Left-handed

People who are left-handed are more dextrous with their left hand than with their right hand: they will probably also use their left hand for tasks such as personal care, cooking, and so on. Writing is not as good an indicator of handedness as it might seem, because many people who write with their right hand use their left for everything else.

Causes of left-handedness

No one knows for certain why the human population is right-handed-dominant, but a number of theories have been proposed.

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Evolutionary theories

The warrior and his shield

This theory attempts to explain left-handedness by the position of a warrior's shield and his heart. Basically, since the heart is on the left side of the body, a right-handed warrior (who holds his shield with his left hand to free the right hand for a weapon) would be better able to protect his heart and therefore more likely to survive.

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There are a number of objections to this theory:

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• The heart is not that far off center. Protecting it with a shield would result in a weak selective pressure.
• There have not been enough generations since the Bronze Age to make a difference.
• Analysis of ancient cave paintings indicate that humanity was right-handed long before the Bronze Age.
• Some believe it predicts that more men would be right-handed than women. However, data indicates that more males are left-handed than females. This objection also demonstrates a misunderstanding of heredity. The theory would predict that fewer left-handed males would survive but says absolutely nothing about ratios of male:female left-handedness after that time. Nor does it explain why there would be either right-or left-handedness to begin with.

Advantage in fist-fighting

A variant of the above argument says that left-handed people have an advantage in fighting without weapons, because of the "surprise" factor. (This fact is well known to boxers.)

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A 2004 study by Charlotte Fauriet and Michel Raymond of the University of Montpellier II in France, published in the Proceedings of the Royal Society, argues that there is such a link. The researchers hypothesize that left-handed inheritance is likely to be associated with violence, because violent left-handed people would be more likely to benefit from the advantage in fighting. They found a positive correlation between murder rates and percentage of left-handed people in several traditional societies: The more left-handed people, the higher the homicide rate.

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The researchers argue that left-handed people are not more violent than right-handed, but that violent left-handed people are more likely to be successful than violent right-handed peoplehttp://www.economist.com/science/displayStory.cfm?story_id=3471297.

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Brain hemisphere division of labour

This is the most commonly accepted theory of handedness. The premise of this theory is that since both speaking and handiwork require fine motor skills, having one hemisphere of the brain do both would be more efficient than having it divided up. And since in most people, the left side of the brain controls speaking, right-handedness would prevail. It also predicts that left-handed people would have a reversed brain division of labour. Lastly, since other primates do not have a spoken language (at least of the type we have) there would be no stimulus for right-handed preference among them, and that is true.

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Objections:

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• It does not explain why the left hemisphere would always be the one controlling language. Why not 50% of the population left and 50% right?
• While 95% of right-handers do indeed use the left side of the brain for speaking, it is more variable for left-handers. Some do use the right for linguistic skills, some use the left hemisphere, and others use both.

On the balance, it appears that this theory could well explain some left-handedness, but it has too many gaps to explain all left-handedness.

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Is left-handedness genetic?

Handedness runs in families, although even when both parents are left-handed, there is only a 26% chance of their child being left-handed. Thus, it is clear that genetics is not the only cause. Handedness must also be influenced by some of the other theories presented here.

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Apparently, the Clan Kerr of Scotland built their castles with counter-clockwise staircases, so that a left-handed swordsman would be better able to defend it. However, a 1993 study found no statistically significant increase in left-handedness among people with the family name Kerr or Carr.

Related Topics:
Clan Kerr - 1993

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Many members of the British royal family are left-handed. Genetics is usually used to explain this.

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Environmental theories

Birth stress

Left-handed people cringe at this theory, because its basic premise is that left-handedness is due to brain damage during the birth process. Some statistics support this theory.

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Difficult or stressful births happen far more commonly among babies who grow up to be left-handed or ambidextrous. Birth stress is also associated with a number of birth defects and complications, including cerebral palsy and autism.

Related Topics:
Cerebral palsy - Autism

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There are objections to the birth stress theory:

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• Throughout history and throughout the world, the level of medicine and technology to assist with childbirth has improved. In spite of that, the proportion of left-handed people has not decreased.
• It does not explain why humans are right-handed by default, with only birth stress making them left-handed. It could, however, explain left-handedness in combination with some of the other theories presented here.

Ultrasound

Scientists in Sweden say they have found evidence ultrasound scans may cause brain changes in unborn babies after they found men whose mothers had tests were more likely to be left-handed. The study suggested scanning produced an extra three left-handed babies per 100 births. This research has come under some criticism since it implies that left-handed people are in some way "brain damaged".

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Parental pressure

This theory explains right-handed dominance by claiming that since the parents who raised us are mostly right-handed, we came to be mostly right-handed and so on.

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Objections:

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• It does not explain how right-handed dominance started in the first place.
• The handedness of children is more closely related to their biological parents than to adoptive parents.
• It does not explain why left-handedness has persisted for so long.
• This theory predicts the existence of isolated societies dominated by left-handed people, although none have been discovered.