Some autistic people may be selectively mute due to anxiety in unfamiliar social situations. If mutism is entirely due to autism spectrum disorder, it cannot be diagnosed as selective mutism as stated in the last item on the list above. The former name elective mutism indicates a widespread misconception among psychologists that selective mute people choose to be silent in certain situations, while the truth is that they often wish to speak but are unable to do so. To reflect the involuntary nature of this disorder, the name was changed to selective mutism in 1994. The incidence of selective mutism is not certain. Due to the poor understanding of this condition by the general public, many cases are likely undiagnosed. Based on the number of reported cases, the figure is commonly estimated to be 1 in 1000, 0.1%. However, a 2002 study in The Journal of the American Academy of Child and Adolescent Psychiatry estimated the incidence to be 0.71%.
Other symptoms
Besides lack of speech, other common behaviors and characteristics displayed by selectively mute people, according to Dr. Elisa Shipon-Blums findings, include:
Shyness, social anxiety, fear of social embarrassment or social isolation and withdrawal
Difficulty maintaining eye contact
Blank expression and reluctance to smile or incessant smiling
Difficulty expressing feelings, even to family members
Tendency to worry more than most people of the same age
Sensitivity to noise and crowdsOn the flip side, there are some positive traits observed in many cases:

Above average intelligence, inquisitiveness, or perception
A strong sense of right and wrong
Creativity
Love for the arts
Empathy
Sensitivity for other people

Causes
Selective mutism (SM) is an umbrella term for the condition of otherwise well-developed children or adults who cannot speak or communicate under certain settings. The exact causes that affect each person may be different and yet unknown. There have been attempts to categorize, but there are no definitive answers yet due to the under-diagnosis and small/biased sample sizes. Many people are not diagnosed until late in childhood only because they do not speak at school and therefore fail to accomplish assignments requiring public speaking. Their involuntary silence makes the condition harder to understand or test. Parents often are unaware of the condition since the children may be functioning well at home. Teachers and pediatricians also sometimes mistake it for severe shyness or common stage fright.Most children and adults with selective mutism are hypothesized to have an inherited predisposition to anxiety. They often have inhibited temperaments, which is hypothesized to be the result of over-excitability of the area of the brain called the amygdala.