The inconsistency and lability of preferences in lower animals probably reflect their lack of an upper-limb dexterity comparable to man's. That is, the range of actions available to them — pushing, reaching for, or picking things up — usually involves whole-limb movements which require an accompanying postural body adjustment. Such movements are often stereotyped and appear to be controlled by lower centres in the brain. In man and the higher primates, an individual's handedness becomes more marked the greater the manipulative skill required in an action, where movements independent of bodily position are controlled by higher cortical brain circuits.
Human handedness, therefore, appears to have developed along with, and to be a feature of, man's uniquely high level of finger dexterity and capacity to make and use tools. Anthropological evidence, although necessarily indirect, suggests that handedness first appeared in the lower Stone Age when tool making became common. Some tools, for example, appear to have been made by right-handers for right-hand use.
By historical times the predominance of right-handedness was well enough known for its origin to be discussed and for sinistrality to be noted as exceptional. Aristotle discusses ambidexterity as a problem for contemporary ideas of the inherent superiority of the right side of the body. The Old Testament notes two cases of left-handedness: a group of 700 sinistral sling shooters in the tribe of Benjamin, and Ehud the Benjamite who stabbed an enemy king with his left hand.
Once established, manual asymmetry quickly attracted a potent mythology associating the right side of the body and the right hand with things that are good, strong, pure, and honourable, while the left in contrast was equated with evil of all kinds. This symbolism has pervaded nearly all cultures (except the Chinese) throughout the ages. Ancient Greeks and Romans regarded the left side as inferior and profane, and in medieval times use of the left hand was associated with witchcraft. Arabs still regard the left as the 'unclean' hand and forbid its use in normal human contact. The odium of the left remains today in terms such as sinister, gauche, and cack-handed.
Right-handedness is of interest to psychology and neurology in that the development of a consistent hand preference in relation to fine motor skills may be linked to the development of hemispheric specialization in the human brain for certain cognitive functions. It is known that in 95 per cent of dextrals language is mediated exclusively by the left hemisphere, that is, by the hemisphere controlling the right side of the body, including the preferred hand. Originally this 'cerebral dominance' for speech was thought to imply that the left hemisphere was the vehicle for all man's highest mental functions. Subsequently, however, it was established that the right hemisphere plays its own dominant role in tasks requiring spatial perception, reasoning, or memory, as in map reading and the recognition of faces, patterns, and melodies. This complementary specialization of the two sides of the brain is, like strong right-handedness, peculiar to man, so they may have evolved together.
Handedness may, however, reflect more directly the cerebral basis of sensorimotor function. Clinical and physiological evidence suggests that in right-handers the left hemisphere has sensory and motor connections to both sides of the body, whereas the right hemisphere is almost exclusively unilaterally connected. The former may thus have a dominant role in integrating activity of the two sides of the body to make a coordinated behavioural unit. If so, voluntary control of the right hand may be easier, and a preference for its use in delicate manipulative skills may emerge as a result.
In accordance with this idea, although it is not yet known exactly what the difference is between the hands in terms of skill, the evidence to date suggests that it centres on timing and the coordination of movements into sequences. Single-finger contractions can be made as precisely or as fast with the left as with the right hand, and with practice either hand may run off pre-programmed sequences of movement (as in typing or piano playing) equally well, provided they do not require adjustment during their execution. But tasks involving visual aiming (such as peg placing or threading a needle) or serial adjustment (as in turning a crank handle or threading a nut on to a bolt) yield consistent differences in performance between the hands. Current theories of the basis of handedness include: (i) increased variability in the left arm's execution of movements, so that they require more frequent correction; (ii) longer time lag in the correction of inaccurate movements when they occur, especially using vision; (iii) greater irregularity in timing of movements by the left hand; and (iv) a greater repertoire of movements available to the right hand because it has had more practice (this last being a rather circular argument).
Another explanation of dextrality is based on the predominant use of the right hand for gesturing, that is, for movements of communication rather than manipulation. This again links it with the development of speech, itself seen as man's most sophisticated form of communication. Certainly the neural motor systems of speech and the right hand are adjacent, and motor activity in the latter is strongly associated with speech generation. Talking in dextrals is accompanied by gestures mainly of the right hand and arm (whereas arm movements in other situations are not specifically dextral), and talking interferes with a concurrent motor activity in the right hand, such as balancing a rod on one finger, more than in the left. Musicians report that conducting rather than fingerwork is the most difficult action to perform with the non-preferred hand, implying that handedness consists of a 'closer, more immediate availability of the right hand as the instrument of the individual's conceptions and intentions'.
Any theory of handedness has to account for the minority of left-handers found in all human societies, a proportion variously reported as from 4 to 36 per cent. The classification of subjects is itself a problem, because most sinistrals are less well lateralized than dextrals. Their preferences and usage are weaker and more changeable both within and across tasks, that is, they may prefer to use the right hand for any actions occasionally or for some tasks regularly. There seems, moreover, to be no hierarchy of preferences, nor any activity common to all sinistrals suitable for selecting them as such (although writing, throwing, shooting, and using scissors have been used for the purpose).
Nor is it clear whether handedness is a continuum (albeit an unusual J-shaped one) ranging from total dextrality to total sinistrality or comprises discrete categories and, if so, whether there are two groups (left and right) or a third 'ambilateral' (mixed-handed) group as well. Much variability in results between different studies of handedness stems from this ambiguity. The great variety of laterality measures used — including grip strength, touch sensitivity, eyedness, and writing hand — also adds to the confusion, especially as many have little relation to dexterity and do not intercorrelate.
On tests of manual speed, accuracy, and steadiness, the performance of pure sinistrals mirrors and equals that of dextrals, both showing a distinct superiority of the preferred hand. In ambilaterals of all kinds (including most self-professed left-and some right-handers) the two hands are more equal in skill, and neither performs quite as well as the preferred hand of the strongly lateralized.
Cerebral specialization and dominance in sinistrals is less clear cut than in dextrals. Speech is still represented in the left hemisphere in 70 per cent of left-and mixed-handers, but 15 per cent have right-hemisphere language and 15 per cent bilateral representation. Sinistrals may suffer aphasia from damage to either hemisphere, but recover from it better than do dextrals. Similarly equivocal results are found for other skills, suggesting a bilateral brain involvement in all mental functions. Some investigators claim pure left-handers are the mirror opposites of dextrals while ambilaterals are a separate group. Others distinguish 'familial' left-handers (with sinistral relatives) from 'non-familials'. Others again classify all sinistral brains as 'imperfectly developed' or 'undifferentiated'.
Theories of the origins and distribution of handedness are of three kinds. (i) Learning-cultural theories claim that it is produced by social pressures or early experience, especially of tools designed for right-hand use. These (mostly older) theories are too imprecise to predict accurately the handedness of any individual or the distribution of handedness in any group. (ii) Genetic models (see genetics of behaviour) suggest handedness is inherited, although the exact mechanism is as yet unknown. Theories include a two-gene model and a right-shift model in which pure right-handedness may be inherited from a dominant gene but, if not, the degree of laterality to either side is determined randomly by environmental factors. (iii) Brain-damage theories hold that neonatal injury shifts cerebral dominance and handedness or prevents normal hemispheric specialization. They explain thereby the higher incidence of sinistrality found in such pathological states as learning disability and epilepsy. Whether they account for left-handedness in the population generally is debatable.
(Published 1987)
— K. A. Flowers
- Bibliography
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