The most obvious behavioral asymmetry observed in people is handedness. Approximately 90% of people prefer using the right hand, with the remaining 10% either preferring to use the left or exhibiting no hand preference. Once a person develops a handedness, it is rare for this to change.

Several theories have been proposed for the factors that determine handedness.1 Broca's original observation, which stated that right-handedness appears to be closely associated with left-hemisphere dominance for language, linked handedness with language lateralization.2 The influence of asymmetries in attention and in the intentional system has also been proposed to account for handedness. For example, whereas the left hemisphere primarily prepares the right-side limbs for movement, the right hemisphere prepares both sides for movement.3 Elementary motor asymmetry has also been proposed to account for handedness. The preferred hand is often the hand that is stronger, more accurate, or can move more rapidly than the nonpreferred hand.4 Unfortunately, the neuronal asymmetry that accounts for these elementary motor asymmetries remains unknown. In 1919, Watson proposed a learning theory that culture and social pressures influence handedness. Although culture and social pressures alone cannot fully account for hand preference, Eacott and Gaffan showed in 1989 that information about the reward history of the stimulus is integrated between the hemispheres before it influences the motor control of the hand. A study on the effects of hand asymmetry by hemispheric brain injury in hemiplegic children suggests the possibility of handedness being able to shift.5

The preferred hand has a better skill performance and is easier to use than the nonpreferred hand.6 However, it is unknown whether the better performance results from frequent use (and thus is a training effect due to cultural or environmental factors),7-13 or from a genetically determined trait lateralized to one hand.14-16 If in right-handers the rate of skill learning is identical in each hand, one might conclude that environmental or cultural factors mainly affect the development of handedness. However, if the handedness is genetically determined, the rate of skill learning will be asymmetric (lateralized to one hand). The purpose of our study was to test this postulate and determine whether motor learning is associated with elementary motor asymmetry.17

In this study we tested whether handedness is related to skill learning and whether motor learning is associated with elementary motor asymmetry, degree of hand preference, strength, or motor speed. Our data showed that, in right-handers, the rate of skill learning was better with the right hand than with the left, a finding that cannot be due to elementary motor asymmetry.

That people prefer to use one hand rather than the other is a remarkable behavioral asymmetry, and means that the preferred hand often performs skills better than the nonpreferred hand.19 Genetic and familial influences, as well as social and cultural factors,20 have been considered to underlie handedness. Better skill performance in one hand can be seen as a training effect from more frequent use of one hand. Social or cultural factors may contribute to this.7-13,20 Alternatively, better skill performance might originate from a genetically determined capacity for lateralized skill-learning, which may also improve skill performance and develop handedness. 14-16 Our data showed that, in right-handers, the rate of skill learning was better with the right hand than with the left. Education level, age, or preference score cannot explain this asymmetry. This finding supports the postulate that handedness is mainly determined by genetic or familial factors. However, this result does not preclude the presence of a practice effect.

Taylor and Heilman (1988) proposed that praxis is related to handedness. Loss of the ability to perform learned skilled movements is often caused by left-hemisphere lesions. If one has lateralized representations and is learning a new skill, the hand opposite to these representations should not only acquire skill more rapidly but also benefit the preferred hand when the acquired skill is learned with the nonpreferred hand. Visuomotor information is transferred via the corpus callosum and this callosal transfer shows left-right asymmetry.21-28 Further study of this pretraining effect remains to be tested, but in our investigation motor learning trials were made randomly for both hands and did not show a difference.

Motor-skill learning is a complex phenomenon that requires harmony of several elementary motor components, such as adequate power, appropriate speed and accuracy, as well as visuomotor integration between the hemispheres.29 Our data showed that the rate of skill learning was not correlated with a greater difference in right-left motor power or tapping rate. An efficient elementary motor system may help in skill learning. However, the development of elementary motor asymmetry30 cannot explain the development of skill-learning laterality. Therefore, although elementary motor components are associated with skill performance, skill learning may independently affect the development of handedness,31 and suggests that improving the motor power or speed may not necessarily affect the motor-skill learning.