As children grow, they develop a strong preference for one of their hands when they want to accomplish certain tasks, particularly writing or drawing. The child becomes right-handed, left-handed or ambidextrous around the age when language is considered acquired – around four years of age – and this remains one of his characteristics for life.
We now know that a child’s manual preference allows us to better understand the functioning and organization of their brain.
The left hemisphere and the right hemisphere of the brain control the motor functions on each side of the body, in an inverse manner. However, the two halves of the brain do not exercise identical control over different physical behaviors; this is why one of the two hands is preferred over the other for the execution of certain tasks. The predominance of one hemisphere over the other to perform certain gestures is called cerebral lateralization.
Scientists believe that this lateralized functioning of the brain serves to prevent the two hemispheres from competing, since only one hemisphere controls a given movement. But it also allows processes as different as language and attention to occur in parallel in the two hemispheres.
In most people, it is the left hemisphere that controls language. And it’s the same region in the left hemisphere that controls hand movements. This is why almost all humans (around 90%) are right-handed when using tools or making gestures.
Evolutionary psychologists believe that tool use and gestures played a large role in the evolution of language. According to them, since vision is our strongest sense, we began to communicate by making gestures. Then it became more efficient to keep our hands free to handle tools, and language took over. The gestural sequences which were used to make and use tools were thus able to prepare the brain to integrate the structure of language.
To acquire complex skills like language, little ones must first master basic sensory and motor skills. For developmental psychologists, this ability to use gestures and manipulate objects sets the stage for the acquisition of the systems necessary for language development.
Right hand, left hand, or both?
Scientists in the first half of the 20th century considered being left-handed to be an anomaly and associated it with a series of dysfunctions – language deficits, mental illnesses -. In fact, many left-handed people born at this time were forced to write with their right hand, in the hope of “converting” them into right-handers.
Today we know that being right-handed or left-handed is not a binary characteristic (left-handers on one side, right-handers on the other), but that there is a sort of graduation, from absolute left-handed to absolute right-handed, with a host of degrees in between.
As they develop motor skills, children use their right and left hands interchangeably to grab objects, because both hands can easily perform these tasks. However, more specific tasks require the “specialized” intervention of the left hemisphere of the brain. This is why most children use their right hand to write.
Writing skills develop over time and the use of the right hand is confirmed while the child learns to hold his pen, moving from a “palm grip” to trace the first shapes on a page to a “tripod grip” (three fingers) which allows him to form letters and connect them. Observing how a child goes about writing tells us where the development of specialized processes in their brain is.
The most recent research shows that children who are “definitely” left-handed or right-handed have good brain lateralization and have no difficulty with language. On the other hand, ambidextrous children (i.e. those who use both hands indifferently to write) encounter more difficulties in the development of language and motor skills.
Ambidextrous people represent 3 to 4% of the general population. In children with autism spectrum disorder (ASD), this figure reaches 17 to 47%.
Children suffering from ASD begin to experience motor difficulties from the age of 7 months. This tends to prove that this type of disorder can be detected very early in children, and that they can have repercussions on cognitive functions as important as language.
Manual preference, what is it for?
The research I am currently conducting aims to understand how children’s hand preference can be used to assess the risk of developing language disorders. Diagnoses of ASD are generally made quite late, when it is noticed that the child cannot speak or understand basic things. These late diagnoses are detrimental to children, who could benefit from therapies and medical interventions earlier in their lives: infants’ brains are incredibly flexible, and if they received the appropriate care on time, it would benefit their cognitive development and to their mental health.
Hand preference is one example among others of sensorimotor laterality in humans. For example, most people use their right hemisphere when reacting to danger. This means that we recognize threatening expressions and faces more easily when they appear in our left visual field rather than our right visual field. A series of classic experiments shows that adults recognize negative emotions more easily in photos of faces when they are presented to them from the left rather than from the right.
By mapping the developmental pathways of sensorimotor biases and cognitive faculties in children, we increasingly understand the relationships between brain organization, brain functions, and behavior.
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