Mutism is a psychiatric disorder most commonly found in children, characterized by a persistent failure to speak in select settings, which continues for more than 1 month. These children understand spoken language and have the ability to speak normally.
In typical cases, they speak to their parents and a few selected others. Sometimes, they do not speak to certain individuals in the home. Most are unable to speak in school, and in other major social situations. Generally, most function normally in other ways, although some may have additional disabilities. Most learn age-appropriate skills and academics.
Currently, Selective Mutism, through published studies, appears to be related to severe anxiety, shyness and social anxiety. Selective Mutism may be associated to a variety of things, but the exact cause is yet unknown.
Information:
http://www.depression-guide.com/mutism.htm
http://www.asha.org/public/speech/disorders/SelectiveMutism.htm#dx
http://en.wikipedia.org/wiki/Selective_mutism
Pictures: Google