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As children grow older, they tend to favor one hand over the other for certain tasks, particularly for writing or drawing. A child’s “handedness” is generally categorized as right, left, or mixed, and tends to settle around the same time they acquire language — about 4 years old. It remains a persistent characteristic throughout our life.

We now know that a child’s handedness says something about the organization and function of their brain.

The left and right hemispheres of the brain control motor action on the opposite sides of the body. Yet, the left and right halves of the brain are not equal in their control of different types of behaviors, which results in a bias of one hand over the other for certain tasks. The dominance of one hemisphere over the other for certain behaviors is called cerebral lateralization.

Scientists think that there are clear reasons for the evolution of cerebral lateralization. First of all, having one hemisphere take control of a process lowers the chance of both hemispheres competing to control a response. It also allows for different processes such as language and attention to operate in parallel across the two hemispheres.

For the vast majority of people, the left hemisphere of the brain is the dominant one used for speech. And the same region of the left hemisphere that controls speech also controls hand actions.

As a result, the majority of the human population (about 90 percent) is right-handed when they use tools, such as pens, and when they make gestures. Evolutionary psychologists speculate that tool use and hand gestures played an important role in the evolution of human speech. One theory suggests that because vision is our primary sense, human communication first emerged as hand gestures. As we became sophisticated tool users, it was more efficient to keep our hands free for tool use and our communication transferred to speech. The structured sequences of hand actions required to make and use tools may also have prepared the brain for language syntax.

In order to acquire complex skills like language, children must first development basic sensory and motor abilities. Developmental psychologists argue that fine motor capabilities like manipulating objects and gesturing sets the stage for acquiring systems required for the subsequent development of language.

Left, right or both?

Early to mid-20th century scientists considered left-handedness to be a developmental abnormality. It was associated with a range of developmental dysfunctions ranging from language deficits to mental health disorders. In fact, many left-handed children of this era were forced to write with their right hands in an effort to “retrain” them.

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.

The skill develops over time and becomes consistently right-handed as children progress from using a “fisted” grip, to make their first marks on a page, to the delicate “tripod” grip required for fine motor actions like forming and joining letters. Observing a child’s handedness for fine motor activities, such as writing, can give us an indication of how well the two hemispheres have developed their specialized processing capabilities.

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.

Ambidextrous individuals make up about 3 to 4 percent of the general population. This figure rises to between 17 and 47 percent in populations of children with autism spectrum disorders (ASDs). Children with ASDs also show motor abnormalities as early as 7 months of age. This suggests that ASDs are likely to be present and observable early in a child’s development, and are likely to have “knock on” effects to the development of higher cognitive functions like language.

How can handedness be useful?

New research that I’m working on is considering how infant handedness can be used as a marker of a child’s risk of developing language disorders. 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. This has been demonstrated in a range of classic psychology laboratory studies where adults are faster to judge pictures of faces expressing negative emotions presented to their left compared to those presented to their right.

Mapping the developmental paths of motor biases and cognitive abilities in children provides a novel way for us to better understand the relationship between brain organization, brain function, and behavior.

Gillian Forrester, Senior Lecturer in Psychology, University of Westminster.