What these papers suggested was that the dogma was completely wrong. In fact, the human brain continues to develop significantly across almost the whole cortex throughout the teenage years, and even into the 20s. This was an intriguing finding, but it also pointed to a massive gap in the field. There were very few papers, little was known, and there were so many questions that had yet to be answered.

I decided back then—in the year 2002, 2003—to change the focus of my research from adult studies on schizophrenia and other mental illnesses to developmental studies. In retrospect, that was a risky maneuver because I'd never done a developmental study before. It was the encouragement of my friend and mentor, Professor Uta Frith, that gave me the confidence to make that change, and also a fellowship from the Royal Society allowed me to take this relatively risky avenue.

In the past 12 or 15 years, a huge amount has been discovered about the development of the human brain throughout the teenage years. Many labs now work in this area, and there's been an explosion of research. We know a lot about the development of the adolescent brain, and I'm going to talk to you about that today, and about the questions that still remain, because there are many.

Most of the work has been done with structural imaging—structural MRI. That is the method that has changed the game in this area of research because before we were able to scan the living human brain with MRI, we weren't able to understand how the brain changes across development. Now we can. We can scan kids of all ages, as long as they keep still, which is not always the case. We're able to look at changes in brain structure, and also changes in brain function across the life span. That technology was the turning point in our understanding of the development of the brain. Now there's a huge amount of experimental behavioral studies on cognitive and socioaffective changes during the teenage years.

In my lab we're particularly interested in adolescent-typical behaviors. What I mean by that are behaviors that you stereotypically associate with teenagers, things like risk taking, heightened self-consciousness, and peer influence. There are a lot of nice examples of these behaviors, and I'm going to read one. This is a letter that was written to the Guardian, which is a British newspaper, a couple of years ago. This is a reader who says:

There's nothing like teenage diaries for putting momentous, historical occasions into perspective. This is my entry for the 20th July, 1969.

'I went to arts center in yellow cords and blouse. Ian was there but he didn't speak to me. Got rhyme put in my handbag by someone who's apparently got a crush on me. It's Nicholas I think. Ugh.

Man landed on moon.'

The reason why that letter is nice is because it illustrates what's important to that girl at that particular moment in her life. Less important that man landed on moon than things like what she was wearing, what clothes she was into, who she liked, who she didn't like. This is the period of life where that sense of self, and particularly sense of social self, undergoes profound transition. Just think back to when you were a teenager. It's not that before then you don't have a sense of self, of course you do.  A sense of self develops very early. What happens during the teenage years is that your sense of who you are—your moral beliefs, your political beliefs, what music you're into, fashion, what social group you're into—that's what undergoes profound change.

We're particularly interested in the effects that peers have on adolescent decision making. It's well known that adolescents do take risks, and they probably take a disproportionate number of risks. However, if you give them an optimal situation, an optimal environment, they don't necessarily take more risks. If they're in a lab and there are no distractions, there are no emotionally motivational salient factors going on, they perform similarly to adults. They take about the same number of risks, depending on what task you use. When you give them some motivational context, for example, a couple of friends standing behind them, that's when you see heightened risk taking in adolescents. You don't just see it in the lab. We all know from epidemiological data and data from car insurance companies that that's borne out in real life, as well. Adolescents, for example, have more car accidents than older people, but the situation in which they have those car accidents is normally when they have a same-age passenger in the car with them.

We're interested in why adolescents are particularly susceptible to peer influence. One of the ways we've looked at this is to look at what happens when adolescents are ostracized by their peer groups. We've done this by using the pretty well-known game called Cyberball, which is a game of catch—a ball game—that you play over the Internet with what you think are two other people. In fact, they're not, they're programmed by the lab. You can program those two other players to either include participants in this ball game, or exclude them from the ball game.

When adults play the Cyberball game and they've been excluded from that game of catch, they feel sad, their mood lowers, they feel more anxious, and you can measure that. This has been done many times by labs around the world. We compared adolescents and adults in this Cyberball game, and we found that exactly the same response was found but even more so in adolescents. Their mood dropped even more than adults' mood did, and they became even more anxious than adults after being excluded from this game. That suggested that adolescents might be hypersensitive to being socially excluded. When you think about that in the context of adolescent decision making, it sheds adolescent decision making and risk taking in a more rational light.

