- Education and Science
Right Versus Left Handedness
Left-handed people have always had rather a hard time of it. Ever since the dawn of history, Man has greeted his fellows with a gesture of the right hand - perhaps holding it up, palm forwards, perhaps offering it for a handshake. Even those whose right hand is not naturally their best-coordinated are expected to fall in with this convention. So, too, must they (in most cultures) write from left to right, use right-handed implements and everyday objects, and generally renounce what appears to be their particular gift of nature.
But why should nature operate in this way? Why should the vast majority of people prefer to use their right hand, while a small but persistent minority dissent? Why, indeed, should there be any preference for one hand at all?
This last question is perhaps the easiest to answer; dominance of one side of the body seems to be a natural consequence of the duplication of bodily organs. A great many organs are duplicated, especially those on the surface of the body. (Under the skin it is a different matter; only a few of the internal organs - such as the lungs, kidneys and sex glands - are doubled.) Where there is duplication, and where only one of the two limbs or organs is necessary to perform a task, one of them naturally tends to become dominant over the other, through practice and use. For example, it would be a great waste of effort to learn to write with both our left and our right hands.
As already hinted, it is not only our hands in which one side becomes dominant. In looking through a keyhole we use our dominant eye; we cock our dominant ear towards a distant caller; we shoot for goal with our dominant foot.
It may not be easy to decide whether you are right- or left-eyed; when you look through a keyhole you should stop and think. In fact, the eye or ear you tend to use may not be the naturally dominant one, for the non-dominant may be more sensitive. For example, a right-eyed person may suffer from bad astigmatism in this eye, thus making him rely more on his non-dominant left eye.
The same sort of thing can happen with handedness, but here it is training during childhood that may upset the natural dominance. This shows up in conflicting statistics on the incidence of left-handedness.
One survey found that 6.2 per cent of men and 3. 9 per cent of women were left-handed, with an average of 5 .1 per cent in the whole population. On the other hand, another study put the incidence of lefthandedness as high as 34 per cent in people who had not suffered parental interference to make them right-handed. Many youngsters are undoubtedly forced to change to right-handedness as soon as their parents see their left preference. So the numbers of natural left-handers may well be underestimated. But, taking many recent surveys into account, the general opinion is that in European countries between 6 and 10 per cent are left-handed.
Another bit of confusion that the surveys have thrown up is that there seems to be no connection between left-handedness and left-eyedness or left-earedness. Left-handers do, however, tend to be left-footers more often than not. In fact, investigators have found that the more tests they make, the fewer people turn out to have pure left-dominance. People who are unadulterated left-dominant in all things are very much a rarity.
Handedness has been, and in some countries still is, of great domestic or religious significance. Even today, it is taboo in certain cultures to eat with the left hand, this hand being 'unclean' because it alone is used for wiping after defecation. There are, however, a few languages which are written from right to left. Are there any more left-banders in these cultures?
It is perhaps surprising that there are not; surprising because to a left -handed person writing from right to left is the more natural way.
There are historical references to tribes or cults where left-handedness has been the norm. However, in the light of modern genetic studies it is apparent that left-handed inbreeding is an impossibility; left marrying left does not necessarily produce left-handedness. The Old Testament tribe of crack-shot slingers the Benjaminites (Judges 20:15-16), who were supposed to be entirely left-handed, probably had a majority of right-handers with a generous minority left-handed. The mere fact that the left-handedness was stressed suggests its unusual existence among the tribe.
Genetic studies have shown that handedness does have some hereditary basis. If the parents are left-handers then the odds are greater that their offspring will be left-handed, too. But this is not the whole story. Heredity is not the cause of left-handedness, but it helps to explain its transmission. Studies of left-handedness in twins has shed light on the inheritance factor. Left-handedness appears to be more common among twins than among the single born. But of the twins only one becomes left-handed.
The incidence of left-handedness in twins is about one in every five sets. This does show that heredity plays some part in the transmission of left-handedness, but environmental factors seem to tip the balance either way. Were the hereditary factors all-important, then both twins should be left-handed. There are no complete explanations as to why there IS an excess of left-handedness among twins; possibly their position or crowding in the womb have some connection.
The actual causes of handedness are not understood. There have been some amusing attempts to explain it, which have little but historical value. One such is the so called primitive warfare theory. This is based on the idea that soldiers go into battle right side foremost in order to offer maximum protection for their heart (left) side. But right dominance appears within the first few months of the child's life, and animals also show right dominance, so this theory can have little scientific significance.
It was also once believed that the body's center of gravity - that we are heavier on one side than the other - caused hand dominance, but this has since been disproved. Eye dominance enjoyed a longer span of favor in the explanation of handedness, but investigators have shown that the incidence of left-handedness is just as great among those born blind.
As already mentioned, right-side dominance is not restricted to the human species. Many animals show dominance, including very low forms of life such as the earthworm. Experiments on rats and monkeys have, in fact, helped to form the most influential theory so far developed on hand dominance. This is its supposed connection with cerebral (brain) dominance.
The human brain is made up of two halves, the cerebral hemispheres, which are connected by a thick bridge of nerve fibers called the corpus callosum.
If this bridge is severed, as has been done by surgical operation in order to prevent the spread of epilepsy, the two hemispheres will then behave just like two separate brains. The patient may be considered to have two seats of consciousness.
