It is 11:30 on an August morning in New York City's Central Park Zoo—breakfast time for the sea lions. A joyful crowd oohs and aahs as trainers put the animals through their paces: catching tossed fish in midair, high-fiving with their flippers, squirting water and torpedoing around the pool. Amid the raucous throng, nine small children watch in wide-eyed silence. When a sea lion zips past at stunning speed, they do not add their voices to the squeals of delight. Some of these children are talking quietly to a camp counselor. Others sit with worried expressions that seem sadly at odds with the scene.

The nine children, ages three to six, are subdued by an anxiety disorder called selective mutism, a condition that often looks and feels like very painful shyness but with a twist. These kids will generally speak—and some will blithely chatter away—when out of the public eye and in the comforting cocoon of their own homes. But in certain settings and most typically in school, they shut down and go silent.

The zoo outing is part of a four-day program called Camp Courage, offered by the Child Study Center at New York University. It is the model for half a dozen summer camp programs for kids with selective mutism. Once thought to be extremely rare, the disorder is now believed to affect between 0.5 and 0.8 percent of youngsters—meaning there is at least one such child in most elementary schools.

These are the kids who never speak in class or whisper to only one or two confidants on the playground. They are the kindergartners who wet their pants, or worse, because they are too mortified to ask permission to use the bathroom. One child at Camp Courage accidentally hammered his thumb during a school craft project and said nothing; a teacher finally came to his aid after noticing the trail of blood.

Pediatricians often dismiss the disorder, which typically emerges when a child begins preschool or kindergarten, as a passing phase that will resolve itself. In the meantime, parents get in the habit of speaking for their child, asking others to accept shrugs and gestures as communication, and explaining their youngster's “shyness” to baffled teachers and neighbors. But selective mutism does not always fade away. Survey findings and clinical experience suggest that many affected children continue to struggle for more than five years, according to child psychologist Richard Gallagher, who heads N.Y.U.'s selective mutism program. A small percentage of children remain mute into high school.

Psychologists and educators familiar with selective mutism now believe intervening to break the mute behavior pattern is important so that it does not compromise a child's academic, social and psychological development. That belief is in keeping with a broader trend toward early intervention in other childhood conditions that affect learning and socialization, such as attention-deficit/hyperactivity disorder, communication disorders and autism. Selective mutism is less well recognized, however, and many kids who would benefit from therapy either receive the wrong kind or none at all. Only in the past few years has rigorous research validated a therapeutic approach to selective mutism. At the same time, scientists are beginning to explore the mysterious—and in some cases, surprising—roots of this once obscure disorder.

Naming the Silence

Spend a few days around children who have selective mutism, and you begin to wonder if they have a hidden on/off switch. In the large, airy classroom where Camp Courage convened, I saw a child stop a conversation in its tracks the minute an unfamiliar therapist tried to join in. Conversely, a boy who had been largely silent all day got into an elevator with his mom at pickup time and began a perfectly ordinary chat about where she had parked the car.

Because these kids are capable of speaking normally, their mute behavior can look willful. In 1877, when German physician Adolph Kussmaul penned what may be the earliest description of selective mutism, he named it aphasia voluntaria (Latin for a “voluntary lack of speech”). In keeping with the idea that the child has chosen silence, psychiatrists called the disorder elective mutism when it first appeared in the Diagnostic and Statistical Manual of Mental Disorders (DSM) in 1980. The name changed in the 1994 edition after research and clinical experience made it clearer that selective mutism was driven more by anxiety than defiance. The current edition (DSM-5), published in 2013, classifies selective mutism as a unique childhood anxiety disorder marked by a failure to speak in certain settings that cannot be explained by communication or language difficulties.

Most children who are treated for selective mutism are also diagnosed with an additional anxiety disorder. Usually this is social anxiety disorder, which involves grave distress in social settings and often a paralyzing fear of making a social mistake. Although most socially anxious kids are withdrawn but not mute, more than half and perhaps as many as nine out of 10 selectively mute children also suffer from social anxiety. A number of clinicians regard selective mutism as a subtype of this disorder.

