The way I see it:

We've always known that something was extremely weird in an adolescent brain. We have all been there (and suffered it!), and some of us have to interact with a teenager on a daily basis. I am talking about dealing with a biological teenager's brain, not with someone that's behaving like one but has long ago overcome his 20's (this is a complete different story, and an argument for a whole new post).

It looks like the prefrontal cortex, which is the most recently evolved part of the human brain, where adult behaviour seem to happen: long-term planning, executive function, impulse control, emotional not being fully developed until your mid 20's.

Combine that with an unbridled overdose of hormones, and you have an explosive Molotov cocktail.

Want to know why? Research shows that this might occurs due to one of the cleverest things that brains ever evolved.

Want to know more? Keep reading. Robert Sapolsky explains it beautifully in his article. A summary of his main ideas, below.

Spoiler alert: There are also benefits of having an adolescent brain. Believe it or not, "their incandescent ability to feel someone else’s pain, to feel the pains of the entire world, to want to right all its wrongs makes adolescents nature’s most wondrous example of empathy."


Adolescence is the time of life when someone is most likely to join a cult, kill, be killed, invent an art form, help overthrow a dictator, ethnically cleanse a village, care for the needy, transform physics, adopt a hideous fashion style, commit to God, and be convinced that all the forces of history have converged to make this moment the most consequential ever, fraught with peril and promise.

For all this we can thank the teenage brain. Some have argued adolescence is a cultural construct. In traditional cultures, there is typically a single qualitative transition to puberty. After that, the individual is a young adult. Yet the progression from birth to adulthood is not smoothly linear. The teenage brain is unique. It’s not merely an adult brain that is half-cooked or a child’s brain left unrefrigerated for too long. Its distinctiveness arises from a key region, the frontal cortex, not being fully developed. This largely explains the turbulence of adolescence. It also reflects an important evolutionary pressure.

The frontal cortex is the most recently evolved part of the human brain. It’s where the sensible mature stuff happens: long-term planning, executive function, impulse control, and emotional regulation. It’s what makes you do the right thing when it’s the harder thing to do. But its neurons are not fully wired up until your mid-20s. Why?

It’s a central tenet of genetics that the genome that we start off life with, back when we were just a fertilized egg, is passed on to every subsequent cell in the body. But if the frontal cortex is the last part of the brain to fully mature, it’s the brain region least shaped by that genome and most sculpted by experience.

Our success as primates, and as individuals, revolves around social intelligence and adapting to the subtleties and idiosyncrasies of the environment. This is the portfolio of the frontal cortex. So if it’s going to develop the ability to do this right, it’s going to have to be profoundly shaped and informed by every smidgen of experience along the way.

Around the onset of adolescence, the frontal cortex is the only brain region that has not reached adult levels of grey matter, made up of neuronal cell bodies. It would seem logical that gray matter levels would increase thereafter. But no, over the course of adolescence, frontal cortical gray matter volume decreases.

This occurs because of one of the cleverest things that brains ever evolved. During fetal development, mammalian brains generate far more neurons than are found in the adult brain. Why? Because the fetal brain holds a dramatic competition. Winning neurons get to migrate to the correct location and form the optimal number of connections with other neurons. Neurons that miss the cut undergo “programmed cell death.” Neuronal overproduction followed by competitive pruning (a process that has been termed “Neural Darwinism”) allows more complex and optimized neural circuitry, a wonderful example of less being more.

The same plays out in the adolescent frontal cortex. At the beginning of adolescence, gray matter volume is greater than it is in adults, and subsequently declines, as less optimally connected neurons are pruned away. Within the frontal cortex, it is the evolutionarily oldest sub-regions that mature first; the spanking new dorsolateral prefrontal cortex, for example, does not even begin to lose gray matter volume until the end of adolescence. This delayed frontal cortical maturation means that adolescents aren’t at adult levels of expertise at various cognitive tasks, like recognizing irony or Theory of Mind—the ability to operate with the knowledge that someone else has different information than you do.

