Characteristic of the human brain in which certain functions (such as language comprehension) are localized on one side in preference to the other. One example is handedness (the tendency to use one hand or the other to perform activities): Since the left and right cerebral hemispheres control the right and left sides of the body, respectively, right-handed people are typically left-dominant in terms of hemispheric control of various motor functions and also with respect to seeing (right-eyed) and language comprehension. Paul Broca first identified the brain centre for articulate speech in what is now called Broca's area. Later researchers discovered that functions involving logical or sequential analysis generally reside in the left hemisphere, while the right hemisphere seems to control processing of spatio-visual information and musical relations. More left-handers than right-handers display a reversal of hemispheric specialization or a more even distribution of functions between the two hemispheres. There is no general agreement about whether laterality is genetically transmitted, developed during gestation, or learned.
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The majority of humans are right-handed. Many are also right-sided in general (that is, they prefer to use their right eye, right foot and right ear if forced to make a choice between the two). The reasons for this are not fully understood, but it is thought that because the left cerebral hemisphere of the brain controls the right side of the body, the right side is generally stronger; it is suggested that the left cerebral hemisphere is dominant over the right in most humans because in 90-92% of all humans the left hemisphere is the language hemisphere.
Human cultures are predominantly right-handed, and so the right-sided trend may be socially as well as biologically enforced. This is quite apparent from a quick survey of languages. The English word "left" comes from the Anglo-Saxon word lyft which means "weak" or "useless". Similarly, the French word for left, gauche, is also used to mean "awkward" or "tactless", and sinistra, the Latin word from which the English word "sinister" was derived, means "left". Similarly, in many cultures the word for "right" also means "correct". The English word "right" comes from the Anglo-Saxon word riht which also means "straight" or "correct." The words "adroit" and "dextrous", both meaning "skillful", come from the French droit and the Latin dexter respectively.
This linguistic and social bias is not restricted to European cultures: for example, Chinese characters are designed for right-handers to write, and no significant left-handed culture has ever been found in the world.
When a person is forced to use the hand opposite of the hand that they would naturally use, this is known as forced laterality, or forced dextrality. A study done by the Department of Neurology at Keele University, North Staffordshire Royal Infirmary suggests that forced dextrality may be part of the reason that the percentage of left-handed people decreases with higher age groups.
Ambidexterity is when a person has approximately equal skill with both hands and/or both sides of the body. True ambidexterity is very rare. Although a small number of people can write competently with both hands and use both sides of their body well, even these people usually show preference for one side of their body over the other. However, this preference is not necessarily consistent for all activities. Some people may for example use their right hand for writing, and their left hand for playing racket sports and eating (see also: cross-dominance).
Also, it is not uncommon that people preferring to use the right hand prefer to use the left leg, e.g. when using a shovel, kicking a football, or operating control pedals. In many cases, this may be because they are disposed for left-handedness but have been trained for right-handedness.
Approximate statistics are below:
Laterality of motor and sensory control has been the subject of a recent intense study and review. It turns out that the hemisphere of speech is the hemisphere of action in general and that the command hemisphere is located either in the right or the left hemisphere (never in both). Around eighty percent of people are left hemispheric for speech and the remainder are right hemispheric: ninety percent of right-handers are left hemispheric for speech, but only fifty percent of left-handers are right hemispheric for speech (the remainder are left hemispheric). The reaction time of the neurally dominant side of the body (the side opposite to the major hemisphere or the command center, as just defined) is shorter than that of the opposite side by an interval equal to the interhemispheric transfer time. Thus, one in five persons has a handedness that is the opposite for which they are wired (per laterality of command center or brainedness, as determined by reaction time study mentioned above).
Laterality in animals is also called limb dominance. Most race tracks are run counter-clockwise, which favors right-side dominant horses, as they take a longer stride with the right foreleg, which helps them turn to the left. Trainers of left eye dominant horses may put a blinder on the left eye to encourage the horse to turn the head slightly to the left and to take a longer step with the right foreleg just as right-side dominant horses do. Parrots tend to favor one foot when grasping objects (for example fruit when feeding). Some studies indicate that most parrots are left footed. Polar bears generally kill their prey using their left paw. Some types of mastodon indicate laterality through the fossil remains having differing tusk lengths.
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