"Over the years, Cho spoke very little to his parents and avoided eye contact," they wrote. "If called upon to speak when a visitor came to the home, he would develop sweaty palms, become pale, freeze, and sometimes cry. Frequently, he would only nod yes or no."
In 1999, when he was in eighth grade, a psychiatrist diagnosed Cho with selective mutism and single episode major depression. Selective mutism falls under the broad ICD-9 diagnostic category of disturbances of emotions specific to childhood and adolescence.
Diagnostic criteria for selective mutism, according to the Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV), include "a 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."
To meet the criteria, the disturbance must interfere with school, work, or social interaction, last at least one month, not be due to lack of knowledge or comfort with the language requited in the social situation, and not be explained by other conditions such as communication disorders, pervasive developmental disorder (including autism spectrum disorders), or psychotic disorders.