The nerve fibers leading from the left-hand side of the body are joined to the right-hand hemisphere. Similarly, the nerves from the right-hand side of the body are joined to the left-hand hemisphere. So movement. in the right-hand side of our bodies - including, of course, the right hand - are triggered from the left hemisphere, and vice versa.
Nature has it that one of these hemispheres becomes a dominant one. Surgical incision of the dominant hemisphere of an animal leads to a shift in dominance. A right-pawed rat becomes left-pawed after such an operation. If we can generalize from animals such as the rat to Man, then this shows that there may be a physiological basis for hand preference, and is a good reason why it is unwise to force a child to change his dominant hand.
The area of the human brain controlling speech is practically always centered in the left hemisphere irrespective of hand dominance. Some rare cases of right-hemisphere speech-center localization are found, and usually -but not always- these are left-handed people. Damage by surgery or accident to the left hemisphere often disturbs the speech processes of the person, a condition known as aphasia.
It was thought at one time that the speech center of the left hemisphere was intimately connected with the dominant hand, but experiments have shown that this is not necessarily the case. Usually, the dominant hand goes with the hemisphere containing the speech center, but scientists have found exceptions to this. It is not really possible to tell whether a person is right or left-brained until either injury or a brain operation occurs causing aphasia.
Damage to the right hand of a right-handed person does not always mean a change in their brain dominance. In such cases, the left hand has to be used because there is no alternative, but the right hand is still their dominant one. A change in brain dominance can occur, however, for there is no structural difference between the two hemispheres. It is most likely if the person involved is young, and even more so if their speech has not fully developed. So a child whose left hemisphere is damaged at birth will develop right brain dominance.
Ambidextrality - equal ability with both left and the right hands -may result from lack of brain dominance in the first place, or from intensive training of the non-dominant limb. Pianists have to practice hard in order to make their left hands just as supple as their right. Often, over-practice of the non-dominant hand occurs, giving rise to greater emphasis to parts of the music where it is not desired. As with all manual skills, training while young is best. Attempts to write with the non-dominant hand are much less of an ordeal during school years than later on in life.
Statistics show that natural ambidextrals number less than 2 per cent of the general population. Brain dominance is a natural state (though its causes are a mystery), and however hard training takes place, nothing can better the actions of the prominent limb. Loss of the dominant hand by injury means that the victim must rely on his non-dominant hand. But skill in the new limb can never hope to match that of the deceased dominant limb.
Are left-handers just as skilful as right-handers?
This very much depends on the task being performed.
Britain's Top Secretary, 1965, Mrs Nancy Hall, was a left-hander, and could type at 80 words per minute and write shorthand at 130 words per minute.
Left-handed sportsmen and women are very common. But there are, many tasks at which the left-hander is put at a disadvantage solely because our civilization is geared to right-handedness.
It has been found that there is a greater incidence of left-handedness among the mentally subnormal. It appears that poor dominance -either weak left-handedness or ambidextrality- tends to go with retarded speech development. Thus, left-handedness is a result of this subnormality- never a cause of it: A forced change in dominance, from left to right-handedness, has been thought to cause stuttering in children. Surveys have shown that there is, in fact, no real connection between the two. Stuttering does not appear in those who have been forced to change dominance through injury.
Stuttering may have a psychological or a physiological cause. A child forced to change his preferred hand may show his emotional disturbance by developing a stutter. At the critical time when speech is developing, any tampering with the child's dominance may cause some damage which might result in a stammer. It is desirable that the child develops a strong dominance as early as possible to ensure adequate speech center dominance.
Dominance appears early in life; some investigators say as early as two to three months. A first sign is in the so-called tonic neck reflex; a potential left-hander will turn the head to the left more often than to the right. By the age of two years, the dominance should be more noticeable. Care must be taken, however, because children at this age tend to go through phases of bi-laterality before settling down to a preferred hand.
Strangely, it is easier to find laterality in boys than girls early in life. All studies have shown that dominance is well settled before school days start.
Experience and training are the best ways of strengthening hand dominance - handling toys or tools, or any situation which requires delicate or sensitive use of the one hand. There is no way of teaching hand dominance at least until we understand exactly what causes it. It will develop of its own accord, the more definite the dominance the better.
The left-hander very quickly accommodates to our right-handed society. Perhaps this is what has tempted some psychologists to accept left-handers as slightly cussed but determined characters! The little things like door handles (always positioned for right-handers) and kitchen sink units (usually with the draining board on the left) are perhaps easier to accommodate to than the old telephone boxes (where writing a message requires the left-hander to go into contortions) or writing checks.
There is a bank in America that issues check books with the stubs on the right-hand side specifically for left handers - the check book can then be held steady with the right hand on the stubs part!
For some strange reason, most refrigerator doors open left-handedly - unlike oven doors which are right-handed. Desks with the ink well on the right-hand side cause many blotches on a young left-hander's school work. Official forms are designed to be filled in with the greatest ease by right-handers; in many of them the spaces to be filled in are all on the right-hand side.
Designers must obviously cater for the majority, and as there are more right-handers to get irate, then the side to cater for is obviously the right-handed one. But perhaps left-handers have more to complain about when their particular gift of nature takes on more unpleasant undertones. For out of leftness came the words gauche and sinister - as opposed to rightness, righteousness and dexterity. To cap it all, there is the Muslim belief that on the day of judgement the good will receive the book recording their deeds on earth into their right hand, and will go to heaven. The wicked will receive their record in their left hand, and go to hell. It hardly seems fair to left-handers.