Handedness Health Article

Basis of handedness

The physical basis of handedness is not well-understood. Through the centuries left-handedness has been attributed to numerous physical, psychological, and supernatural causes.

Each hemisphere of the brain has some specialized functions, a poorly-understood phenomenon called brain lateralization. In the late nineteenth century Paul Broca, a French neurosurgeon, identified an area of the left hemisphere that has a major role in the production of speech. Carl Wernicke, a German neurologist, identified another region in the left hemisphere that was responsible for language comprehension. Broca suggested that people's handedness was the opposite of their language-specialized hemisphere, so that a person with left-hemisphere language specialization would be right-handed. Thus until the 1960s, handedness was believed to be indicative of brain lateralization. Between 70 and 90 percent of humans have language specialization in their left hemispheres. The remainder may have right-hemisphere specialization or no real distinction between the two hemispheres in language specialization. However, among lefthanders, about 50 percent process language on both sides of their brains, 10 percent process language primarily in their right brains, and the remainder process language primarily in their left brains.

The 1987 Geschwind-Behan-Galaburda (GBG) Theory of Left-Handedness suggested that left-handedness was a result of some brain injury or trauma or chemical variations in the fetal environment, such as high levels of the male hormone testosterone.

For decades during the twentieth century scientists argued about whether there is a genetic basis for handedness. Children of left-handed parents have a 50 percent chance of being right-handed and 18 percent of identical twins differ in their handedness. Furthermore, right-handed twins are equally as likely as their left-handed twins to have left-handed children. A 2003 study appeared to identify a single gene that controls both handedness and the direction that hair spins on the scalp. An individual possessing at least one copy of the dominant form of this gene is both right-handed and has a clockwise hair spiral. However, when an individual has two copies of the recessive form of the gene—one copy from the mother and one copy from the father—the gene does not determine handedness. Thus 50 percent of these individuals are right-handed and 50 percent are non-right-handed. Furthermore, these individuals have a separate 50 percent likelihood of hair that spins clockwise or counterclockwise.

Handedness determines few if any lateralized behaviors other than fine finger dexterity. However, one study showed that right-handers preferred turning to their left side and non-right-handers preferred turning to their right side. Turning to the right or left is strongly correlated with turning toward the side of the brain that has less dopamine, an important brain hormone.