Researchers believe that the side of the preferred hand (which is, right versus left) is produced by biological and, most likely, genetic causes. According to studies, evolutionary natural selection has produced more people with speech and language control in the left hemisphere of the brain. Because the left hemisphere also controls the movements of the right hand—and especially the movements needed to produce written language—evolutionary development resulted in a population of humans that is biased genetically toward individuals with left hemisphere speech/language and right-hand preference. Approximately 85 percent of people are right-handed.
The genetic proposal to explain hand preference claims there are two alleles, or two manifestations of a gene at the same genetic location, that are linked with handedness. One of these alleles is a D gene (for dextral, meaning “right”) and the other allele is a C gene (for “chance”). The D gene is more frequent in the human population and thus more likely to occur as part of the genetic heritage of an individual. It is this D gene that promotes right-hand preference in the majority of humans.