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 indicater of handedness as it might seem, because many people who write with their right hand use their left for everything else.
Approximately 10-13% of the population is left-handed. People who can use both hands equally well are ambidextrous. True ambidexterity is rare.
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:
- 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.
- It predicts that more men would be right-handed than women. However, data indicates that more males are left-handed than females. Analysis of ancient cave paintings indicate that humanity was right-handed long before the bronze age.
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.
- 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.
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.