JMS Pearce
Hull, England, United Kingdom

 

Handedness (chirality) refers to the preferential use of one hand over the other. It is a matter of degree; it is seldom absolute. Population left and right preference existed in the Neanderthals (lived from 400,000 to about 40,000 years ago) onwards. Only homo sapiens amongst the great apes shows strong and consistent population level biases, indicating that both direction and strength of right-handedness may have emerged after divergence from the last common ancestor. Geschwind pointed out:

the superiority of one cerebral hemisphere in acquiring certain kinds of learned behavior—is a distinctive biologic attribute of man, and is, as far as is now known, either absent or poorly developed in any other mammal.¹

 

Vertebrate and invertebrate “handedness”

Laterality and handedness exist in other vertebrates but are also weaker than in man.² Right-handedness—evident in manipulative tasks, reaching, and throwing—only slightly exceeds left-handedness in chimpanzees and gorillas, but orangutans show population-level left-handedness.³ By contrast, 90% of humans are right-handed. There is much variation within and between species.

Many flatfish have both eyes on one side of the head (right-sided in plaice, left in turbot). Most gastropod mollusk shells have an aperture on the right-hand side of the base with a right-handed spiral or dextral shell, but some are left-coiling species (e.g. lightning whelk, Sinistrofulgur perversum). In chicks and pigeons, the development of visual asymmetries depends on asymmetrical light stimulation during ontogeny. Spiders display lateralized attack behavior and bees use the right antenna to associate smell with a food reward. There are numerous examples which show that laterality may relate more to hemispheric differences in language, sensory, and motor processing than to limb preferences. Thus, a brain can be lateralized without that lateralization being manifested as handedness.⁴

In non-humans, limb preference in the individual is associated with aspects of behavior, defense against predators, and feeding. Many aspects of the mechanism remain unknown.

 

Human handedness and language lateralization

The development of hemispheric asymmetries (lateralization) is controlled both by genes and environment. Hand preference is first evident in ten-week human embryos who begin to suck their right thumb and wave their right arm more than the left. Lateralization is much stronger in humans than in primates or other species. It applies not only to language and handedness but to several other perceptual and cognitive skills. Across cultures, about 90% of people are right-handed (dextrals) for most manual tasks whereas 10% prefer the left hand (sinistrals) with more males than females; mixed-handedness is common but only 1% have no clear preference—so-called “ambidextrous.” Different assessment tasks produce different estimates, and the nature of the task tested and its difficulty influence the degree of lateralization observed.

In healthy subjects there is a consistent and almost linear relationship between the degree of handedness and the direction of language dominance.⁵ The cerebral hemisphere that contains the neural networks for speech is known as the dominant or major hemisphere; it is the left hemisphere in almost all right-handed people. However, like handedness, cerebral dominance is not absolute. The right-hemisphere is dominant for language in 4-7% of right-handers⁵ and in about 30% of left-handers. The evolutionary and biological reasons for this difference are uncertain.

 

Etiology

Interestingly, the normal DNA double helix is right-handed. And strangely, neutrinos are overwhelmingly left-handed and antineutrinos are similarly right-handed. In man, left-handedness is sometimes familial and concordance of handedness is greater in monozygotic twins than dizygotic twins,^6,7 with an estimated heritability of 25%.⁸ But it is far from complete, since over 20% of monozygotic twins with identical genotypes are discordant for handedness. If the left cerebral hemisphere is damaged early in life, the right hemisphere may take over the speech functions of the left; thus, infantile right hemiplegia is not associated with dysphasia. The idea that left-handedness is commonly caused by brain damage in utero or infancy has been shown to be untrue. This leads to the important distinction between so-called natural and pathological left-handedness.⁹ Childhood training can force naturally left-handed children to use their right hands for writing and other tasks; they are called “shifted sinistrals.”

