The Surprising Science of Adolescent Brains: Jennifer Pfeifer (Transcript)

Read the full transcript of researcher Jennifer Pfeifer’s talk titled “The Surprising Science of Adolescent Brains” at TEDxPortland, August 18, 2025.

Introduction

JENNIFER PFEIFER: If you’re too rebellious, party animals, maybe glued to their TVs or Ataris, why can’t they be more like adults? Who, of course, have exactly the right amount of mental health issues and other vices, right? Of course not. But when it comes to young people, it’s a problem. And we look for something to blame. Is it their biology? Maybe the latest technology?

I’m here to tell you what the science says. Adolescence isn’t a problem to be solved. It’s a transformative period of growth and opportunity. And we all have a role to play in helping them unlock its potential. So, let’s dive into the science of adolescence.

Who Is an Adolescent?

First things first, who is an adolescent? I can’t see very well. I’m sure there are some in here. You’re probably thinking teenagers, right? Thank you. Actually, no. Scientists say adolescents last from about 10 to 25 years of age. Why? Because of how it’s defined. With a biological beginning in puberty and a social ending when they gain adult rights, roles, and responsibilities.

The Biology of Puberty

Let’s start with a biological beginning and something that probably nobody in this room is excited to talk about except me. Puberty! It’s the best. Okay. Let’s take a quick guided tour. Puberty is a natural process everybody goes through starting around age 10, give or take a year, when our brains release hormones that ultimately change a lot about our bodies.

We can’t see what’s going on inside our bodies, but there are a lot of outward puberty signs. From skin, hair, and voice changes to body smells, shapes, and size. One puberty change that might be a surprise is sleep. Puberty kick starts a sleep phase delay, which means an adolescent’s biological clock doesn’t make them feel tired until one or two hours later than before. Combine that with everything going on in their social lives, and ta-da! You have a new night owl.

Now, if you spend any time around middle schoolers, you’ll know the range in these puberty signs is huge, especially between boys and girls. Because on average, girls start puberty about one or two years earlier than boys. So you can thank puberty for so many awkward middle school dances.

The Impact of Early Puberty

When you see the signs that puberty is starting, something really important to know is this. Going through puberty earlier than your same-age, same-sex peers is linked with more depression risk, especially in girls. But the surprising thing we’ve learned about that is it’s not the hormones increasing that risk. It’s how young people, girls, feel their bodies are changing compared to others. That’s because having your body change so dramatically changes how you feel about yourself. And that matters for your mental health.

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It also changes how society sees you. One way this shows up is in something called adultification. When we see more grown-up-looking bodies, we treat them differently. We have different expectations. For example, older boys assume early maturing girls are more interested in things like sex. And adultification also happens in our education and justice systems where it’s extra harmful.

The bigger picture here is we love to blame hormones, but biology matters less than how young people see themselves and how society sees them during this stage of life. That’s actually a good thing. We can’t avoid hormone changes forever, but we can make puberty a little easier by combining greater awareness of some of these puberty-related risks and biases I just talked about with open communication.

Look, if you can talk candidly about puberty to a middle schooler, you have set the foundation for honest conversations not just about physical health, but all kinds of important things to come.

Understanding Adolescent Brains

Besides hormones, another slice of adolescent biology we misunderstand is their brains. The idea that teenagers have immature brains, and that’s why they make such bad decisions, it’s everywhere. Where’d that come from? In the 1990s and 2000s, neuroscientists learned that adolescent brain changes don’t really level out until your mid-20s. This brain science was so striking that it helped shape multiple Supreme Court decisions, most importantly, ending the death penalty for minors. Now we’ve basically… Yeah, applause for that. Science that matters.

Now we’ve basically embraced the idea that the adolescent brain is still developing. That’s fantastic, but there’s a risk that the same science used to protect youth can be misapplied to limit their rights, like to vote or make their own health care decisions.

So I have a really important message to share. Decades of research show adolescents can make good decisions about big things. Around age 16, their decision-making abilities basically match those of adults. When they have the time and space to think carefully. It’s interesting, right? I just told you the adolescent brain is still changing to the mid-20s, but these key decision-making abilities are online earlier.

