Split-Brain Functioning

One of the most apparent asymmetries related to the human brain is hand use preference. Differences in abilities between the hands reflect asymmetries in the cerebral hemispheres' functions. Studies show that about 90% of people across cultures are right-handed, while non-human animals tend to be divided pretty evenly in terms of limb preference. The question of why human beings show an overwhelming favoring of the right hand compared to other animals has been the subject of much theorizing and assumes greater importance when one understands that an individual's handedness has been found to correspond in complex ways to how various functions are distributed between the left and right hemispheres.

Handedness is very generally defined as the almost exclusive use of one hand for such activities as writing and other one-handed behaviors. There is much individual variation however in the frequency, strength, and efficiency of differential hand use, and an individual's handedness can be assessed in a number of ways. While asking an individual which hand they tend to favor might seem the simplest approach, this does not tell the researcher about an individual's possible common variations in hand preference across different activities which the individual may neglect to report. For instance, while someone may write with their right hand, they may throw balls, or use scissors with their left hand.

The most common method used to assess handedness is questionnaires that ask the individual which hand they use across a number of different behaviors. Findings from these questionnaires indicate that those who show a preference for the right hand use it more consistently across tasks than those who show a left hand preference. Those who preferred their left hand did not prefer it as consistently as right handers, instead they tend to also use their right hand in a number of behaviors. Some researchers believe direct behavioral observation of the individual performing a number of activities is the most precise method of determining handedness.