Theories of Handedness - Genetic Theories
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The idea that handedness may be hereditary dates all the way back toCharles Darwin who, although right-handed himself, was puzzled by his left-handed son, noting that his brother, mother and grandfather were all also left-handed. Research by John Santrock and others has shown that adopted children are actually more likely to share the handedness of their birth parents than that of their adopted parents.
Left-Handeness in Families
Other studies have also indicated that, at least to some extent, handedness does indeed run in families. However, great care should be taken with blanket assertions to this effect, especially where they are based on anecdotal examples.
For example, much has been made of the prevalence of left-handedness within the British Royal Family, with the usual assumption that this is due to genetic factors. Some websites claim that theQueen Mother, Queen Elizabeth II, Prince Charles and Prince William are all left-handed, in an unbroken line, although in fact the Queen and Prince Charles are almost certainly right-handed (arguably, Charles had a “left-handed marriage”, though: one between people of unequal social rank). When the rate of incidence of left-handedness within a particular family is high enough, it can look as though the trait is inherited, even it if it actually arises by chance. It has also been pointed out that such a powerful family is much less subject to the usual societal pressures to conform to the norm, and this may be just as much a deciding factor as the possibility that left-handedness has been inherited within the family.
Another oft-quoted example of inherited left-handedness is that people with the Scottish surname Kerr or Carr, who are possibly descended from the famously left-handed Kerr family of the Middle Ages, are more likely to be left-handed than the average. However, this piece of popular wisdom has also been conclusively disproved.
Detailed Studies
The two major studies into the frequency of left-handed children according to the handedness of the parents are by Chris McManus and Phil Bryden (1992) and by Walter McKeever (2000), both of which show that left-handed children are more likely to be born to left-handed parents than right-handed parents. The McManus study indicates that about 9% of the children of two right-handed parents are likely to be left-handed, about 19% of children of one left-handed and one right-handed parent, and about 26% of the children of two left-handed parents. The equivalent figures in the McKeever study are 16%, 20% and 40%, substantially higher but still indicating a similar significant genetic effect. In both studies, left-handed mothers are more likely to have left-handed children than left-handed fathers.
A detailed 2002 study by Dan Geschwind, working with brain scans of identical and fraternal twins, has provided more evidence of a genetic component to handedness. Geschwind showed that heredity can shape the brains of left-handed twins differently from right-handers (developing, among other things, more symmetrical and balanced hemispheres, as described in more detail in the section on Handedness and the Brain), and that identical (monozygotic) twins, which have 100% identical genes, tend to be more similar in their handedness than fraternal (dizygotic) twins, although all twins have a higher-than-average likelihood (almost twice as likely) of being left-handed.
Right Shift Theory
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The most widely-accepted genetic model for handedness is usually known as the Right Shift Theory, and was initially developed by the British psychologist Marian Annett back in 1972. The theory posits a hypothetical RS+ gene that develops the motor cortex and speech processing systems of the left side of the brain, leading to a preference for the right side of the body (a “right shift”). The other form of the gene, known as RS-, which is carried by left-handers, does NOT however cause a “left shift”, but rather is indifferent to the direction of both language and motor dominance. The theory works if the genes areadditive (or co-dominant), rather than the more commondominant/recessive model.
According to this theory, then, left-handers do not inherit their hand preference per se, they merely inherit a lack of neurological biastowards a dominant left hemisphere. There is, then, no gene for left-handedness as such, merely the absence or presence of a gene for right-handedness, the overall effect of which is to shift the distribution of handedness in the population as a whole towards the right end of the spectrum.
Because genes come in pairs of alleles, the handedness genes can fall into one of three combinations: RS+RS+, RS+RS- and RS-RS-. The majority of people therefore inherit at least one right shift gene, which acts to weaken the right hemisphere as well as to strengthen the left, resulting in some degree of right-handedness. Those who receive theRS-RS- combination of genes could go either way or even both ways (left-handedness, mixed-handedness, ambidexterity). Among those completely lacking the right shift gene, random events in childhood (such as a mild injury, parental coaching, repeated imitation of an older sibling, etc) may be enough to influence handedness one way or the other, regardless of the genetic underpinning. So, genetics make less of a difference to the hand preference of left-handers than they do to right-handers, who have more dominant handedness genes.
The resulting probabilities of handedness under this theory work out to be remarkably close to those observed statistically, and it neatly explains many observations that are very hard to account for in other models. The theory even purports to explain howidentical twins can end up with different hand preferences, despite having similar brain structures. It also explains why three-quarters of the children of two left-handed parents turn out to be right-handed, and how two right-handed parents can still have a left-handed child (e.g. if at least one of them has an R+R- combination).
Marian Annett refined her theory over time into what she called the Balanced Polymorphism Theory, which posits the existence of the R- gene as a kind of counter-balance to the more dominant RS+ gene, its job being to ensure that the species does not become too lopsided in any one area, and to prevent the brain from tilting too far towards the right hand/left brain end of the continuum.
Dextral/Chance Theory
Another (similar, but slightly refined) genetic theory has been proposed by Chris McManus (and by Amar Klar, who has developed a similar theory). McManus hypothesizes a “dextral” (D) allele, which strongly biases handedness in favour of the right hand and control of speech toward the left cerebral hemisphere, along with an alternative “chance” (C) allele, which is presumed directionally neutral. According to this theory, the DD genotype produces only right-handers, the CC genotype produces a random mixture of 50% right-handers and 50% left-handers, and the heterozygotic genotype DC produces 25% left-handers and 75% right-handers. The incidence of left-handedness within “left-handed families” actually appears never to rise above 50%, which supports the idea that what is being inherited is not left-handedness as such, but rather an absence of handedness, so that handedness is then determined by chance.
Some commentators have argued that the D gene was the principle factor in the separation of humans from the other apes, some 2-3 million years ago. Michael Corballis, who supports McManus’ gene theory, believes that these uniquely human alleles emerged somewhat later, due to a random mutation at some point in the evolution of Homo sapiens in Africa around 150,000 to 200,000 years ago, and spread quite quickly through the population thereafter. He also claims that its heterozygotic effect (the inheritance of a dissimilar pair of genes for any hereditary characteristic) could explain the apparent stability in the relative proportions of left-handers and right-handers. Corballis goes further in suggesting that the evolution of human speech implies an association between speech and gesture, and so our brain lateralization for speech may itself be responsible for our asymmetric hand use.
Gene Discoveries
Annett’s RS+ and RS- genes and McManus’ D and C genes all remain hypothetical, but with the leaps and bounds that are occurring in genetic mapping, many scientists believe it is only a matter of time before they are identified. Or, it is quite possible that two or more genes working in tandem may be involved, as is the case with so many other body systems.
In 2007, a group of Oxford University researchers discovered a gene called LRRTM1 (Leucine-Rich Repeat Trans-Membrane 1), the first gene linked to increased odds of being left-handed, a variant of which also slightly increases the risk of psychotic mental illnesses such as schizophrenia. However, the link to left-handedness only appears when the gene is inherited from the father’s side. Whether this will turn out to be, or be some component of, the right-shift gene of the theory, remains to be seen.
Another Oxford group in 2010 identified a link between a variant of a gene called PCSK6 and handedness in children with language-related disorders like dyslexia. The gene appears to have an effect on the initial left-right patterning of the embryo, which in turn influences the development of brain asymmetry and thus handedness. The work also provides the first genetic evidence to support a much speculated link between handedness and language-related disorders.
