Language hemisphere dominance is ostensibly related to hand dominance, but this is more of a supposition than a statement. Approximately 90 to 95 percent of the general population is right handed; that is, they innately choose the right hand for intricate, complex acts and are more skillful with it. The preference is more complete in some persons than in others. Most individuals are neither completely right handed nor completely left handed but strongly favor one hand for more complicated tasks. The reason for hand preference is not fully understood. There is strong evidence of a hereditary factor but the mode of inheritance is uncertain. Learning is also a factor; many left-handed children are shifted at an early age to right (shifted sinistrals) because it had been a perceived handicap to be left-handed in a right-handed world. Most right-handed persons, when obliged to use only one eye (looking through a keyhole, gun sight, telescope, etc.), sight with the right eye, and it has been stated that eye preference coincides with hand preference. Even if true, this still does not account for hemispheral dominance. It is, however, noteworthy that handedness develops simultaneously with language. The most that can be said at present is that localization of language and a preference for one eye, hand, and foot, as well as praxis, are all manifestations of some fundamental, partly inherited tendency for hemispheric specialization.
There are slight but definite anatomic differences between the dominant and the nondominant cerebral hemispheres. Yakovlev and Rakic, in a study of infant brains, observed that the corticospinal tract coming from the left cerebral hemisphere contains more fibers and decussates higher than the tract from the right hemisphere. More pertinent to language, the planum temporale, the region on the superior surface of the temporal lobe posterior to Heschl gyri and extending to the posterior end of the sylvian fissure, is slightly larger on the left in 65 percent of brains and larger on the right in only 11 percent (Geschwind and Levitsky). LeMay and Culebras noted in cerebral angiograms that the left sylvian fissure is longer and more horizontal than the right and that there is a greater mass of cerebral tissue in the area of the left temporoparietal junction. CT studies have shown the right occipital horn to be smaller than the left, indicative perhaps of a greater right-sided development of visuospatial connections. Also, subtle cytoarchitectonic asymmetries of the auditory cortex and posterior thalamus have been described; these and other biologic aspects of cerebral dominance have been reviewed by Geschwind and Galaburda particularly as they relate to developmental dyslexia (see Chap. 27).
Left-handedness may result from disease of the left cerebral hemisphere in early life; this probably accounts for its higher incidence among the cognitively impaired and brain injured. Presumably, the neural mechanisms for language then come to be represented during early development in the right cerebral hemisphere. Handedness and cerebral dominance may fail to develop in some individuals; this is particularly true in certain families. In these individuals, defects in reading as well as the faults of stuttering, mirror writing, and general clumsiness are frequent.