life span it is always in the direction of left-handers having a poor survival rate.

Table I

Conditions That Two or More Research Reports Have Shown to Be Associated with a Disproportionately High Number of Left-Handers

Alcoholism

Allergies

Asthma

Attempted suicide

Autism

Bed-wetting

Brain damage

Chromosomal damage

Celiac disease

Crohn's disease

Clinical depression

Criminality

Deafness

Depression

Drug abuse

Drug hypersensitivity

Emotionality

Epilepsy

Excessive smoking Eczema

Extremely high intelligence

Negative conditions associated with left-handedness

Hashimoto's thyroiditis Hayfever

High and low extremes in numerical ability

Homosexuality

Hypopigmentation

Immune disorders

Impulsive aggression

Infection susceptibility

Juvenile delinquency

Juvenile-onset diabetes

Language problems

Learning disabilities

Lower intelligence scores

Manic-depressive psychosis

Mental retardation

Migraine headaches

Myasthenia gravis

Neural tube defects

Neuroticism

Poor school performance

Positive conditions associated with left-handedness

Good divergent thinking ability High musical ability

Poor spatial ability Poor verbal ability Predisposition toward aggression Psychosis Reading disability Reduced adult height Reduced adult weight Regional ileitis

Schizophrenia or schizotypal thinking

School failure

Sleep difficulty

Slow maturation

Slow physical development

Strabismus

Stuttering

Sudden heart attack death Transsexuality Ulcerative colitis Urticaria

High spatial ability

Figure 6 The relative risk of accidents is higher for left-handers in every category of accident studied.

Although many left-handers were dying of factors traceable to the problems in Table I, there was a surprise in the data: Left-handers were five times more likely to die of accident-related injuries. This brings us back to the issue that our technological world and the built environment have been designed for the safety and the convenience of the right-handed majority. There is evidence that left-handers are more likely to suffer injuries while playing sports, driving in traffic, working with tools and motorized equipment, working with kitchen and home implements and devices, and even while engaged in military activities. Figure 6 shows the relative risk of various injuries as a function of handedness based on data from four different studies. Notice that in all cases, left-handers seem to be at considerably higher risk. Obviously, these injuries are the direct result of a cultural and societal bias toward facilitating activities done with the right hand that we discussed earlier. These right-handed biases continue in today's world, even though we now know that they place the left-hander at higher risk of injury.

X. ARE LEFT-HANDERS AND RIGHT-HANDERS REALLY DIFFERENT?

When we examine all the data associated with handedness, it becomes obvious that left-handedness is not just a minor behavioral deviation shown by about 1 in every 10 humans. Handedness does not appear to be a marker for genetic differences as previously thought, neither is it necessarily a sign of difference in brain organization. The evidence indicates, however, that for about half of all left-handers, handedness does represent a sign that may indicate some level of neurological and developmental pathology. From the standpoint of behavioral medicine, we may view left-handedness as a risk factor that may well predict susceptibility to illness, physiological deficits, psychological problems, and perhaps even a shortened life span.

See Also the Following Articles

BRAIN DEVELOPMENT • BROCA'S AREA • EVOLUTION OF THE BRAIN • HAND MOVEMENTS • LATERALITY • WERNICKE'S AREA

Suggested Reading

Bishop, D. V. M. (1990). Handedness and Developmental Disorder.

Lippincott, Philadelphia. Corballis, M. C. (1991). The Lopsided Ape: Evolution of the

Generative Mind. Oxford Univ. Press, New York. Coren, S. (1993). The Left-Hander Syndrome: The Causes and Consequences of Left-Handedness. Random House, New York. Coren, S. (Ed.) (1990). Left-handedness: Behavioral implications and anomalies. Advances in Psychology, No. 67. North-Holland, Amsterdam.

Springer, S. P., and Deutsch, G. (1995). Left Brain, Right Brain. Freeman, New York.

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