Despite its sometimes dramatic symptoms, selective mutism is often overlooked. In 2002 R. Lindsey Bergman, a child psychologist at the University of California, Los Angeles, studied the prevalence of this disorder among 2,256 kindergartners and first and second graders in a large California school district. She noted that most of the children who matched DSM criteria for the disorder (based on detailed input from their teachers) had not been previously identified. “Teachers just think the kids are super, super shy,” Bergman says. “And at this age, the teacher is more worried about the child who is acting out and not staying in his seat.”

Another common issue is mislabeling. In her clinical practice, Bergman has seen youngsters with selective mutism who have been incorrectly diagnosed with a speech and language problem or an autism spectrum disorder. Sue Newman, co-director of the Selective Mutism Foundation, says that she frequently hears about misdiagnosed children who have been placed in educational settings designed for kids with autism or speech disorders that not only fail to address their mutism but may make them feel more self-conscious and anxious.

Even when the problem is correctly diagnosed, finding help (and a qualified therapist who accepts health insurance) is a challenge. According to Britanny Roslin, who is one of three N.Y.U. child psychologists at Camp Courage, “a lot of clinicians don't want to work with these kids, because they don't know what to do. You can be sitting across from a kid for years without speaking.”

Why They Go Quiet

A child's mute behavior can come as a shock. Susan* still gets emotional recalling events on her son's third day of kindergarten when a teacher came up to her spouse at pickup time and cheerily asked, “So, is he ever going to start talking?” Evan* was exuberant and verbal at home and had been vocal during three years of preschool. “We were totally flabbergasted,” she remembers. “We were both up all night.” Only in hindsight did they see the warning signs: Evan's refusal to say hello to waiters, store clerks and adult neighbors. The couple, who live in New York City, recognized their son's symptoms from online descriptions of selective mutism and quickly made contact with N.Y.U.'s program.

Clinicians who work with kids with the disorder say that parents often describe their kids as having been cautious and socially reticent since infancy. These characteristics—what psychologists call a behaviorally inhibited temperament—are typical of 15 to 20 percent of babies and toddlers. “They are hesitant to interact with peers—they withdraw from social situations and are highly vigilant,” says developmental psychologist Nathan Fox of the University of Maryland. Although most go on to be perfectly fine, behaviorally inhibited children have a 30 percent increased risk of developing an anxiety disorder, especially social anxiety.

As with most psychiatric disorders, the causes of selective mutism are not well understood, but a genetic component seems likely. Studies have found that anxiety disorders of various types tend to run in the families of affected kids. Bergman says, “When I see parents of kids with selective mutism, about 75 percent of the time I can say, ‘Which one of you was like this as a kid?’ and one will say they either did not speak in class or sat there in fear that they would be called on.”

A 2011 study involving 106 children with the disorder offers a hint to its genetic origins. University of California, San Diego, psychiatrist Murray Stein and his colleagues found preliminary evidence that a variation in a gene called CNTNAP2 raises the risk of the disorder. The study also discovered that the same gene variation was associated with symptoms of social anxiety in a group of 1,028 young adults. The CNTNAP2 gene codes for a protein that is produced in the developing cortex of the brain and plays a role in brain cell connectivity. Intriguingly, variants of the gene have been implicated in autism and certain language impairments—findings that suggest the gene might have a part in a variety of social and communication disorders.

One research group in Israel has found evidence that many children with selective mutism have a hearing abnormality that affects the way they perceive their own voice. In a series of small studies published between 2004 and 2013 involving a total of 75 subjects with selective mutism, psychologist Yair Bar-Haim of Tel Aviv University and his collaborators found that roughly 50 percent of affected children have some kind of problem with their “efferent” auditory system. This system—which involves the middle ear, brain stem and cerebral cortex—normally attenuates the sound of one's own voice, which, as Bar-Haim says, is otherwise loudly “bone-conducted directly into our own brain.” Quieting down our voice helps us tune into our environment while speaking. Bar-Haim's findings could help explain why some children with this disorder complain that their voice sounds funny or loud to them. If he is correct, then treating anxiety alone will be insufficient for many cases of selective mutism.

Indeed, numerous factors can contribute to anxiety about speaking. According to osteopathic physician Elisa Shipon-Blum, director of the Selective Mutism Anxiety Research and Treatment Center in Jenkintown, Pa., developmental delays, learning disabilities, speech and language issues, and sensory processing challenges can cause a child to shut down in a noisy, overstimulating classroom. “We may see a deficit in their narrative skills—their ability to tell a story, to tell you what they read in a book or what a movie is about,” Shipon-Blum says.

