Moms who think their teenagers “tune them out” are probably right. But rather than a sign of rebellion, that behavior signals something to be celebrated: a healthy growing pain in family life.

Tuning out is the result of a biological signal that helps teens separate from their parents and become more independent as they mature, according to research from Stanford School of Medicine just published in the Journal of Neuroscience. The researchers used brain scans to show how children respond — or don’t — to their mother’s voice.

Functional magnetic resonance imaging showed how different voices light up a child’s brain — with big changes in the response occurring around the time a child turns 13.

Up to that point, brain scans show “an explosive neural response to their mother’s voice, activating reward centers and emotion-processing centers in the brain,” according to ScienceAlert.

Mom’s voice is recognizable to babies in the womb, said Percy Mistry, co-lead author and a research scholar in psychiatry and behavioral sciences at Stanford. “The mother’s voice is the sound source that teaches young kids all about the social-emotional world and language development.”

But, in the teenage brain, mom’s dulcet tones don’t get their own neural fireworks.

“Just as an infant knows to tune into her mother’s voice, an adolescent knows to tune into novel voices,” said lead author and psychiatrist Daniel Abrams in a news release. “As a teen, you don’t know you’re doing this. You’re just being you. You’ve got your friends and new companions and you want to spend time with them. Your mind is increasingly sensitive to and attracted to these unfamiliar voices.”

The scans showed that in teens the parts of the brain that light up in response to rewards and stimuli respond more to unfamiliar voices than to a mother’s voice. The scientists call it “an aspect of healthy maturation.”

“A child becomes independent at some point and that has to be precipitated by an underlying biological signal,” said senior author Vinod Menon, professor of psychiatry and behavioral sciences. “That’s what we discovered: This is a signal that helps teens engage with the world and form connections which allow them to be socially adept outside their families.”

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Teens are different

In 2016, the research team showed kids were very accurate in identifying their own mother’s voice and that her voice triggers parts of the brain that other voices do not in younger children, including reward centers and areas that process emotions and visual signals. In younger kids, mom’s voice also triggers neural networks that determine if incoming information is important. That voice helps kids develop language and learn about social-emotional things.

The new study built on previous work and focused on children 13 to 16.5 years old with at least an 80 IQ who were being raised by their biological mothers. None of the children in the study had been diagnosed with neurological, psychiatric or learning disorders.

Mothers were recorded saying the nonsense words, as were two women the children didn’t know. The teens then heard the recordings, and they recognized their own mother’s voice with 97% accuracy. After that, from the MRI scanner they listened to the recordings as well as other sounds like a running dishwasher, included to see how their brains reacted to both social and nonsocial sounds.

In teens, voices got more reaction in several parts of the brain, compared to children ages 7 to 12. And the response to voices increased with the teens’ age. The study said that the change was so clear the researchers could guess a child’s age by reaction to the voices.

But in teens, unfamiliar voices got a bigger response than mom’s in the reward processing system and in the region that is involved in assigning value to social information. The researchers found no difference between boys and girls.

The researchers said the study, like their previous work, creates a framework in which to look at developmental shifts among adolescents with social impairments, including autism. In 2016 they noted that children with autism and other clinical disorders may not be as able to recognize “biologically salient” voices.