Having false beliefs is understanding that others can have a view of the world which is inaccurate and is different to ours.
In a false belief task developed by Wimmer and Perner, children were asked where a character (Maxi) would look for his chocolate if he left it in a blue cupboard but his mother moved the chocolate to a green cupboard without him knowing.
Results showed that most 3 year old children incorrectly answered the green cupboard. However, the 4 year olds mostly answered correctly (the blue cupboard). By the age of 6 all children gave the correct answer of the blue cupboard.
This shows that from age 4 onwards, children begin to develop a theory of mind, and by the age of 6 all children have a theory of mind.