The Adolescent Brain: What the Science Says and What It Doesn’t

Ask a group of adults about teenagers and you will usually hear about risk taking, moodiness, late nights and an obsession with friends. Over the past twenty years, brain imaging research has added a biological layer to these observations. Some of it has been useful. Some of it has been turned into slogans that the research does not support.

A brain still under construction

Until the 1990s, many scientists assumed the human brain was largely finished by late childhood. Longitudinal MRI studies, which scan the same young people repeatedly over years, changed that view.

Kathryn Mills and colleagues (2016) brought together four such studies from different countries, covering 391 participants aged 8 to 30 and more than 800 scans. They found a consistent picture:

• The volume of cortical grey matter, which contains the cell bodies of neurons and their local connections, was highest in childhood and declined steadily through the teenage years, with the decline slowing in the twenties.
• White matter volume, which reflects the insulated fibres connecting brain regions, increased into mid-to-late adolescence before levelling off.

The decline in grey matter is often explained as the pruning of unused connections and other refinements that make the brain more efficient, though MRI cannot show these processes directly. The changes are gradual and continue past the teenage years.

The “brain finishes at 25” myth

You may have heard that the brain “finishes developing at 25”. No single age marks the end of development. The data show change slowing during the twenties, at different rates in different regions and for different people. A 25th birthday is not a biological finishing line, and the brain continues to change in response to learning across adult life.

A sensitive period for social learning

Sarah-Jayne Blakemore, a cognitive neuroscientist who built her research group at University College London, has led much of the UK research on the adolescent brain. Her book Inventing Ourselves (2018), which won the Royal Society Science Book Prize, is an accessible starting point.

In a 2014 review with Kathryn Mills in the Annual Review of Psychology, Blakemore asked whether adolescence might be a sensitive period for social and cultural learning. The brain regions involved in understanding other people’s thoughts and intentions, sometimes called the “social brain” network, continue to develop through adolescence. The authors suggest this may make young people especially tuned to their social environment, at an age when they are working out where they fit among peers.

Rejection hurts more

One line of evidence comes from experiments using a computer game called Cyberball, in which a participant throws a virtual ball with two other players who, unknown to the participant, are programmed to stop including them. Catherine Sebastian, Essi Viding, Kipling Williams and Blakemore (2010) found that adolescents reported a greater drop in mood after being left out than adults did.

To an adult, a group chat that goes quiet or a seating plan that separates friends may look trivial. For a thirteen-year-old it may feel considerably worse.

Risk and peers

A common claim is that teenagers take risks because the “brakes” in the brain’s prefrontal cortex are immature. One well-known experiment points to the importance of social context.

Margo Gardner and Laurence Steinberg (2005) asked 306 participants in three age groups, adolescents (13 to 16), young adults (18 to 22) and adults (24 and over), to complete risk questionnaires and a computerised driving game. Some participants did the tasks alone; others did them with two same-age friends in the room. Being with peers increased risk taking in all groups, but the effect was much stronger for the adolescents and young adults than for the adults.

The study does not reduce teenage risk taking to an unfinished brain. The same young person may choose differently alone than with friends watching, and the presence of peers, and the reward of being seen by them, can tip the balance.

Sleep

Teenage sleep is one area where biology and daily life collide. Stephanie Crowley, Christine Acebo and Mary Carskadon (2007) reviewed evidence that, around puberty, the body clock shifts later. The evening rise in melatonin comes later, and the pressure to sleep builds more slowly across the day, which makes it easier to stay awake at night.

Teenagers’ sleep need does not fall to match. In Carskadon’s work, sleep need stays roughly stable across adolescence, while early school starts cut into the time available. A teenager who cannot fall asleep before 11pm and has to get up at 6.30am is chronically short of sleep, which affects mood, attention and learning.

That is not a reason to give up on bedtimes. It is a reason to understand why “just go to bed earlier” is harder than it sounds, and to protect sleep where possible.

Avoiding neuromyths

Popular accounts of the teenage brain sometimes slide into determinism: teenagers behave badly because their brains are unfinished, so nothing can be done. That reading does not follow from the research.

• Brain differences are averages across groups. Individual teenagers vary widely.
• Brain development interacts with experience. Environments, relationships and opportunities shape how the brain develops.
• Adolescence is a period of opportunity as well as risk. The same sensitivity to social feedback that makes rejection painful also makes young people responsive to belonging, mentoring and purpose.

In school

• Treat social belonging as part of learning. Pupils who feel excluded by peers may struggle to concentrate however good the lesson.
• Be careful with public correction and public praise. Being singled out in front of peers carries more weight at this age.
• Use peers deliberately. Collaborative tasks, peer mentoring and older pupils supporting younger ones put the social drive to good use.
• Consider timing. Where timetables allow, avoid placing the most demanding new learning very early in the day.
• Watch for changes in friendship groups, online life or withdrawal, and share concerns with the pastoral team.

At home

• Talk about risk without peers present, and help your teenager plan in advance how they will handle pressured situations: a lift home, a code word, a reason to leave.
• Take friendship problems seriously, even when they seem small.
• Protect sleep: agree screen-free time before bed, keep phones out of the bedroom overnight if you can, and try to keep weekend wake times not too far from weekday ones.
• Offer independence in steps. Teenagers need practice making decisions, with a safety net.

If you notice persistent low mood, withdrawal, self-harm or changes in eating or sleep that worry you, speak to your child’s school and your GP. The GP can refer to NHS Child and Adolescent Mental Health Services (CAMHS) where appropriate.

Further reading

• Blakemore, S.-J. (2018). Inventing Ourselves: The Secret Life of the Teenage Brain. Doubleday.
• Blakemore, S.-J. and Mills, K. L. (2014). Is adolescence a sensitive period for sociocultural processing? Annual Review of Psychology, 65, 187-207.
• Crowley, S. J., Acebo, C. and Carskadon, M. A. (2007). Sleep, circadian rhythms, and delayed phase in adolescence. Sleep Medicine, 8(6), 602-612.
• Gardner, M. and Steinberg, L. (2005). Peer influence on risk taking, risk preference, and risky decision making in adolescence and adulthood: an experimental study. Developmental Psychology, 41(4), 625-635.
• Mills, K. L. et al. (2016). Structural brain development between childhood and adulthood: convergence across four longitudinal samples. NeuroImage, 141, 273-281.
• Sebastian, C., Viding, E., Williams, K. D. and Blakemore, S.-J. (2010). Social brain development and the affective consequences of ostracism in adolescence. Brain and Cognition, 72(1), 134-145.