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Left-handed

Table of contents

1 Statistics of left-handedness 2 Causes of left-handedness 2.1 Evolutionary theories 2.1.1 The warrior and his shield 2.1.2 Brain hemisphere division of labour 2.1.3 Is left-handedness genetic? 2.2 Environmental theories 2.2.4 Birth stress 2.2.5 Parental pressure 3 Social stigma and repression of left-handedness 4 Left-sidedness 4.3 In humans 4.4 In animals 5 Source 6 External link

Statistics of left-handedness

Approximately 10-13% of the population is left-handed. People who can use both hands equally well are ambidextrous. True ambidexterity is rare. See laterality.

Generally, males are three times more likely to be left-handed than females. Statistically, one twin of a pair has a 20% chance to be left-handed. Gay people may be up to 39% as likely to be left-handed as straight people (Habib, 2000).

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.

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 warrior holding his shield with his left hand would be better able to protect his heart than if he held it with his right. Thus, a greater mortality of left-handers would explain the prevalence of right-handedness today.

There are a number of objections to this theory:

1. The heart is not that far off center. While it is on the left side of the body, it is still fairly central in location. Protecting it with a shield would only result in a weak selective pressure, and there have not been enough generations since the bronze age.
2. It predicts that more men would be right-handed than women. However, data indicates that more males are left-handed than females.
3. Analysis of ancient cave paintings indicate that humanity was right-handed long before the bronze age.

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.

Apparantly, the Clan Kerr of Scotland built their castles with counter-clockwise staircases, so that a left-handed swordsmen 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.

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

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. Unfortunately, some statistics do back this theory up.

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.

There are objections. You can breathe easy now:

1. 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. (In a sense, it has increased because more people see left-handedness as the benign trait it is.)
2. 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.

Throughout history being left-handed was considered as negative - the Latin word sinister meant "left". Hence the many negative connotations associated with the word "left-handed": clumsy, awkward, unlucky, insincere, sinister, malicious, and so on. There have been, however, many famous left-handed people, and the associated right brain hemisphere that is said to be more active in left-handed people, has been found in some circumstances to be associated with genius and is correlated with artistic and visual skill.

Until very recently in Taiwan, left-handed people were strongly encouraged to switch to being right-handed (or at least, switch to writing with the right hand). It is more difficult to write legible Chinese characters with the left hand than it is to write Latin letters. Remember that "easy" and "difficult" depend on the person using those terms, so your writing may be neater. Because it is supposedly easier to write when moving your hand towards its side of the body, it is easier to write the Roman alphabet with your right hand than with your left. Conversly, Arabic and Hebrew, which go from right to left, would be easier to write with the left hand. Again, "easier" and "harder" are subjective.

It is possible that sun worship relates to the association of the left with evil. People in the northern hemisphere, looking south, would see the sun rise on their left, move rightwards across the sky, and set on their right. In the southern hemisphere the opposite happens. Among cultures from the southern hemisphere, right-handedness is still dominant. No study on left-side connotations from those cultures has been done.

However, since most sun-worshipping cultures see the setting sun as it dying or vanishing, the right side would indicate the negative associations associated with a setting sun. This is the opposite trend from that

Left-sidedness

In humans

Studies show that left-handedness does not necessarily correspond with "left-sidedness" (using your left foot to kick with, for example). The same thing holds with "eyedness."

In animals

Most primates also exhibit a preference for using one hand over the other although their populations are not right-hand preferential.

Source

• Habib, Marlene (2000). http://www.canoe.ca/Health0007/06_hands.html
• http://www.sciencedaily.com/releases/2000/07/000710071931.htm

• The Gay Gene: Two Trait Profiles--Human Handedness & Human Sexual Orientation