Machine Gun Kelly says he has 'Peter Pan syndrome.' Why some adults fear growing up.

Machine Gun Kelly says he suffers from "Peter Pan syndrome." But what exactly is it?

While discussing his upcoming movie "Taurus," the "Bloody Valentine" singer, 32, said he, like his movie character, is "just a broken boy who has Peter Pan syndrome."

"You don't want to grow up," he said Tuesday while speaking to The Hollywood Reporter’s Awards Chatter podcast. "You're a lost boy, but you have Wendy, and Wendy doesn't want to be with the lost boy, but she's like, 'Come back home and be a grown-up with me.'"

Peter Pan syndrome is not an actual medical diagnosis, experts say. It's a popular metaphor named after the timeless fairytale to describe adults who feel like "kids at heart," characterized by a fond nostalgia for their youth and a fixation on living life spontaneously.

"Even celebrities like Machine Gun Kelly can have all the trappings of success but still feel like a helpless child inside," says Dr. Gauri Khurana, a child and adult psychiatrist based in New York. "Adulthood can be challenging, and it is common to feel anxious about one’s ability to fully take care of themself."

Oftentimes, she says, this childlike innocence stems from an array of factors, including anxiety and childhood trauma. But it's not always a bad thing.

"Everybody likes Peter Pan because he embodies this newness, potential for growth and hope for the future," says Nathan Brandon, a licensed psychologist specializing in anxiety. "You don't have to get rid of the youthful nature altogether, but it is important to find a healthy balance … because there's the negative side of an adult who refuses to meet the challenges of life."

'Peter Pan complex' describes adults who don't want to grow up

Those with "Peter Pan syndrome," also called failure-to-launch syndrome, typically possess a playful outlook on life.

"Someone with Peter Pan syndrome may be young at heart, optimistic, and can see joy in the simple things," says Lauren Cook, clinical psychologist and founder of Heartship Psychological Services. However, "they can also struggle with commitment, responsibility, and facing obligations that they don't feel like doing because it's uncomfortable for them." Other common signs include romanticizing escapism, the expectation of being taken care of and an overreliance on others.

These behaviors, however, "exist on a spectrum from mild to severe, persisting over time and over varied situations," Khurana says. "This means a person may not seem childlike in one situation, for instance when a parent dies, but can in other aspects of their life, like taking care of themselves."

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What are the causes of Peter Pan syndrome?

Though there is little research on Peter Pan syndrome, experts speculate some adults may use it as a coping mechanism to avoid the stress of adult responsibilities such as parenting, paying the bills or searching for jobs. Unresolved childhood trauma can also play a role in emotionally stunting someone (In the past, Kelly has talked and rapped about when his mother left at age 9).

Brandon says those with Peter Pan syndrome can also be a product of overprotective parents who "encourage their kids to stay a child for as long as they can because the adult world is difficult."

As a result, "they may fail to develop the skills necessary for adulthood, which then causes them to become overly dependent."

Is there treatment? How to overcome the fear of growing up

The biggest step in overcoming Peter Pan syndrome is identifying the root cause of these worries. Cook encourages those to "lean into the discomfort," with the help of counseling.

"Build self-esteem and confidence with a professional therapist and learn to meet long-term goals in your life, career and relationships," Brandon say. "Taking on more responsibility in your life can seem daunting, but it's key to making positive changes."

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This article originally appeared on USA TODAY: Machine Gun Kelly says he has Peter Pan syndrome. What is it?