Years ago, when a young child couldn't respond verbally to a question, parents called it "cocking a deaf ear."
Now that inability to speak has a medical name — selective mutism — and some suggest that it may have led Seung Hui Cho to cock a gun and shoot 32 victims and then himself at Virginia Tech in April.
Cho was treated for selective mutism as a high school student in the Fairfax County, Va., schools, according to media reports this week.
His emotional condition was considered so severe — he was virtually unable to speak in social settings — that he was placed in a special education program, and a plan was devised to help him.
The condition was one of an array of psychological disorders that the troubled student displayed throughout his short life.
Selective mutism is a type of social anxiety disorder, though some research suggests a biological link to the amygdala, the part of the brain that governs emotions and the "fight or flight" mechanism.
Those who suffer from the disorder "feel afraid," Dr. Elisa Shipon Blum, director of the Selective Mutism Anxiety Research and Treatment Center in Jenkintown, Pa., told ABC News.
"Many of them say, 'The words won't come out,'" she said. "Their voice box feels like it's closing up. Their stomach hurts. Their body won't let them speak."
In a classic case, the child can be a chatterbox at home and with familiar friends, but be paralyzed and speechless in group settings or around strangers. Like a panic attack, the behavior becomes learned, especially when parents speak for the child.
Most selective mutes "self correct" or start talking on their own between grades 3 and 5, due to peer pressure, according to Bill Garrison, professor of pediatrics at University of Massachusetts Medical School.
"The boys do not, however, become highly verbal," he said. "They just begin to talk enough so that they are not noticed," Garrison said.
The cause of the disorder is not known, though it may be inherited. It can strike at any age, but most develop sufferers will develop selective mutism before the age of 5.
It is more common in girls than in boys, according to Shipon Blum, and about 70 percent of those with the disorder have an immediate family member who also struggles with it.
Some studies say it is twice as common as autism, affecting about 1 percent of the population, according to the American Academy of Child and Adolescent Psychiatry.
"Selective mutism implies that the child refuses to speak, but not speaking is the end result," Shipon Blum said. "It's a social communication impairment, an anxiety."
Her research points to several risk factors: language difficulties — including bilingualism, as in the Cho case — and speech and sensory issues. Pressure from parents and teachers can exacerabate the disorder.
The difference between ordinary shyness and selective mutism is the degree to which the child is functioning, said Shipon Blum.
"A shy child can function socially, even if they are not the most popular or outgoing in the class," she said. "They may not be emotionally thrilled with themselves, but they function."