We all know a child who is hesitant to speak. While shyness is an accepted personality trait, in an estimated one percent of children, that social anxiety is debilitating.

Health reporter Erin Billups has more on a relatively unknown condition called selective mutism.

Five-year-old Sam Henick is like most kids: he gets excited about new toy prizes. "I got two 'Star Wars!'"

But most people who meet side don’t see a bubbly little kid, because he often doesn't talk. His mother, Julie Henick, just thought he was slow to warm up in social situations -- that is until Sam's teachers told her he hadn't uttered a word in his pre-school class.

"I thought they were a little crazy because I couldn’t believe that my son wasn't speaking at school. But he wasn’t, the whole year," said Henick. "I showed them a video of him speaking at home and they were blown away. They were just like, 'Wow, this is a different Sam.'"

Henick took Sam to a psychologist and he was diagnosed with selective mutism. The American Psychiatric Association officially classified it as an anxiety disorder in 2013.

"It's a diagnosable condition where a child who is otherwise really chatty at home, is inhibited and often unable to talk in situations like school or playdates, or activities, sometimes for an entire year or years,” said Dr. Rachel Busman, a child psychologist that leads the Anxiety Disorder Center at Child Mind Institute.

Sam began weekly behavioral therapy. Two years later, he's functioning better.

This summer marks his second year participating in Child Mind Institute's "Brave Buddies" camp. It’s a week-long intensive that creates a safe place for kids with selective mutism to practice socializing.

Dr. Busman says not talking needs to be “un-practiced.”

"Imagine how many times someone might ask a child a question: 'Where do you put your backpack? Did you bring your lunch today? Who are you going to play with?' And every question is met with an inability to answer. It becomes really practiced," Busman said.

Selective mutism is not tied to any childhood trauma or developmental disorders like autism. It is believed to be a consequence of the child's environment, natural temperament, and a family history of anxiety.

In hindsight, Henick realized she struggled with social anxiety well into adulthood. Becoming a fitness instructor eventually forced her out of her shell.

But she says Sam's future is already looking brighter. "His teachers were telling me that they are amazed that this is the same child because he was speaking. He was raising his hand," she said.

Early intervention is key, says Dr. Busman, because it gives children tools to engage in school, reducing the chance they'll fall behind.