The hand is rated by students of the history of civilization as one of the most important determinants of man’s rise from savagery. The loss of even a finger is a handicap recognized in such times of stress, as when men are drafted for war. With the great majority of people, the two hands are unequal in strength and accuracy, the right being the major member. With a small minority of children there is, however, a predisposition to use the left hand, instead of the right hand, as the major member. This is a special condition which must be taken into account by educators.
According to different investigators, the proportion of left-handed children ranges from 2 to 6 per cent. The disagreements arise from the variety of criteria used and of populations sampled. The median figure of 4 per cent seems, for several reasons most probable, as the general proportion of left-handedness.
Many theories as to the origin of handedness have been formulated. It has been argued that handedness is not innate, but acquired from the mother’s habitual method of carrying the infant on one arm rather than on the other, so that one of its arms is pinioned against the mother habitually, and gets comparatively little exercise. The theory has been advanced that since the heart is the most vital organ of the body, and is located on the left side, the shield to protect it was held by the left hand, permitting the right hand to attain greater dexterity with the spear, the advantage thus acquired being transmitted to offspring. Also, it has been proposed that the center of gravity of the viscera, the position of the subclavian arteries, cerebral asymmetry, and greater blood supply to one cerebral hemisphere may be, respectively, the origin of handedness. All of these theories are unsatisfactory, for reasons which have been well stated by the original investigators. There remains to be considered the proposal that handedness is determined by ocular dominance. The right eye is the better seeing eye in about 96 per cent of people. As vision develops long before muscular coördination in the infant, the proposal is that the hand is brought to coöperate with the dominant eye. The disproof of this theory is that among the congenitally blind the proportion of right-handed to left-handed is not materially different from that among seeing persons.
The origin of handedness is, therefore, not understood, and it is not known why about 4 per cent of the population should show dominance of the left hand. It must be considered that handedness is of many degrees, from extreme right-handedness, through ambidexterity, to extreme left-handedness. All right-handed persons are not equally right-handed, and all left-handed persons are not equally left-handed.
Trustworthy studies of the heredity of handedness indicate that it is inherited. Ramaley studied 610 parents and 1130 children, and arrived at the conclusion that left-handedness is inherited (as a Mendelian recessive), and is potential in about one-sixth of the population.
It is obvious that modern appliances are adapted to the right-handed, and that right-handedness is regarded generally as “the way to be.” Teachers and parents feel it their duty to compel the child to use the right hand.
Studies of left-handed children who have been “changed over” through education or accident to the right hand, and of right-handed children changed over through accident to the left hand, lead to the conclusion that among them there is more nervousness and a greater number of speech defects than would be allowed by the usual course of events. Stammering is evidently a complication in some cases of modified handedness. The physiology of this connection is obscure. In view of the fact that speech defects occur to so great an extent in “changing over,” and that we do not know the physiology of handedness, it seems by all means wisest not to try to modify handedness where it is very pronounced. A very right-handed person, fortunate in being with the majority, may, by using for a week his left hand instead of his right, get an idea of what is suffered by a very left-handed child being compelled to use the right hand.
It has been reported that there is an undue proportion of left-handed persons among criminals, mental defectives, and the insane. These reports require careful verification. Criminals, mental defectives, and the insane have been much more carefully scrutinized than have the superior in intellect and character, or even than the average population. The present writer has, during recent surveys, noticed left-handed performance repeatedly in very gifted children, but has not computed the proportion. Until further scrutinies have been made, it cannot be said positively that left-handedness is correlated with organic inferiority.
Perfectly satisfactory tests of handedness have not yet been agreed upon. Jones proposed some years ago to measure congenital handedness by means of a brachiometer. This is an instrument for measuring the bones of the forearm, and by its use Jones hoped to detect handedness “at the moment of birth” as well as on any subsequent day of life. These hopes have not been realized in the findings of others who have given the method fair trial, as Beeley did. Tapping, with the wrist movement, tapping with fingers, tracing, spontaneous rubbing, throwing and picking up, winding, and cutting with scissors are the most promising among tests so far tried out, to discover whether a child is congenitally left-handed. Gripping, as with the dynamometer, does not seem to correlate so well with known facts, as do the other tests of movement.
Left-handedness as an element in individuality becomes conspicuous in school procedure especially in writing, drawing, shop work, or any work where the hand is an important factor in the performance. It may become conspicuous in vocational endeavor, either as an asset or a handicap. In a few kinds of performance, such as baseball or tennis, left-handedness gives an advantage, all other things being equal. In most professional pursuits (with the possible exception of dentistry and surgery because of manufactured appliances) left-handedness is a matter of indifference. In work with machines left-handedness is likely to be a handicap, because machines are “right-handed.” Even scissors, eggbeaters, typewriters, and other common appliances of office and home are “right-handed.”