Selective mutism (SM) is an anxiety disorder in which a person who is normally capable of speech cannot speak in specific situations or to specific people. Selective mutism usually co-exists with shyness or social anxiety.[1] People with selective mutism stay silent even when the consequences of their silence include shame, social ostracism or even punishment. Selective mutism affects about 0.8% of people at some point in their lives.

Children and adults with selective mutism are fully capable of speech and understanding language but fail to speak in certain situations, though speech is expected of them.[4] The behaviour may be perceived as shyness or rudeness by others. A child with selective mutism may be completely silent at school for years but speak quite freely or even excessively at home. There is a hierarchical variation among people with this disorder: some people participate fully in activities and appear social but do not speak, others will speak only to peers but not to adults, others will speak to adults when asked questions requiring short answers but never to peers, and still others speak to no one and participate in few, if any, activities presented to them. In a severe form known as "progressive mutism", the disorder progresses until the person with this condition no longer speaks to anyone in any situation, even close family members. Selective mutism is by definition characterized by the following:[5] • Consistent failure to speak in specific social situations (in which there is an expectation for speaking, e.g., at school) despite speaking in other situations. • The disturbance interferes with educational or occupational achievement or with social communication. • The duration of the disturbance is at least 1 month (not limited to the first month of school). • The failure to speak is not due to a lack of knowledge of, or comfort with, the spoken language required in the social situation. • The disturbance is not better accounted for by a communication disorder (e.g., childhood-onset fluency disorder) and does not occur exclusively during the course of autism spectrum disorder, schizophrenia, or another psychotic disorder.