 

Consequences of handedness

In both the scientific and popular literature, personality traits and physical, perceptual, and cognitive skills have been associated with handedness. Left-handers may struggle with scissors, screws, fountain pens, musical wind instruments, golf clubs, and many everyday objects designed for right-handers. Left-handedness is disparaged by such words as gauche, sinister, and maladroit. It is debatable if the prevalence of left-handedness is higher in people with better verbal and mathematical ability. After much past discourse, we can conclude that human handedness does not affect longevity. It correlates statistically with various cognitive and psychiatric disorders, though correlation does not equate with causation.

 

Cerebral hemisphere dominance

While handedness, language, and cerebral dominance at first seem simple, their evolution, etiology, and consequences are complex. Lateral asymmetry (lateralization) in man includes handedness, use of feet, visual, and auditory perception (e.g., dichotic listening).¹⁰ Human right-handedness, indicating a left-hemisphere (LH) bias, is associated with language bias to the left hemisphere. There are structural asymmetries in humans and other higher species for both the inferior frontal gyrus (Broca’s area) and the posterior temporal lobe (Wernicke’s area), which may underlie the functional specializations for language. Functional magnetic resonance imaging (fMRI) shows that 96% of right-handers and 76% of left-handers have LH language dominance; many linguistic tasks however, elicit bilateral activity. Functional transcranial Doppler sonography shows similar results.⁵

Wernicke-Geschwind model: Saying a written word.11

The recognition of the dominance of the left cerebral hemisphere for speech dates back to Marc Dax, whose thesis presented in 1836 but not published till 1865, was entitled Lesions of the left half of the brain coincident with forgetfulness of the signs of thought. Hughlings Jackson, Broca, and Wernicke extended his work in pivotal publications, hence the anatomical designations Broca’s and Wernicke’s areas. (Fig 1)

Natural handedness is important for neurological diagnosis, because if a right-handed patient is rendered dysphasic, this indicates a lesion of the left hemisphere. Since 70% of left-handers are also left hemisphere dominant, dysphasia commonly occurs in them after a left hemisphere lesion. Dysphasia is generally less severe, with a better prognosis among left-handers regardless of the hemisphere damaged.

The control of both handedness and language can also shift to the right hemisphere after longstanding left-hemisphere lesions. This indicates the plasticity of brain function, a feature notable in both individual and in social demands driving behavioral lateralization.

Traditionally, human handedness is determined by two alleles, one for right-handedness and one for left-handedness.¹² Wiberg and colleagues have identified genome-wide significant loci for human handedness and genes associated with brain development.¹³ Handedness was not controlled by a single genetic locus. Similarly, using data from the UK Biobank, a genome-wide association meta-analysis showed forty-one loci associated with left-handedness and seven associated with ambidexterity. These several genetic pathways included regulation of microtubular proteins and brain morphology. There are also suggested, genetic correlations between left-handedness and dyslexia, schizophrenia, and bipolar disorder.¹⁴

In a highly complex and often long-winded literature, variation of test methods explains several conflicting results. For instance, the nature of the task and the context tested influences manual choice. Population-level right-handed bias is greater for manipulative activities than for communicative gestures.¹⁵ This led to the plausible theory that human right-handedness is a trait which resulted from tool use that was inherited from an ancestor common to both humans and great apes.

Human left hemisphere dominance progressively decreases from right-handed, to mixed-handed and left-handed people. Right hemisphere language dominance increases linearly with the degree of left-handedness, from 4% in strong right-handers to 27% in strong left-handers. These results demonstrate that the relationship between handedness and language dominance is a natural phenomenon.⁵ Hand preference based on a dominant hemisphere suggests that populations or species with a majority of either left- or right-handed individuals have adapted to cope with particularly important environmental factors using acquired patterns of behavior.

 

Discussion

Hand preference is but one manifestation of lateralization. It is polygenic and depends on multiple genetic factors, environmental pressures to adopt right-handedness, and in some, minimal brain damage in utero or infancy. Many unsolved issues underlie the search for evidence of handedness in primitive tools and fossil morphology, and in live tests of manual tasks and related behavior in animal species.

