Four facts about teenage brains all neuroscientists agree on.

Biology determines the beginning of adolescence.

The tick-tick-tick of the hypothalamic clock signals the beginning of the transition from girl to woman. But the end of adolescence has no clear biological demarcation. When exactly do girls become adults? When they’re physically able to have a baby? Or when they actually have a baby? When they turn eighteen? Finish school? Leave home? Get their first full-time job? Buy a house? Get married?

With puberty starting earlier than it used to and the traditional cultural markers of adulthood somewhat irrelevant, ‘adolescence’ could last decades!

Despite its current unclear cultural or social endpoint, adolescence is a distinct period of brain development. It’s when brain becomes particularly vulnerable to reorganisation by sex hormones and when the cognitive, emotional and social circuits refine and mature. Tracking alongside the brain changes are the stereotypical changes to the thoughts, emotions and behaviours of the brain’s teenage owners.

Adolescence is a significant neurological transition

Most scholars agree on a few facts about the teenage brain

1. Early childhood is marked by increased cortical grey matter volume, which peaks in late childhood. Throughout adolescence, the frontal, parietal, and temporal cortices experience a reduced volume and thickness, a process that stabilizes in the early twenties.
2. White matter increases in volume and organization into adulthood, peaking at around age 30. White matter is white because of myelin, which insulates axons, thus accelerating signaling between different brain regions.
3. Not all parts of the brain mature at the same rate during adolescence. For example, the gray matter volume of the caudate, putamen, and nucleus accumbens decreases with age, whereas the volume trajectories of the amygdala, cerebellum, hippocampus, pallidum, and thalamus displayed an inverted U-shaped pattern (that is, they expand slightly then shrink again).
4. Sex differences in the trajectory of brain development emerge during adolescence. Typically, girls achieve their maximum grey matter volume (cortical and subcortical) a bit earlier than boys. However, this variation is mainly influenced by the onset of puberty, which, on average, begins for girls about a year earlier than for boys. So, boys eventually catch up.

Grey matter thins as connections are refined

The prefrontal cortex (PFC), as its name suggests, sits at the front of the brain and is sometimes called the brain’s CEO because it acts as an experienced decision-maker. The PFC is involved direction attention, judgement calls, emotion regulation, strategy, impulse control, working memory and social cognition – navigating complex social relationships such as discerning friend from ‘frenemy’ from foe.

During childhood, PFC grey matter thickens as neurons make new connections or synapses. Much like a tree growing extra branches, twigs and roots.

However, during the teens, the PFC gets thinner.

PFC thinning is due to pruning away of unwanted neural connections (‘twigs and branches’) which partly depends on experience in a ‘use it or lose it’ manner. Connections that are being used are strengthened, and those that aren’t used are pruned away. Loss or ‘thinning’ of grey matter always sounds unhealthy, but as is often the case with the brain, less is more. Think of it as streamlining, pruning, tuning, or refining connections.

Losing grey matter during adolescence (or any time of life) might sound bad, but in the teenage brain’s case, this isn’t a loss; it’s a smart move! The brain keeps the connections that are important — the ones being used — and gets rid of the rest.

MRI scans have revealed ever-increasing volumes of white matter in the tracts leading to and from the PFC during adolescence. Recall that axons are insulated by myelin, which speeds up signalling between brain regions. Myelinated axons transmit impulses up to 100 times faster than unmyelinated axons; faster transmission means faster mental processing.

Jay Giedd, a neuroscientist at the University of California, San Diego, has found that increased white matter means the connections between brain regions become stronger and more numerous. The brain matures by becoming more interconnected and more specialised.

To use Giedd’s metaphor, brain maturation is not so much a matter of adding new letters to the alphabet but one of combining existing letters into words, words into sentences and sentences into paragraphs.

“These changes ultimately help the brain to specialise in everything from complex thinking to being socially adept.”

Subcortical brain areas shrink, and some grow

Changes to white and grey matter structure are not restricted to the PFC.

Subcortical brain structures start to mature right around puberty, and development is more or less complete by the mid-teenage years.

Subcortical brain regions are those deep inside the brain under (‘sub’) the cortex in the frontal and temporal lobes. They include the limbic system (hippocampus and amygdala), and the corpus striatum, which is made up of four parts: nucleus accumbens, caudate, putamen and globus pallidus. They’re connected to the PFC and to each other by bundles of axons.

In teens, the amygdala and hippocampus get bigger during puberty. At the same time, the four structures making up the corpus striatum all shrink. Maturation of subcortical structures is fastest in late childhood and early adolescence but the rate of growth or shrinkage slows right down after age sixteen.

Is mental illness emotional development gone wrong?

One theme in adolescent brain research can be summed up by the phrase ‘moving parts get broken’. Because the teenage brain is undergoing such extensive remodelling, it’s thought that it’s easy for the wiring to go awry, predisposing girls to depression, anxiety and eating disorders.

The emotional life of teenage girls is complex both inside and out. It’s clear that ‘inside’ hormones are fluctuating and neural networks are fine-tuning. But we tend to forget the dynamic shifts taking place ‘outside’.

Many emotions teens experience are novel.

Especially those taking place in new social contexts. There is a first time for everything and that includes your first crush (especially if it’s unrequited), experience of jealousy, being left off the party invite list, and getting likes on Instagram. The newness of these experiences heightens their emotional impact. Sometimes this is positive – falling in love for the first time is pretty wonderful! Sometimes this is negative – being rejected by your first love is rather painful.

Girls’ preference for emotional intimacy increases during their early teenage years.

Intense ‘bosom buddy’ friendships form around the same time girls ‘break up’ with their families. This leaves girls potentially vulnerable if those friendships break down or are less than ideal. Loss of friendship is a significant and overlooked potential cause of stress.

While the ‘wrong’ friendships can lead teens astray, the ‘right’ kinds of friends are powerful and protective. I had one such dear friend all through high school (we’re still close now and I’ve called on her psychiatric training to inform sections of this book). While writing this chapter we talked about how our devoted friendship buffered us from the tumult so many of our peers experienced. We know the happiest teens aren’t the ones who have the most ‘friends’ on Instagram but the ones who have a few close supportive friendships, and sometimes that means having one terrific friend.