Scientists have gained an insight into why children who are taught two languages from a young age have the best chance of growing up bilingual and fluent in both.

Teams from Brown University and King’s College London worked together on the study, published in The Journal of Neuroscience, in order to gain an understanding of the interaction between asymmetry in myelin and the development of language in early childhood. To do this, they took brain scans of 108 children aged between one and six years old who had typical cognitive development.

What does the science say?

Using the scans, the scientists examined the growth and positioning of myelin. This is insulating cellular material that forms a sheath around the axons of the body’s central nervous system. Although it is something that begins to develop in pregnancy, infants do not have much present in the brain when they are born, which means their brain is more flexible at this stage. Myelination accelerates between the ages of four and six and continues until adolescence.

By examining the scans, the scientists concluded that the locations where myelin develops become fixed around the age of four. As a result, the effect of outside influences on skills such as language are far stronger before this has occurred, when the brain is most pliable.

At the start of the study, researchers predicted they would witness the development of more myelin on the left side of the brain – where language abilities are housed – as the children learnt this skill, but in fact it was constant across the brain. However, it was found to be more influential on language development before the age of four. Because of this, the study helps explain why the bilingual children with the greatest fluency in two languages are likely to have been immersed in both from birth.

Not just language development

However, it’s not only language development that the studies provide an insight into. Dr Jonathan O’Muircheartaigh, of King’s College London, tells the BBC the results may also be relevant to the study of autism and other developmental disorders. “Since our work seems to indicate that brain circuits associated with language are more flexible before the age of four, early intervention for children with delayed language attainment should be initiated before this critical age,” he adds.

The study is the first of its kind to examine the relationship between language and the brain’s structure. Because early signifiers of disorders like autism include a delay in the development of language, the research could one day lead to a breakthrough being made in the diagnosis and treatment of autism and related disorders.

A University of Arizona-led study published in the Association for Psychological Science’s journal in March revealed that those who are brought up bilingual can switch between their two languages by differentiating between the unique sound systems of each. As a result, they can listen and react in the correct way to the language they hear – even if the person they are speaking to switches between the two.

The development of language in humans remains one of science’s greatest mysteries. Debate abounds not just as to how infants acquire the rules of syntax, but also as to how and why humans began speaking in the first place.

Noam Chomsky is among the world’s leading linguists. He acknowledges that how humans acquired language is one of life’s most intriguing mysteries, but suggests it may be as a result of a gene mutation in a single human ancestor.