According to the Selective Mutsim Foundation, "selective mutism is a psychiatric disorder most commonly found in children, characterized by a persistent failure to speak in select settings that persists for more than one month. These children understand spoken languages and have the ability to speak normallhy. "In typical cases, they speak to their parents and a few selected others. Most are unable to speak in school and other major social situations."
There is no clear cut answer to what causes selective mutism but according to the Selective Mutism Foundation, "through published studies, selective mutism appears to be related to severe anxiety, shyness and social anxiety."
Shyness is actually a normal personality trait where a person voluntarily withdraws from others, particularly unfamiliar people. Everyone has some degree of shyness.
According to SelectiveMutism.org, "shyness is not a psychiatric disorder, like SM. People who are shy are able to function adequately in society. People with physchiatric disorders such as SM do not adapt well to social situations nor are they able to communicate effectively with others."