At around the age of four we suddenly do what three-year-olds are unable to do: put ourselves in someone else's shoes. Researchers at the Max Planck Institute for Human Cognitive and Brain Sciences (MPI CBS) in Leipzig and at Leiden University have shown how this enormous developmental step occurs: a critical fiber connection in the brain matures.
If you tell a 3-year-old child the following story of little Maxi, they will most probably not understand: Maxi puts his chocolate on the kitchen table, then goes to play outside. While he is gone, his mother puts the chocolate in the cupboard. Where will Maxi look for his chocolate when he comes back? A 3-year-old child will not understand why Maxi would be surprised not to find the chocolate on the table where he left it. It is only by the age of 4 years that a child will correctly predict that Maxi will look for his chocolate where he left it and not in the cupboard where it is now.
The researchers observed something similar when they showed a 3-year-old child a chocolate box that contained pencils instead of chocolates. When the child was asked what another child would expect to be in the box, they answered "pencils," although the other child would not know this. Only a year later, around the age of four years, however, will they understand that the other child had hoped for chocolates. Thus, there is a crucial developmental breakthrough between three and four years: this is when we start to attribute thoughts and beliefs to others and to understand that their beliefs can be different from ours. Before that age, thoughts do not seem to exist independently of what we see and know about the world. That is, this is when we develop a Theory of Mind.
The researchers have now discovered what is behind this breakthrough. The maturation of fibers of a brain structure called the arcuate fascicle between the ages of three and four years establishes a connection between two critical brain regions: a region at the back of the temporal lobe that supports adult thinking about others and their thoughts, and a region 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. Interestingly, this new connection in the brain supports this ability independently of other cognitive abilities, such as intelligence, language ability or impulse control.