Handedness is the preference for using either the left or the right side of the body for certain things. People are described as left-handed or right-handed when they prefer to write with their left or their right hand. They may prefer the use of certain hands for certain tasks.
Handedness is caused by the lateralisation of brain function. Lateralisation is not only found in humans: it is found in many kinds of animals. Purposeful movements of the limbs are controlled by the pyramidal motor system in the cerebral cortex. As the motor nerve fibres move down through the lower part of the brain, they cross over to the opposite side of the body. So, the movements on each side are controlled by the opposite side of the brain.
Handedness seems to follow from the brain hemisphere division of labor. Since in most people the left side of the brain controls speaking, right-handedness predominates. This theory predicts that left-handed people have a reversed brain division of labor.
However, many animals also have handedness. For example: elephants often have preferences for whether they swing their trunks to the left or the right. Honeybees have right antennas that are more sensitive to smells. Parrots can be left- or right-footed, and some don’t mind (they are “ambidextrous”). Animals as different as chickens and minnows like to look for food with one eye and look out for predators (animals that might eat them) with the other. This seems to help them to do two things at once.
There is a simple theory for animals which learn difficult skills. It is efficient to learn difficult skills for one hand, rather than learn skills less well for two hands. Consider how much time it takes a young person to write well. Then, if they want to write with the other hand, it takes almost as long again to reach a similar standard. This general rule applies to many skills which an early human might value, such as throwing a spear or rock. The extra benefit from handedness would have influenced past survival, and resulted in the cerebral cortex developing a bias for handedness.
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References[change | change source]
- Annett, Marian 2013. Handedness and brain asymmetry: the right shift theory. London: Taylor Francis. ISBN 9780415648264
- Non-purposeful (involuntary) movements are not controlled by the cerebral cortex. They are controlled by the more ancient parts of the brain in the brain stem.
- Rosenbaum, David A. 1991. Human motor control. San Diego, CA: Academic Press, p. 411. ISBN 0-12-597300-4
- Banich, Marie 1997. Neuropsychology: the neural bases of mental function.
- Article “Southpaws” by Nora Schultz: New Scientist 1 May 2010 pages 36-39.