Many systems are designed to favor users' preferences. For example, designers may make design decisions to ensure that critical tasks are performed by users with their preferred hand. Theoretically, this should help to speed-up performance and reduce errors. Most would agree that most users are right-handed, but what percent of users are truly right handed? If a user is right-handed are they also right-footed, right-eyed and right-eared?
The best available research shows that about 87% of adults are right handed. The remaining 13% contain people that are either left-handed or can use either hand without a strong preference for one or the other (Porac and Coren, 1978). The preferences for the right foot, right eye or right ear, are as follows:
80% prefer the right foot,
69% prefer the right eye, and
56% prefer the right ear.
However, the correlations among the above tend to be quite low:
Hand/foot - .36
Foot/ear - .36
Foot/eye - .35
Hand/eye - .34
Hand/ear - .26
Eye/ear - .26
To show how confusing this can be when designing to accommodate users' performance and preferences, consider the results of the following study on hearing. In this study, all participants were right-handed. Most participants (86%) exhibited the best listening performance using the right ear. Also, most subjects (71%) preferred listening with their right ear, some with their left ear (17%), and a few had no preference (12%). However, when their hearing loss was measured using an audiometer, about two-thirds of the subjects heard best with their left ear (least hearing loss), while only 15% heard best with their right ear, and 18% showed no differences between ears (Reib and Reib, 1998).
In most systems, simple tasks that require little or no specialized training or practice tend to be carried out equally well by either hand (e.g., reaching for objects). In certain highly skilled activities, where both hands may receive an equal amount of training and practice, both sides can achieve close to the same high level of proficiency (e.g., typewriting and piano playing). It is in activities where there is an advantage for one hand or the other to achieve a high level of competence, that one side tends to be used more consistently than the other (e.g., eating, handwriting, playing the violin or cello).
Some past studies of handedness found a higher incidence of right-handedness in females, and in older people. Recent studies report no differences due to gender or age. The incidence of right-handedness in children is as follows:
Both parents are right-handed - 91%
One parent is right and one is left - 81%
Both parents are left-handed - 74%
Designers should be aware that there is a strong cultural bias in favor of using the right hand in many countries. For example, the incidence of left-handedness in writing is less than 1% in China and Japan. One study reported that in Japan the following percentages of children used the left hand for writing:
1st grade - 5.1% of boys and 2.9% of girls
8th grade - 0.2% of boys and 0% of girls
Another factor that could affect the design of systems relates to using the "right vs. left brain." Most true right-handers (up to 99%) show strong lateralization of speech representation by having speech represented in the left cerebral hemisphere. However, left-handers are more likely to exhibit a bilateral representation of speech. Only about 70% of left-handers have speech represented in the left cerebral hemisphere.
The genetics and evolution of handedness, Corballis, M.C., Psychological Review, 104 (4), 714-727 (1997).
On the biological basis of human laterality: II. The mechanisms of inheritance, Morgan, M.J. and Corballis, M.C., The Behavioral and Brain Sciences, 2, 270-276. (1978).
The growth and inheritance of laterality, Porac, C. and Coren, S., The Behavioral and Brain Sciences, 2, 311-313 (1978).
Human laterality: A multidimensional approach, Porac, C., Coren, S., Steiger, J.H. and Duncan, P., Canadian Journal of Psychology, 34(1), 91-96 (1980).
Ear preference: Association with other functional asymmetries of the ears, Reib, M. and Reib, G., Perceptual and Motor Skills, 86, 399-402 (1998).
Handedness and speech: A critical reapparisal of the role of genetic and environmental factors in the cerebral lateralization of function, Provins, K.A., Psychological Review, 104 (3) 554-571 (1997).
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