Selective mutism is a severe anxiety disorder where a person is unable to speak in certain social situations, such as with classmates at school or to relatives they don't see very often.
It usually starts during childhood and, left untreated, can persist into adulthood.
A child or adult with selective mutism doesn't refuse or choose not to speak, they're literally unable to speak.
The expectation to talk to certain people triggers a freeze response with feelings of panic, rather like a bad case of stage fright, and talking is impossible.
In time, the person will learn to anticipate the situations that provoke this distressing reaction and do all they can to avoid them.
However, people with selective mutism are able to speak freely to certain people, such as close family and friends, when nobody else is around to trigger the freeze response.
Selective mutism affects about 1 in 140 young children. It's more common in girls and children who are learning a second language, such as those who've recently migrated from their country of birth.