Why Are Most People Right-Handed, Do You Know It?

 

Scientists have searched for an answer to this question for more than a century and have yet to find a definitive answer. In your own thinking about the matter, you have probably concluded that there are three possibilities:

• Handedness is completely determined by genes.
• Handedness is completely determined by learning.
• Handedness is determined by both genes and learning.

If you are drawn to the first hypothesis, consider the finding that only 82% of identical twins, whose genotypes (genetic makeup) are identical, have the same hand preference ( Klar, 2003 ). If handedness were completely determined by genes, then identical twins’ phenotypes (actual characteristics) would always be the same for handedness. Therefore, handedness cannot be entirely
explained as a function of our genes. Does this mean that handedness is determined by learning? Not necessarily.
The learning hypothesis cannot explain why handedness appears very early in infancy, long before children are exposed to formal instruction that requires them to use one hand or the other ( Rönnqvist & Domellöf, 2006 ). Moreover, the proportions of left-handers and right-handers in the human population have been about the same for thousands of years ( Hopkins & Cantalupo, 2004 ). In
fact, these proportions are evident even in the skeletons of humans who died long before writing was invented as well as Neanderthals ( Steele & Mays, 1995 ; Volpato et al., 2012 ). The key to understanding the evidence on handedness is to adopt the view that both genes and learning are at work in the development of hand preferences, but not in the way that you might expect. In most of us, right-handedness is completely determined by our genes, but in a few of us handedness, whether
left or right, is influenced by learning. Sound confusing? To clarify, here is the most current thinking on the genetics of handedness.

Researchers suspect that right-handedness is determined by a single dominant gene, R ( Francks et al., 2003 ). If an individual receives a copy of R from one or both parents, then she will be right handed. The frequency of R in the human population is extremely high, scientists believe, because it is tied to the genes that support left-lateralization of language function in the brain. (Remember, the left side of the brain controls the right side of the body.) It makes sense that motor functions are linked to language, experts claim, because producing language requires activity in both the language centers and motor cortex of the brain. Putting both on the same side of the brain facilitates the

rapid development of neural connections between the two without having to go through the slowly developing membrane between the two hemispheres (corpus callosum). But what happens to the relatively small proportion of humans who do not receive a copy of R from either parent?

The dominant gene for right-handedness, R, is complemented by a recessive gene, r ( Francks et al., 2007 ). You might think that the phenotype of an individual who receives a copy of r from both parents would include left-handedness, but, in reality, the phenotype that is associated with rr is
non-handedness. In individuals with the rr genotype, learning shapes handedness. Because most people are right-handed, and the tools that humans have developed for use in fine motor activities (e.g., scissors) favor righties, there is a considerable amount of cultural pressure on those who lack innate handedness, those with the genotype rr, to become right-handed. Nevertheless, some
of them do develop left-handedness. Why? Researchers believe that other genes come into play as well. Specifically,if a person possesses genes that cause language functions to lateralize to her
right rather than her left cerebral hemisphere, then she is also likely to be lefthanded. Here again, the lateralization of handedness follows the lateralization of language function. To make matters more complicated, in 2007, scientists discovered a gene that pushes us in the direction of left-handedness when we receive it from our fathers.

When we get the gene from our mothers, it seems to have no influence on hand dominance ( Francks et al., 2007 ). Finally, although genetics appears to play a complex, but important, role in the development of hand preferences, the capacity of individuals to adapt to severe injuries to or the loss of the dominant hand demonstrates the adaptability of the brain with regard to motor functions. Thus, as the trait of hand preference illustrates, nature and nurture are often linked in complex ways. Remember this the next time you are involved in a debate with someone about whether a
given trait is either genetic or learned.