So I think that it’s time we reframe our view of adolescent brains. Don’t think of them as immature. They’re perfectly suited to meet the needs of young people. Their brains prime them to explore, learn quickly from rewards, and be sensitive to social status. It’s easy to think of those things as liabilities, but that would be a mistake. They’re actually powerful assets at a time in life when building identity, independence, and new relationships outside the family really matters.

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Let’s acknowledge all the amazing things that adolescents can do. And to be super concrete, if you find that a young person’s right to vote or make their own health care decisions is on the ballot in the future, don’t forget, research shows these are exactly the things they can do well by age 16.

The Smartphone and Social Media Debate

I thought I studied the biggest and baddest monsters of adolescence, and then smartphones entered the chat. Forget biology. The most explosive story right now is that social media is destroying this generation.

It’s true that youth anxiety, depression, and suicide risk are at alarming levels, but are smartphones to blame? Anyone want to know what science says? Okay, you ready?

Social media use is one of the least influential factors shaping youth mental health. I know, this isn’t what we’re being told, and the kicker is, I know it probably doesn’t feel true if you parent or teach adolescents, but here’s the thing, science doesn’t care if something feels true.

What the Research Really Shows

Let me tell you the facts. Scientists have a way of statistically summarizing information across all the studies on a topic. It’s called a meta-analysis. Meta-analyses are great because they can keep you from getting science whiplash, you know, when the headlines ping-pong back and forth with each new shiny study.

Anyways, there are a lot of them on this topic, and they consistently show really small effects. Excessive time on social media is linked with maybe 15% higher levels of mental health problems in youth at most. That’s like nudging from 20%, which is the current baseline risk for adolescent depression, up to 23%. The effect’s so small, it almost gets lost in the noise.

And in fact, when studies do a good job accounting for the many known risk factors in adolescent depression, that’s exactly what happens. The effect basically disappears. This makes sense if social media use is like a marker of youth mental health problems, a correlate, and not the cause.

The Real Factors That Matter

Now those other risk factors, those should be on your list, because they have way more impact. Take relationships. Whether you’re 16 or 60, they matter for your mental health. For example, youth who are bullied have double the risk of developing depression. That’s a way bigger effect. But an adolescent with high-quality friendships, they can spend four or five hours a day online and still have over a 90% chance of great mental health and well-being.

Four to five hours a day kind of freaks you out. That’s the current national average for 13 to 19-year-olds. What about families? The good news is they can protect against bad outcomes from even the worst experiences, including bullying. But parent mental health problems more than triple the risk of adolescents developing them too. 3.5 times greater risk.

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I don’t say this to blame parents. My point is just that parents matter way more than phones. Look, as someone deeply invested in the well-being of young people, both as a scientist and as a mom, I wish the answer were as simply shutting off the phones. But the science is clear. We won’t be able to solve this by focusing on social media alone. In fact, young people often turn online first for mental health resources and support.

What We Can Do

So what do we do? Big system-level changes are needed to fix this, no question. But we’re not powerless. If you’re worried about youth mental health, build their resilience. Listen. Provide support. Help them learn that feelings and failures are a normal part of adolescence because they’re a normal part of life.

I love how this message is for all of us. It’s for parents especially. Even the best parents fail sometimes. We have hard days and big feelings, but too few of us get the support that we need. So if you’re struggling with your own mental health, know that asking for help makes you a great parent because one of the best predictors of this generation’s well-being is the mental health of those who care for them. In other words, put your own oxygen mask on first. Let’s… Yeah.

Changing the Narrative

Let’s land this thing. Adolescence lasts from 10 to 25 years of age when bodies, brains, relationships, and mental health are all changing. But it’s not the biological or psychological changes alone that define adolescence. We do. We are all in this together. That means we have to stop doom-shaming young people. If we keep saying this generation is being destroyed, how can we expect them to believe in their own potential? Thank you.

Instead, we need to respect their growing strengths, create space for them to contribute at home, at school, and in the community. Honestly, we need young people just as much as they need our love and support. Their ability to adapt to our rapidly changing world, it’s a superpower.

It’s time to change the narrative. Adolescents, they’re not a problem to be solved. They represent our brightest future. Thank you. Thank you.