"Selective mutism is described in the 2000 edition of the Diagnostic and Statistical Manual of Mental Disorders(DSM-IV-TR: pp.125-127).
Children with selective mutism may also show:
- anxiety disorder (e.g., social phobia)
- excessive shyness
- fear of social embarrassment
- social isolation and withdrawal
Many times a child with selective mutism has or is experiencing:
- an anxiety disorder
- inner self/self-esteem issues
- a speech, language, or hearing problem
- trauma
Associated behaviors may include no eye contact, no facial expression, immobility, or nervous fidgeting when confronted with general expectations in social situations. These symptoms do not indicate willfulness, but rather an attempt to control rising anxiety."
http://www.selectivemutism.org/