For the most part, children who have selective mutism are too young to offer their own explanations for their behavior, but a preoccupation with making verbal or social mistakes seems to be central for many. Danica Cotov, a recent college graduate from New Jersey who struggled with mutism for 16 years, gives this account: “I lived in constant fear of being judged by my peers, who I was certain would think negatively of me. I had a constant stream of thoughts and worries running through my head.” After years of silence, she dreaded the fuss that would be made if she ever did speak up.

Finding Help

At Camp Courage, each of the nine children was working toward a specific goal. For Cindy,* an elfin girl with big, brown eyes and a honey-colored braid, it was to use her “full voice” instead of a whisper. Evan was working on allowing anyone besides Gallagher, with whom he chatted easily, to hear his voice. Campers earned points by participating in group games such as Go Fish that required simple, predictable utterances (“Do you have any zebras?”). At the end of each day, they could pick a big or small prize depending on how many points they had racked up.

Psychologists at N.Y.U. and elsewhere typically treat selective mutism with a modified version of therapies shown to be effective for other anxiety disorders and phobias. First they encourage kids to speak with parents in a clinical setting and eventually to speak with the therapist. In close collaboration with teachers, they gradually move the child through a hierarchy of behaviors—from nonverbal exchanges to mouthing words to whispering and then using a full voice—in circumstances where he or she would ordinarily be mute. They also work on widening the circle of people to whom the child will speak. At school, for example, teachers may be coached to permit silent nods, then one-word answers prompted by simple, limited-choice questions (such as “Is the answer 5 or 7?”). Along the way, kids earn rewards for speaking up. The idea is that gradual exposure to speaking will defang their fears.

In 2013 Bergman and her colleagues published a study on this type of treatment. She divided 21 children with selective mutism, ages four to eight, into two groups. One group was placed on a 12-week waiting list. The other group received 24 weeks of an intervention that included 20 hour-long private sessions with a therapist and assignments designed to gradually increase the child's exposure to speaking in feared settings—mainly school. Therapists worked closely with teachers and parents, who were taught how to continue to help the child once the experiment was over. Independent evaluators, who did not know which kids had the intervention, rated their progress.

After 12 weeks, a quarter of the children receiving treatment showed major improvement, whereas none on the waiting list improved. After completing the full 24 weeks of therapy, 75 percent of the treated children had progressed in their speaking behaviors, and two thirds of them no longer met the criteria for selective mutism. Three months later they were found to have maintained their progress. Aside from a small-scale drug study published in 1994, Bergman's research appears to be the first ever randomized controlled study of a therapy for the condition.

Getting Results

In Gallagher's experience, the children who respond fastest to therapy are those who are social, despite the disorder. “They look like they want to be involved with other kids,” he says. “They play, they use a lot of gestures, they have friends.” More challenging are those who have some features of autism and lack motivation to engage with others, and youngsters with symptoms of a broader social anxiety who appear distressed or uncomfortable even when playing. Several studies have shown that some children improve when given a selective serotonin reuptake inhibitor such as Prozac, which reduces anxiety. Adding elements of cognitive-behavioral therapy can help older kids learn to use reason to make their fears seem more manageable.

When school got under way last fall, Cindy's parents were pleased with her progress. “She's initiating conversation with her teacher,” her mother says. “That didn't happen last year.” For Evan, progress is slower. “When we walk into school, he's saying, ‘Hey, everybody,’ to the kids and has his rambunctious, easygoing personality,” Susan reports. “Then when we say, ‘Gotta go, have a great day, love you,’ that's when he stops talking. He's missing out on so much of school, and [his teachers] are missing out on so much of him.”

Few studies have followed children with selective mutism throughout childhood and adolescence, so no one can say with authority how long the disorder typically lasts, what percentage will remain socially anxious or what traits predict a good outcome. One point does seem clear: training teachers, special education personnel, and speech and language specialists to better recognize selective mutism and to intervene more effectively could help many children. Once they have begun to speak, most remain timid, but for others, Bergman says, “it's almost like they've had the flu. They go on to be the most gregarious people, and you can imagine that years later their families will say, ‘Oh, my gosh, remember when Mary didn't talk?’”

*Not their real names