In an adult, the frontal cortex steadies the activity of parts of the limbic system, a brain region involved in emotion; in contrast, in the teenage brain, the limbic system is already going at full speed, while the frontal cortex is still trying to make sense of the assembly instructions. One result of this imbalance is that emotions are more intense. Stick people in a brain scanner and show them pictures of faces expressing strong emotions.

But in the teenager, the frontal cortical response is muted, and the amygdala’s response is augmented. That means emotional highs are higher, and the lows are lower. This is shown in studies of limbic pathways that release dopamine, a neurotransmitter central to anticipation of reward and pleasure (cocaine, for example, works on this limbic dopamine system). Average dopamine levels in adolescents and adults do not differ; what differs are patterns of dopamine release.

The delayed maturation of the frontal cortex also helps explain the defining feature of adolescence, namely the weird predilection for bungee jumping. During risky decision-making, adolescents show less activation of some key sub-regions of the frontal cortex than do adults, and among adolescents, the less activity in these regions, the poorer the risk assessment.

And adolescents are bad at risk assessment in a particular way, shown by Sarah-Jayne Blakemore of University College London. Ask test subjects to estimate the likelihood of some event happening to them and then tell them the actual likelihood of it happening.

So adolescents are lousy at risk assessment and take more risks. But there’s more to the story of those skeletons in Moaning Caverns. It’s not the case that adolescents and adults have an equal desire to do the same dumb-ass thing, and the sole difference is that the fully mature frontal cortex in the latter prevents them from doing so. Adolescents feel the allure of jumping off things. Middle-aged adults just recklessly cheat on their diets. Adolescents not only take more risks, they seek more novelty.

The slowpoke frontal cortex is not the only explanation for teen behavior. Another factor comes into play that keeps that teen brain off balance, namely gonadal hormones like estrogen and progesterone in females, and testosterone in males. This helps explain why adolescence is more turbulent than childhood—the frontal cortex is immature at both ages, but the tsunamis of hormones haven’t started in pre-adolescents. Hormones have numerous effects on the function of both the limbic system and frontal cortex. Testosterone decreases the ability of the frontal cortex to communicate with and rein in the amygdala. Not surprisingly, landmarks of adolescent maturation in brain and behavior are less related to chronological age than to time since puberty.

As adolescence dawns, the frontal cortex’s efficiency is diluted with superfluous connections failing to make the grade. The limbic system is fully online and dopamine is careening all over the place. Meanwhile, the brain is being marinated in the ebb and flow of gonadal hormones.

But adolescence isn’t always as dark as it’s made out to be. There’s a feature of adolescence that makes up for the stupid risk-taking and hideous fashion decisions. And that’s an adolescent’s frenzied, agitated, incandescent ability to feel someone else’s pain, to feel the pains of the entire world, to want to right all its wrongs. Adolescents are nature’s most wondrous example of empathy, where the forcefulness of feeling as the other can border on nearly being the other.

This is the picture of adolescents with their hearts on their sleeves, limbic systems going full blast, and their frontal cortices straining to catch up with some emotional self-regulation. When I see the best of my university students in that agitated, optimistic state, I always have the same thought: It used to be so much easier to be like this. Having this adult frontal cortex of mine probably enables me to do good in a more efficacious, detached way. The trouble, though, is the same detachment makes it so much easier to decide that it’s really not my problem.

So what is the adaptive advantage of human brain development evolving this way? Potentially, there is no advantage. Perhaps the human frontal cortical maturation is delayed because it is the most challenging construction project the brain undertakes. In this view, wiring up something like the visual cortex can pretty much be wrapped up in the first year of life, but doing the same in the frontal cortex takes another quarter century. This seems unlikely. The frontal cortex has the same basic structure as the rest of the cortex, uses the same neurotransmitters and types of neurons. On a nuts and bolts level, its maturation is slower than necessary, suggesting that there has indeed been active selection for the delay, that there is something advantageous about it.

If the frontal cortex is the last part of the brain to fully mature, it is by definition the brain region least shaped by that genome and most sculpted by experience. With each passing day, the frontal cortex is more the creation of what life has thrown at you, and thus who you become.


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