Whenever you make a decision you weigh various pros and cons, various advantages and disadvantages. With something like speeding down the motorway, or texting while driving, you might think, "I'm going to get to my meeting on time if I speed," or you might get a kick out of speeding. On the other hand, you might crash, you might get caught by the police. You're weighing up these pros and cons, but there's also the social factor. We know that people behave differently in groups compared with when we're on our own, and having someone else observe your behavior changes your behavior on cognitive tasks. What we think is happening is that, in adolescence, that social influence is particularly heightened. This is a framework that my student, Kate Mills, and I have been working on recently.

If you take, for example, smoking. Say you have a 13-year-old girl, and all her friends are smoking. For her, what is the more risky decision, saying yes to a cigarette when she knows the risks associated with smoking—as all 13-year-olds do these days—or saying no and potentially ostracizing herself from her peer group? We think that because of the hypersensitivity to being ostracized by the peer group, saying no is probably more of a risk for adolescents.

One of the things coming out of adolescence studies is this idea that these stereotypical behaviors we associate with adolescents—risk taking and peer influence—are there for a reason. There's probably a good reason why adolescents care so much about being included by their social group and take more risks when they're with their friends. I'm not an evolutionary biologist, but it makes sense when you think about the need, the drive to become independent from one's parents, to go and explore the environment, and to affiliate with your social group during this period of life. One thing I'm not saying is that risk taking is bad, or that peer influence is bad. It's probably an important and adaptive process that we all need to go through in our transition between childhood and adulthood.

One of the questions that we've been interested in looking at is why and how is it that social influence has its effect on decision making and behavior in adolescents? There are lots of theories about why this is, and I'm not going to go into any detail. One of the things that we're interested in is the development of the social brain. What I mean by that is the network of brain regions that are involved when we do theory of mind—when we think about other people's minds, their intentions, their beliefs, their desires, their emotions.

There is a circumscribed network of brain regions that are activated when we do a mentalizing task. What various labs around the world have found is that that network of brain regions undergoes significant development, both in terms of structure—in gray matter and white matter development—and also in terms of function during adolescence. Specifically, a number of labs now have replicated the effect that a certain region of the social brain called the medial prefrontal cortex, is more active in adolescents than in adults when thinking about other people's minds. Even though adolescents and adults in these studies are just as good at the mentalizing task, they use a different level of activity in medial prefrontal cortex in order to do the task. Again, we don't know why this is, but we think it might have something to do with the cognitive strategy, or the mental approach to the problem. The way they solve these problems might require different levels of activity in the regions of the social brain network.

The development of the social brain during adolescence suggests that during this period of life the brain is particularly susceptible to social pressure, but also to the social experiences that adolescents have around them, and the social opportunities that are given to them. This brings me on to one of the major questions. There are many, many questions that still remain to be looked at in this area, but one of them is whether adolescence represents a sensitive period for brain development. You'll hear a lot of people talking about adolescence as representing a second sensitive period of brain development, but we don't have very much data on that.

We know from studies on early development of the brain, both from humans and non-human animals, that the brain undergoes different "sensitive periods" of development, meaning there are periods of development where the brain is particularly susceptible to certain types of environmental stimuli. We know lots about this in domains such as sensory input, and also language input in the first few years of life, just to name a couple of examples.

Given that the brain is undergoing a lot of development during adolescence, particularly in areas like prefrontal cortex and other cortical areas, many people have suggested that this might represent a window of opportunity—a second sensitive period for learning in cognitive and social domains. There is very little evidence on this yet. It makes a lot of sense, but it's still an open question. It's something that we and other labs are currently looking at, but again, there's not much to say about it yet because there are a couple of studies suggesting it might be in some domains.

If it's true that adolescence represents a sensitive period of brain development in some areas of cognition and social behavior, then that has implications for things like education; when to teach what, what's the best moment to teach calculus, or algebra? It also has implications for the social environment; should adolescents be experiencing certain types of social interaction experiences during that period of life? It has implications for things like the legal treatment of teenagers. At the moment, if teenagers do something naughty they are incarcerated with other teenagers who have done something naughty. Yet we know that they are particularly susceptible to peer influences. Is this the most rational thing to do? It's probably not a particularly productive solution.

If adolescence is a sensitive period for brain development, that is a double-sided coin because although it represents a period of opportunity in which the brain is particularly susceptible to acquiring new information in certain domains, it also might represent a period of vulnerability, in which the brain is particularly vulnerable to certain environmental inputs.

