Interestingly, it seems that there is not one gene which contributes to determining whether people are left-handed or not. Most likely, different genetic influences are involved.
Of course, children learn from their caregivers, so one may expect that if one of the parents is left-handed, the child might become left-handed just by imitation. One reason why this cannot be the full story is that hand preference can already be observed before birth. This observation of fetal handedness is at odds with the finding that some children seem to switch their hand preference, at least up to the age of 2 [ 3 , 4 ].
Perhaps some people are left- or right-handed at birth, whereas others develop their preference later on, during the first years of life. Are you confused about what makes a person left-handed or not? So are we! Whatever the exact causes may be, this is not a simple story, but a very complex interplay of genes, environment, and chance.
Let us leave this topic for now, and have a look at the left-handed brain. Sometimes, people are amazed to hear that the brains of left-handers are different from those of right-handers. But it is clear that they should differ in some respects: left-handers use their hands and feet differently than right-handers do, and they do this often over the course of a lifetime. It is only natural that the parts of the brain that control movements should be different in left-handers and right-handers Figure 1.
Compare this to skilled musicians: they practice fine movements a lot, and this influences their brains 1. To study exactly how the brains of left- and right-handers differ, we need to look at brain lateralization. Brain lateralization refers to the fact that the left and right sides of the brain are not the same. The two sides differ in their anatomy, and also in what they do.
By the way, lateralization is also found in the rest of the body: the two arms look slightly different, and are capable of doing different things; the heart is lateralized to one side of the body cavity, etc. Left-handers are distinct from right-handers in that they tend to have less lateralization in the brain. A well-known example of this is language, which is mainly a function of the left side of the brain in right-handers 2.
Although left-sided language areas in the brain are still important in left-handers, the difference between left and right sides tends to be less strong. A similar example comes from face perception, which is rightward-lateralized in the majority of people, meaning that mainly right-sided parts of the brain are responsive to faces.
Again, left-handers tend to use both left and right regions of the brain relatively often when they see a face [ 5 ]. Does this mean that left-handers think differently? Another gene, LRRTM1 , has been associated with an increased chance of being left-handed in people with dyslexia a condition that causes difficulty with reading and spelling.
It is unclear whether either of these genes is related to handedness in people without these conditions. Studies suggest that other factors also contribute to handedness. The prenatal environment and cultural influences may play a role. Like many complex traits, handedness does not have a simple pattern of inheritance. Children of left-handed parents are more likely to be left-handed than are children of right-handed parents.
However, because the overall chance of being left-handed is relatively low, most children of left-handed parents are right-handed. Identical twins are more likely than non-identical twins or other siblings to be either right-handed or left-handed, but many twins have opposite hand preferences. Genome-wide association study of handedness excludes simple genetic models. Heredity Edinb. Epub Sep PubMed: PLoS Genet. Brandler WM, Paracchini S. The genetic relationship between handedness and neurodevelopmental disorders.
Trends Mol Med. Epub Nov The molecular genetics of hand preference revisited. Sci Rep. It was associated with a range of developmental dysfunctions ranging from language deficits to mental health disorders.
Today, we understand that handedness is not a binary characteristic left or right , but rather, it exists along a gradient that ranges from strongly left-handed to strongly right-handed. As they start to develop their motor skills, children may use both the left and right hands equally for simple actions such as reaching for objects. This is because both hands can accomplish the task with ease. Yet, for the majority of the population, more complex tasks require the specialized processing properties of the left hemisphere of the brain.
For example, the majority of children choose their right hand for writing. Recent research suggests that children who are strongly left or right handed also have good cerebral lateralization and typical language production. On the other hand, mixed-handedness not developing a dominant hand has been linked with atypical development of motor and language abilities.
Children with ASDs also show motor abnormalities as early as seven months of age. Current diagnoses of ASDs tend to occur relatively late, when children fail to produce and understand basic language. Late diagnoses can limit benefits that may come from early interventions and therapies. Infant brains are incredibly flexible and the right kinds of early intervention may improve later cognitive development and mental health. Handedness is not the only bias of our motor skills that comes from cerebral lateralization.
A majority of the population has a right hemisphere of the brain that is dominant for responding to danger. This means that we are faster at recognizing threatening faces and expressions when they appear to our left side in the left visual field compared to our right side.
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