A crucial but unsolved problem is why homo sapiens is so predominantly right handed while in most other animal species, left and right preference occurs in approximately equal numbers. Teleologically, this trait has clearly evolved and therefore “must” favor Dawkins’ selfish gene, and hence survival. Handedness and laterality, to be useful, should provide advantages in reproduction, feeding, and defense against predators. Changing environments in time might demand changing patterns of manual use and neural connections. A gradual evolutionary adaptation of lateralization and limb preference should facilitate survival. There is, however, sparse evidence of biological advantage in human handedness, right or left.

Population-level asymmetries are plainly present in human and nonhuman primates, but what evolutionary forces cause them and the mechanisms that underlie their expression remain unknown. Annett suggested that at some point in evolution, a putative genetic factor named the “right-shift factor” displaced speech function to the brain’s left hemisphere, thus conferring advantage to the left hemisphere for speech and handedness.¹² What caused this shift she did not specify, and there is little evidence for significant biological advantage in right- or left-handed humans.

 

References

1. Geschwind N. Cerebral Dominance and Anatomic Asymmetry. New Engl J Med 1972; 287:194-195.
2. Fitch WT, Braccin SN. Primate laterality and the biology and evolution of human handedness: a review and synthesis. Ann. N.Y. Acad. Sci 2013; xxxx:1–16.
3. Hopkins WD. Neuroanatomical asymmetries and handedness in chimpanzees (Pan troglodytes): a case for continuity in the evolution of hemispheric specialization. Ann N Y Acad Sci. 2013;1288(1):17-35.
4. Rogers LJ. Hand and paw preferences in relation to the lateralized brain. Philos Trans R Soc Lond B Biol Sci. 2009;364(1519):943-954.
5. Knecht S., Dräger B., Deppe M., Bobe L., Lohmann H., et al. Handedness and hemispheric language dominance in healthy humans, Brain2000;123 (12): 2512–2518.
6. Dahlberg, G. Twin Births and Twins from a Hereditary Point of View, Stockholm 1926.
7. Medland S.E. Genetic influences on handedness: data from 25,732 Australian and Dutch twin families. Neuropsychologia. 2009;47:330–337.
8. Medland SE, Duffy DL, Wright MJ, Geffen GM, Martin NG. Handedness in twins: joint analysis of data from 35 samples. Twin Res Hum Genet 2006; 9: 46–53.
9. Brain WR. Speech And Handedness. [The Bradshaw Lecture RCP] Lancet 1945;246:837-842.
10. Lishman WA, McMeekan E.R.L. Handedness in Relation to Direction and Degree of Cerebral Dominance for Language. Cortex 1977;13(1):30-43.
11. Kuhnke, P. “The Functional Neuroanatomy of Speech Perception Insights from Electrocorticography on an Epilepsy Patient Watching a Movie.” (2014). https://www.semanticscholar.org/paper/The-Functional-Neuroanatomy-of-Speech-Perception-on-Kuhnke/fec14c0fa06b8d2e5c65d7f09623d9fab066732e/figure/2
12. Annett M. Handedness and brain asymmetry: The right shift theory. London, England: Erlbaum. 2002.
13. Wiberg A, Ng M, Al Omran Y, et al. Handedness, language areas and neuropsychiatric diseases: insights from brain imaging and genetics. Brain. 2019;142(10):2938-2947.
14. Cuellar-Partida, G., Tung, J.Y., Eriksson, N. et al. Genome-wide association study identifies 48 common genetic variants associated with handedness. Nat Hum Behav 2021;5, 59–70.
15. Cochet H, Byrne RW. Evolutionary origins of human handedness: evaluating contrasting hypotheses. Anim Cogn (2013) 16:531–542.

 

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JMS PEARCE, MD, FRCP, is emeritus consultant neurologist in the Department of Neurology at the Hull Royal Infirmary, England.

 

Fall 2021  |  Sections  |  Neurology