I've been briefly talking, in no detail at all, about average teenage brains. Most of the data we have comes from averaging over teenage brains, but there is no average teenage brain. There is no average teenager. The individual differences are much greater than the averages. We're only just starting to look at individual differences and how individual differences in both genetics and environment influence brain development. We know that they both do. We know that genetics—your genotype—and also your environment influence your brain development. Environmental influences are almost infinite and difficult to study, but there are things like stress, alcohol, drugs, your social group, your family environment, your culture, your peers, who you hang out with, all these things invariably will be shaping the way the adolescent brain is developing. They also might play an influence in triggering the onset of mental illnesses in people who are genetically predisposed to them.

Now I'm going back to where I began: mental illness. It's not just schizophrenia that has its onset during this period of life. It's also many other mental illnesses. Most mental illnesses—depression, anxiety, eating disorders, addictions—have their onset at some point during adolescence. There is something about adolescence that means this period of life is a window of vulnerability to these illnesses.

My aim 12 years ago was to map out the development of the adolescent brain and then move on to brain development in teenagers who become schizophrenic. I've just made just a tiny input into the former, and I'm not anywhere near doing work on schizophrenia yet, but other labs around the world have started to do that. I've mentioned the NIH study particularly, and there are lots of labs, but I have a collaboration with Jay Giedd at the NIH and he is one of the pioneers of this area, and they have done research on longitudinal studies looking at brain development in kids who then go on to develop some mental illness, or developmental disorder. The data is quite new, it needs to be replicated. It's interesting, but it's quite varied.

The one take-home message is that it is critical to look at development in these disorders. Rather than taking a snapshot of what the brain looks like in an 18-year-old with depression compared to 18-year-olds who don't have depression—the brain might look similar by that age—what's critical is the way it gets there; there can be different developmental trajectories that end up at more or less the same point. The analogy I'd give is, you might use the motorways, the freeways, or the A roads, ending up at the same point, but you take a different route to get there. That seems to be what's critical in a lot of these developmental disorders and mental illnesses. Looking at development, and not just taking a snapshot in time is important.

Finally, I wanted to talk about prevention of mental illness because, like I said, adolescence might give us a window of opportunity, not just for things like education and learning, but for intervention. There's a dogma in social policy and educational policy that the first three years of life are the critical window where you have to get in and intervene. What this research on the brain is suggesting is that the brain continues to develop; it is plastic, but in a heightened way, right throughout the teenage years, so it's not too late during the teenage years to intervene in the cases where people might need some extra help.

One of the interventions that is important is an intervention that prevents, or at least reduces the onset of mental illness in people who are susceptible to it. That's something that a lot of people are thinking about at the moment. How do you do that? Is a universal approach better than a targeted approach? One of the areas that we are about to start working on is looking at whether mindfulness meditation, as a universal treatment in schools, has any affect on wellbeing and lowering anxiety and stress, but also reducing the onset of mental illness in teenagers. There is some promising data on that from other people's labs. We haven't begun our studies on that yet, but we are about to next year.

One of the things that I have learned over the last 10 or 12 years of researching in this area is that it's critical to include your research subjects in every aspect of your experimental design. This is not something that I had done previously. There's a tendency for adults to think that they know best for teenagers when teenagers know probably a lot more than we do about what's best for them in terms of their education, in terms of their social environment, what they want to do.

It applies to experiments as well. These days we always include teenagers in the designs of our experiments, the designs of our stimuli, and I'll give you a couple of trivial examples where that's helped. Firstly, talking to teenagers about actual phenomena that they experience has led us to design experiments, or apply for grants to research those phenomena. We probably wouldn't have remembered what it was like when we were teenagers, and also things have changed. Each generation is different.

There was one experiment we did where we had a stimuli with a load of objects on it, and one of those objects was a tape—a cassette—and involving teenagers at a very early stage of our research made us realize that no teenagers know what a tape is. They don't recognize it, so we changed that. We're interested in peer influence, and we found from involving teenagers in the designs of our studies that what matters to them is not being observed by a peer, but having a peer monitor their behavior. Being told that this friend of yours is going to sit behind you, and after you've done this task, they're going to fill in a questionnaire about how you did, that's what matters to them. Again, we couldn't have guessed that from the adult literature, because there's no indication that that is a significant factor in adults. That's what we do now on the basis of suggestions by teenagers. 

