People are 90% right-handed and 10% left-handed {handedness, behavior}. Few people are ambidextrous.
animals
Mammals have paw preferences but 50% for left or right.
age
Handedness begins at 24 months.
brain
Until age three or four, brain hemispheres have little specialization.
In right-handers, left cerebral hemisphere has sense and motor connections to both body sides, and right hemisphere connects to only one side. In left-handers, cerebral lateralization is less. Adult human skulls are asymmetric.
brain: language
Left-handers typically have left-brain language region, but 30% have language regions on both brain sides. 95% of right-handers have left-brain language region.
brain: evolution
Human cerebral dominance probably started 300,000 years ago, when human skulls first appear asymmetric.
left side
In right-handers, left side has fewer skills, poorer timing and coordination, more variability, and more frequent and slower corrections.
hand usage
Right-hander typically supports and orients object in left hand, without using visual feedback, and performs fine movements with right fingers, using visual feedback {hand usage}. Most people use right hand for gesticulation.
causes
People inherit handedness. Handedness also results from social pressures or early experience, especially with objects designed for right-handers. Brain damage before or after birth can shift cerebral dominance or prevent hemispheric specialization. Subnormal and epileptic people have more left-handedness.
causes: twin
Perhaps, left-handedness is because there was a twin in utero. Twenty percent of twins are left-handed.
intelligence
Performance by right-handers and left-handers is equal on all tasks. No special ability or disability distinguishes left-handers.
Subnormal people have more left-handedness.
disease: dyslexia
Dyslexics often are neither strongly right-handed nor left-handed.
disease: epilepsy
Epileptic people have more left-handedness.
disease: synesthesia
Synesthesia is more with left-handedness [Stein and Meredith, 1993] [Stein et al., 2001].
disease: early death
Perhaps, left-handers die nine years earlier.
People can use right and left hands equally well {ambidexterity}|. Ambidextrous people typically have less skill on their better side than left-handers or right-handers on their better side.
Left-handed people {left-hander} are 4% to 36% of people in different races and cultures.
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Date Modified: 2022.0225