There are a variety of theories about what causes left-handedness. Lorin Elias has a page on his website (link currently not working) describing many of them, and provides good counter-arguments. I would encourage anyone interested in this topic to read about it more fully, as it is still far from being a solved puzzle. The primary ideas about left-handed causes include genetic and birth stress.
Theories
Heredity and GeneticsGenetic theories so far are inconclusive. Although environment may have an effect, statistically there is approximately (studies vary) a 9% occurance of left-handers born to two right-handed parents, 19% with one right and one left-handed, and 26-35% with two left-handed parents. But, if the cause is genetic, why do two right-handed parents have any left-handed children? And what about the findings that left-handers seem to run in some families?
- Dr. Amar J. S. Klar has a theory that there is a right-handed gene, which he calls RGHT. He contends that the absence of this gene would yield left-handedness.
- Hull University psychologist Marian Annett has said that humans may have hand skills which vary from completely right to completely left, with a possible genetic factor. This spectrum of skill has also been interpreted by Drs. Corballis and Morgan as "an inherited absence of the predisposition to be right-handed."
- Dr. Jerre Levy and geneticist Thomas Nagylaki contend that two genes play a part in controlling handedness and language hemisphere location.
- In a study conducted by Stanley Coren, and replicated by Philip Bryden, the degree of handedness was analyzed. The study indicated that, rather than a gene for left or right-handedness, perhaps a genetic factor influences how strongly or weakly handed an individual will be.
- Clare Porac, a professor of psychology at Pennsylvania State University who studies handedness, explains what causes some people to be left-handed, and why are fewer people left-handed than right-handed:
"Researchers who study human hand preference agree that the side of the preferred hand (right versus left) is produced by biological and, most likely, genetic causes. The two most widely published genetic theories of human hand preference argue that evolutionary natural selection produced a majority of individuals 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 notably the movements needed to produce written language--millennia of 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. These theories also try to explain the persistent and continuing presence of a left-handed minority (about 15 percent of humans).
The genetic proposal to explain hand preference states that there are two alleles, or two manifestations of a gene at the same genetic location, that are associated 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 population and is more likely to occur as part of the genetic heritage of an individual. It is the D gene that promotes right-hand preference in the majority of humans. The C gene is less likely to occur within the gene pool, but when it is present, the hand preference of the individual with the C gene is determined randomly. Individuals with the C gene will have a 50 percent chance of being right-handed and a 50 percent chance of being left-handed.
These theories of hand preference causation are intriguing because they can account for the fact that the side of hand preference of individuals with the C gene (most left-handers and some right-handers) can be influenced by external cultural and societal pressures, a phenomenon that researchers have documented. These theories can also explain the presence of right-handed children in families with left-handed parents and the presence of left-handed children in families with right-handed parents. If the familial genetic pool contains C genes, then hand preference becomes amenable to chance influences, including the pressures of familial training and other environmental interventions that favor the use of one hand over the other. The proposed genetic locus that determines hand preference contains an allele from each parent, and the various possible genetic combinations are DD individuals who are strongly right-handed, DC individuals who are also mostly right-handed, and CC individuals who are either right-handed or left-handed. These genetic combinations leave us with an overwhelming majority of human right-handers and a small, but persistently occurring, minority of left-handers."
The theory of birth stress can encompass factors such as age of the mother, difficult birth, or injury in utero.
- Paul Bakan, a Canadian psychologist, put forth the idea that since left-handed children are born to parents who both are right-handed, then left-handedness must be caused by stress during birth, or injury to the left hemisphere.
- His theory is countered with the thought that if there is pathological left-handedness, where are the pathologically caused right-handers?
- Bakan's idea was that left hemisphere injury would result in language deficits, and left hemisphere dominance then would default to the right hemisphere. If this process is true, right-hemisphere injury would result in right-handers with spatial ability deficits; is anyone looking for them?
- Murray Schwartz countered Bakan's hypothesis with his own more detailed study (using hospital records in addition to the maternal anecdotal reports upon which Bakan relied) and the results indicated that only one birth stress factor had a relationship to later left-handedness.
- Paul Satz studied what he terms PLH, or pathological left-handedness due to early brain injury. He found related changes in these cases, such as:
- The hemisphere for speech being shifted (from left to right) almost three times more than non-brain injured left-handers.
- This hemispheric speech shift also causes problems with spatial abilities by displacing these functions that would normally develop in the right hemisphere.
More discussion on brain hemispheres and handedness is in the next section.