THE REALITY CLUB

JOSHUA KNOBE: I like the point that you were making about how these behaviors on the part of teenagers that might seem irrational can be seen as rational. I was just wondering why that same argument doesn't also apply to parents. These kids are in a situation where they could do the seemingly safe thing, or they could do the seemingly risk-seeking thing. If you think about it, the actual risk of the safe thing is great, because the risk is a reputational risk. You can see why adolescents would evolve to do these seemingly risk-seeking things that are actually the safe thing because they avoid having reputational punishments.

Why do we, as parents, not evolve in the same way? Why is it that if I saw my daughter doing a safe thing I wouldn't pressure her to do a more badass thing so that she would avoid the possible reputational punishments? Why is it that, as parents, we don't have that exact same mentality: "You must seek the risk-seeking behavior, or else you're going to suffer these reputational punishments?”

BLAKEMORE: We're not very good at taking the perspective of people who are different from us, and we've forgotten how important it was to impress your friends, or not to be ostracized by them. Not all teenagers are like this, and again, I want to emphasize that there are individual differences. Some teenagers never take any risks, some teenagers aren't susceptible to peer influence. I bet, around the room, certainly for me, if you think about your teenage years you did take risks when you were with your friends, not when you were on your own. Now you probably wouldn't. You just wouldn't. If a group of your colleagues were smoking cigarettes outside and they offered you one, you wouldn't mind saying, "No thanks, I don't smoke," as an adult. It's easy to forget the social pressure of not being excluded by your group, and the importance of what that feels like.

KNOBE: You're thinking of it as happening it at the proximate level, not at this ultimate level. The question I was asking was why didn't we just evolve as parents to pressure our kids to engage in risky behavior?

BLAKEMORE: Well, I guess risk taking has to be constrained. Risk taking is a good thing. If we didn't take risks, where would we be? On the other hand, it can be dangerous and can result in accidents or even death. If you look at mortality rates across the life span, the number one cause of death in adolescents is from risk taking; it's from accidents. That's not true at any other period of life. Risk taking is a good thing unless it goes too far. You need some constraints over risk taking, and that's where parents come in. That's probably evolutionarily important as well.

HUGO MERCIER: I have a quick question about the evolutionary history of the theory. I guess you could have two thoughts. One is that throughout our evolutionary history, there was this significant period of our lives during which we had to form cliques to find partners—sexual partners, cooperation partners—so it was important, as you are saying, to get along with them and to give in to peer pressure. Because it was always around the same time that that happened throughout our history, our brains have taken that in, and now they reflect that history. Another possibility—both are evolutionarily consistent—would be that throughout our history, there had been times in people's lives in which they found themselves in these situations in which they have to make new friends, or new partners—that can happen when you're 40, it can happen when you're 15, it can happen at any age—so instead of having this maturational period of adolescence, our brains would be equipped to behave in the way that adolescents behave now, in any context in which it's the best thing to do. Could you think of people who are conscripted in the army, or start a new job, or move to a new country, and you find yourself in a situation that is similar to that adolescence, and would you think that they can revert, or they can become more adolescent-like?

BLAKEMORE: I don't know of any research looking at that. It probably exists, but I don't know any data. My guess would be that the brain is plastic throughout life. You can change, you can revert, you can behave differently in different contexts, but the large amount of brain development going on during adolescence, although it's protracted in some areas and continues right throughout the 20s and 30s, it is stabilizing around then. I suppose if you're going to attribute these changes in behavior during adolescence to changes in the brain structure and function, then those changes are not going to be as profound in adults, even if you find yourself in a situation which might demand that.

MERCIER: Could it be an artifact of the fact that most of the adults we've scanned happened to have stable lives? If you look only at adults who need to change plans often because they're moving often, or they don't have a stable partner, maybe you find that their brain keeps being adolescent-like for longer.

JENNIFER JACQUET: I have the same inverse question, then. Imagine a 17-year-old who has been a kidnapped bride, and now has two children, and isn't going through the same peer pressure. Would their brain show the level of stability then? Are there cross-cultural studies?

