handedness: "Which hand are you writing your notes with today?

"handedness" turns out to be a little more complicated than first glance suggests:

Humans usually vary in their relative fine motor skill between the two hands; most commonly individual are more skilled, quicker, and stronger with their right hand. As a rough estimate, most young to middle aged adults are approximately 10% faster on simple motor tasks (such as finger tapping, poking targets with a pin, or inserting pegs in holes) with their right ("dominant" hand) and are approximately 10% stronger on grip strength. There is wide variability in the difference among people; and several factors (strength training, injury) can alter this balance. In some individuals the difference in speeded responding, coordination, and strength is much less than usually seen; in a few individuals there may be little or no difference in some of these aspects; and in some individuals (possibly 10%) the left side of the body is "dominant" (e.g., "faster, stronger, better", a line from an old sci-fi show on TV).

Measuring "dominance" can be difficult--because different approaches to measurement can give (usually slightly) different answers: can can ask the person which hand they write with for instance, or watch them to see which hand they use. We can also be interested in which hand they use to hold a spoon/fork with (but some cultures hold some implements in the non-dominant hand), to throw a ball with, to catch with, to hammer a nail, etc. We might also become interested in which eye they sight a gun or rifle with, and which foot they use to kick a ball. Usually the answers will be the same for a person (writes "right", sights "right", kicks "right") but sometimes there will be differences.

Hand writing, for instance, usually shows a "natural" preference in most children, with approximately 90% coming to write with their right hand and about 10% their left. But this preference can be overridden, to some degree, by training (and punishment). You may have known some, for example, who accident broke their preferred arm/hand while in school and had to learn to write with the other hand. It's frustrating but learnable, as many people have discovered. Most children will return to the "natural" preference as soon as circumstances allow. And there are a few individual who seem equally talented with each hand, "ambidextrous."

Handedness is an interesting topic in its own right but often comes up because we are interested in the brain, especially how the brain is organized with respect to language comprehension and production. It turns out that the vast major of right-handed people are strongly "left dominant" for language. And it turns out also that the majority (70+ to 90% depending on who you read) of left-handed individuals are also "left dominant" for language. The other 10 to 30 percent of left handed individuals have either strong right localization of language functions or some degree of bilateral representation of language in the cerebral hemispheres.