Researchers have found why at around the age of four children suddenly do what three-year-olds are unable to do -- put themselves in someone else's shoes.

This enormous developmental step occurs as a critical fibre connection in the brain matures, according to a study published in the journal Nature Communications.

For the study, the research team from Max Planck Institute for Human Cognitive and Brain Sciences (MPI CBS) in Leipzig, Germany and Leiden University in the Netherlands analysed MRI data and behavioural data of 43 normally developing 3- and 4-year-old children.

"Our findings show that the emergence of mental state representation is related to the maturation of core belief processing regions and their connection to the prefrontal cortex," said the researchers. The study showed that maturation of fibres of a brain structure called the arcuate fascicle between the ages of three and four years establishes a connection between two critical brain regions.

One region is at the back of the temporal lobe that supports adult thinking about others and their thoughts, and the other is in the frontal lobe that is involved in keeping things at different levels of abstraction and, therefore, helps us to understand what the real world is and what the thoughts of others are. 

Only when these two brain regions are connected through the arcuate fascicle can children start to understand what other people think, the study said. Interestingly, this new connection in the brain supports this ability independently of other cognitive abilities, such as intelligence, language ability or impulse control, according to the researcher.