BLAKEMORE: That's an excellent question. Adolescence is often defined as the period of life between puberty and the end of adolescence is defined at the age at which you attain a stable, independent role in society. With that definition—which I like, a lot of people use it—that includes variations between cultures. In our culture it's normal to be an adolescent—using that definition—right into the 20s, even in the 30s we may not feel particularly stable and independent yet. In other cultures, like the cultures you are mentioning, kids are expected to become financially independent, get jobs, earn money as soon as they go through sexual maturity, or even before. Girls are expected to get married, have babies as soon as they can. Some people have argued that adolescence doesn't exist; it's a Western invention about 100 years ago.

There are three reasons why that's not a completely watertight argument. Firstly, if you look at cross-cultural studies you can see, even in cultures that vastly differ, increases in risk taking, and increases in peer influence, and self-consciousness in those different cultures. Secondly, there are studies in animals showing that even animals undergo a period of heightened risk taking and heightened socialization during their adolescence, post-puberty. If you look at historical descriptions of adolescents, even from thousands of years ago, or in Shakespeare 400 years ago, you see similar descriptions of this age group as the way we stereotypically describe today as taking risks, making bad decisions, and being particularly influenced by their peers.

I'm not saying that culture does not influence the development of the brain, of course it does. Not very much is known about that at all—how brain development looks in these different cultures—but I'm sure it will be subtly different, and people will measure that when they start these studies. There is a lot of overlap between cultures.

MICHAEL MCCULLOUGH: I've been thinking a lot about changes of state in biological systems. Something like the skull, there are so many functions contained in it and yet it has to grow from a small size to something much larger over time. To get all of those functions to coordinate, to create a change from the infant skull to an older skull, the genetic architecture that enables that to happen without devastating trauma is mindboggling. When it doesn't happen, it is truly devastating.

The same with something like sleep, going from a waking state to a sleeping state. We think of it as just unplugging the cord, right? It turns out, for people with chronic sleep disorders who cannot fall asleep there are many things that have to go right. We usually take for granted that they will go right. Or even the assignment of the primary sex characteristics during development, so many things have to go right.

I'm thinking about something like schizophrenia, which is different from lots of other mental disorders inasmuch as it is a truly devastating disease, where evidently, many things that have to go right, at least one of them hasn't gone right. If that premise is true, why does it take the shape it does? It's a menu you pick your features from, but why those features and not others? How do those features so reliably come out of a developmental failure—failure in a change of state—that leads most children to another social way of being in the world? Why does this generic failure, if everything would go right, lead one way, and what does that tell us about what the actual target is normatively?

BLAKEMORE: It's an interesting question, and that's what I was interested in when I was studying schizophrenia back in the day. I was interested in delusions of control and auditory hallucinations. This is where patients think that their movements are being controlled by someone else, or a machine, or they're hearing voices inside their head. My question wasn't why, it was how come that doesn't happen for all of us? When I move and pick this up, how do I know that's my own movement, and that was a movement caused by my intention? That's amazing. How does that happen? Why doesn't that go wrong in all of us? That's exactly the question I was interested in, but not from a development point of view at that point, from a phenomenological point of view. Also, from a mechanistic point of view, how do we achieve that?

All these things are on a spectrum, and yes, people with schizophrenia, there is a clear cut set of symptoms, and it's severe, but each one of those symptoms, most of us will have experienced to a tiny extent at some point. You only have to look at the effects of psychotropic drugs to just push the brain over into temporary paranoia or hallucinations to see it's all a fragile state, and it's not a black and white qualitative difference. It's a quantitative difference. All of these things are on a spectrum, and you can measure that with things like schizotypy questionnaires. The question is why are some people pushed over the edge into this, sometimes permanent, situation of constantly experiencing delusions and hallucinations?

MCCULLOUGH: Or command hallucinations, for example. It's so tempting to try to draw a parallel between those and where the normal, typical pattern of development should take you, which is appropriate approval and respect from your peers. Having people that you can influence, and that can influence you in ways that are going to be adaptive through the rest of adulthood.

BLAKEMORE: One interesting theory of adolescence, back in the '50s, by Peter Elkind, was this idea that teenagers have an imaginary audience. They think they're being watched and judged by other people much more than they are. That is similar to the state that people with schizophrenia describe how their life feels, that they're constantly being watched and observed. I'm not saying there is an overlap or there is a similarity, but there is something that is similar, and the question is how do most teenagers not go over the edge into paranoia?