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| |    What Is 'Handedness'? by Robert Needlman, M.D., F.A.A.P. reviewed by Laura Jana, M.D., F.A.A.P.
While many people are all right-sided or all left-sided for these functions, others have a combination of left and right dominance. Hand dominance also can vary depending on the task: Some people write with the pen in their right hand, but throw, swing a tennis racket, unscrew the lid of a jar, and do virtually everything else left-handed. Choosing sides Because handedness is not just about hands, it's useful to talk more generally about laterality, which is the specialization of a particular side of the body to perform a particular task. Another way to think about laterality is that it is the opposite of symmetry, which is the idea that the left and right sides are mirror images of each other. Symmetry is clearly only skin deep. On the inside of the body, the left-right differences are obvious: The liver sits all the way over on the right, the spleen all the way on the left. The right lung has three lobes, while the left has only two, to make room for the heart--which sits more or less in the middle of the chest, but is somewhat pushed over toward the left. (A very few people have their organs in reverse position, with, for example, their heart sitting more over to the left of their chests. It's very rare for the reversal to be complete. More often, some organs are switched, while others are in their usual position or near it.) Degrees of handedness One way that researchers determine handedness (and foot, eye, and ear preference) is to ask questions like the ones at the beginning of this article. Another way is to actually test hand skill--for example, by timing a person putting pegs in holes or finger tapping. By measuring the difference in skill (accuracy or quickness) between the right and left hands of many people, you can see that handedness isn't a matter of either-or, but actually a matter of degree. In thinking about handedness, you need to ask two related questions: Which functions do you consistently do with one side of your body or the other? And for any given function (say, throwing a ball or screwing in a light bulb), how consistently do you do that with the preferred side? Some actions are more "handed" than others. For example, when drawing or writing, children almost always use the same hand to hold the pencil every time. But when they go to touch their nose, many of them (30 to 40 percent, depending on age) use either hand; and in placing beads in a bottle, up to 60 percent swap hands without thinking about it. Among adults, some of the least handed activities are carrying a suitcase, holding a dog's leash, and lifting the lid off a box; among the activities most likely to be always done with the same hand are hammering, throwing a ball, hitting a tennis ball, and, of course, writing. And if you have never paid much attention to the subject of handedness, consider this fact: Less than 50 percent of adults always use the same hand while stirring with a spoon, but more than 80 percent stick to their preferred hand while eating from a spoon. In general, activities that are more hand-specific are ones that either require a lot of practice and fine detail (writing) or the coordination of large muscle groups for a sudden, smooth action (throwing a ball). The fact that these sorts of activities take up a lot of brainpower provides a clue about the relationship between handedness and the brain. More information: Talk about:
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