HANDEDNESS. Why the vast majority of human beings do most things in a right-handed way is a problem which has induced voluminous discussion. Parson, in a recent book, gives some 4o pages of titles of books, pamphlets and articles dealing with various aspects of the problem.
Theories of Handedness.—Theories relative to handedness vary in their treatment of it as an acquired or a native trait. Those who believe it an acquired habit advance many theories to show how it originates. Some relate it to the way a child is held in infancy; others to social training and imitation. Watson, the behaviourist, thinks it a socially conditioned response ; forcing the child to eat with the right hand is a potent conditioning factor. The theorists who consider handedness a congenital trait seek to connect it with structural features of the human body. For in stance, an appeal has been made in explanation to the unequal visceral distribution in the two halves of the human body and the consequent displacement of the centre of gravity ; also to a sup posed inequality in a blood supply to the brain, the left cerebral hemisphere, which controls the right half of the body, receiving the greater supply of blood. Many authorities accept the view that right-handedness is due to the functional predominance of the left brain ; others cite the dominance of the right eye as explana tory. Ocular dominance and the pre-eminence of the left brain hemisphere must then be explained.
A theory intermediate between explanation of handedness as a habit or dependent upon innate structure, states it originated in primitive warfare, where the stick or sword was wielded by the right hand in order that the left arm and hand might be used for defensive covering of the heart or for holding a shield for this purpose. This racial habit was, according to this theory, transmitted to later generations.
Percentage of of right-hand edness as a native trait is often associated with emphasis on the fact that left-handedness appears to be hereditary in certain family lines. The percentage of left-handedness among normal individuals is estimated to run from 4 to 8%. The percentage is much higher among inmates of institutions for the feebleminded and the psychopathic.
Left-brainedness.—Right-handed persons are left-brained and left-handed persons right-brained. The motor nerves that innervate the two halves of the body as they descend from the cerebral hemispheres cross over from one side to the other. Some evidence exists that the left hemisphere is more highly organized than the right. Attempts to determine the relative weight of the two hemispheres encounter many sources of error and the results are inconclusive.
Bilaterality.—The problems of handedness should be restated in terms of laterality of function in general. People are right and left footed as well as right and left handed. They also are right eyed or left-eyed in the sense that in unilateral sighting one eye is preferred to the other. About 7o% of right-handed persons are also right-eyed; about 5o% of left-handed persons are left-eyed. Discrepancies between preferred hand and eye, described as crossed dextrality or sinistrality, may originate from interference with natural handedness or arise from development of ocular faults. Possibly they may be due to asymmetrical functioning of the nervous system. In any case they appear to be connected with certain psychopathic tendencies.
Bilaterality of Structure and Twinning.—It is probable that we must look to investigations on the biology and physiology of twinning for an ultimate explanation of handedness, since cur rent studies suggest that bilaterality of structure is a form of twinning. In the twinning relation one twin usually gains a physiological ascendency over the other. Unilaterality of function might arise similarly in man. One would anticipate, however, all degrees of asymmetry between the two body halves. The goal of the biological process would appear to be the development of a bilaterally symmetrical individual.
BIBLIOGRAPHY.—Widely dispersed lists of titles may be found in Bibliography.—Widely dispersed lists of titles may be found in journals, medical, psychological, physiological and anthropological. For summaries, see B. S. Parson, Left-Handedness (1924) and J. E. Downey, "Types of Dextrality and Their Implications," in Amer. Jour. Psychol., vol. xxxviii. (1927)• (J. E. D.)