Under Construction — The Resilience Lab

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Study 01 · The Resilience Lab — Under Construction
Is the teenage brain really “unfinished” until 25?
“The brain finishes developing at 25.” → No study found a finish line.
Childhood: two to three times more connections than adults. Through the teens and twenties, unused ones are pruned.
123,984 — brain scans in the largest brain-chart study
≈ 29 — the age white matter volume peaked — not 25
Chein et al., 2011: With friends watching, teens took more risks — and their reward regions lit up.
Van Hoorn et al., 2016: With friends watching, teens also gave more.
You can choose your audience.
Your brain isn’t behind schedule. It’s under construction — and you’re on the crew.

Make a prediction

Researchers had teens play a driving game alone, then again while friends watched from the next room. What do you think happened to their risky choices?

Committing to a guess before you learn the answer helps you remember the answer.

Fewer risks — nobody wants to look reckless in front of friends.No real change — a game is a game.More risks for the teens — but not for the adults.

How sure are you?

Not sureVery sure

The most famous number about your brain

Search online and you’ll find it everywhere: the brain “finishes developing at 25”. It turns up in articles, school talks and arguments about curfews. It sounds precise. It isn’t.

What the research actually shows is more interesting: your brain doesn’t have a finish line. Different parts change on different timetables — some peak in early childhood, some in your teens, some in your late twenties — and brains keep changing for the rest of life.

Researchers have pointed out that “brain maturity” depends on which part you measure, and how (Somerville, 2016). No single study found that brains are “done” at 25. The number spread because it’s easy to remember.

Fig. 1 — Pruning and insulating

In childhood, the prefrontal cortex carries two to three times more synaptic spines than it will in adulthood. Through the teens and twenties, unused connections are trimmed and busy ones are wrapped in myelin. Drag the slider to age the brain.(Petanjek et al., 2011)

Two kinds of change

Brain scans that follow the same young people for years track two big shifts (Giedd et al., 1999). Grey matter — cell bodies and their branching connections — grows fast in childhood, then slowly thins. That thinning isn’t losing ability. Much of it is pruning: the brain trimming connections that aren’t being used, so the busy ones work better.

White matter — the long-distance cables, wrapped in a fatty insulation called myelin — keeps growing for much longer. In the largest brain-chart project so far, built from 123,984 scans, grey matter volume peaked at about age 6 and white matter volume at about age 29 (Bethlehem et al., 2022).

Notice what that does to “finished at 25”. If one part of the brain peaks around 6 and another around 29, there was never one finish line to begin with.

Words worth knowing

Synapse: A connection point where one brain cell passes a signal to the next.
Pruning: Removing connections that aren’t used, so the ones that are used work better.
Myelin: Fatty insulation around nerve fibres that makes signals travel faster.
Reward regions: Areas such as the ventral striatum that help the brain work out how good an option might feel.
fMRI: A scan that tracks changes in blood flow to show which brain areas are working harder during a task.

Evidence

The evidence: friends in the room

Two studies, the same question, two different methods.

Developmental Psychology2005Experiment

Peer influence on risk taking, risk preference, and risky decision making in adolescence and adulthood: An experimental study

Gardner, M., & Steinberg, L.

The question

Do people take more risks when friends are with them — and does age matter?

What they did

306 people in three age groups — 13–16, 18–22, and 24 and older — were randomly assigned to do risk tasks alone or with two friends their own age. One task was a driving video game.

What they found

Everyone took more risks in groups than alone, but the peer effect was stronger for the teens and young adults than for the adults. Risk taking also decreased with age.

What this does NOT mean

It doesn’t mean friends make you reckless. It means the same choice can feel different when people you care about are there.

Developmental Science2011Brain imaging

Peers increase adolescent risk taking by enhancing activity in the brain’s reward circuitry

Chein, J., Albert, D., O’Brien, L., Uckert, K., & Steinberg, L.

The question

What changes inside the brain when friends are watching?

What they did

40 people — 14 teens (14–18), 14 young adults (19–22) and 12 adults (24–29) — played a stoplight driving game in an fMRI scanner, once alone and once while two friends watched on a monitor from the next room. The friends were told not to cheer them on.

What they found

Only the teens took significantly more risks when watched. Their reward regions (ventral striatum and orbitofrontal cortex) were more active with friends watching, and that activity predicted risky choices. The self-control regions did not change with the audience.

What this does NOT mean

“Reward regions lit up” doesn’t mean teens can’t control themselves. The control regions worked the same either way; what changed was how rewarding the risk felt.

Try it

Try it: the Stoplight task

A simplified version of the game used in the study. Drive through 10 intersections a round (the study used 20). When a light turns yellow, choose: brake and lose a little time, or go — and risk a crash that costs more. You’ll play two rounds. In the second, you’ll have an audience.

Compare your two rounds. Did the audience change anything? One person playing one game is noisy data — which is exactly why the researchers tested many people, many times.

The myth

“Teenagers are basically adults with broken brakes.”

What the evidence says

The famous “accelerator and brakes” picture (the dual-systems model) captured something real, but researchers have argued it is too simple: the brain works as connected networks that develop together (Pfeifer & Allen, 2012). And in the stoplight study, the self-control regions worked the same with or without an audience. What changed was how rewarding the risk felt.

Pfeifer & Allen, 2012 · Chein et al., 2011

Evidence

The other side: braver in good ways

If an audience can make a risk feel more rewarding, can it make generosity feel more rewarding too?

Social Cognitive and Affective Neuroscience2016Brain imaging

Neural correlates of prosocial peer influence on public goods game donations during adolescence

Van Hoorn, J., Van Dijk, E., Güroğlu, B., & Crone, E. A.

The question

Can peers push teens toward generosity?

What they did

61 teens aged 12–16 played a sharing game in a scanner, deciding how many tokens to give their group. Sometimes a group of same-age “peers” was online watching; sometimes those peers gave likes for big donations.

What they found

Teens gave more when peers were watching, and more still when peers liked generous choices. Social-brain regions were more active with peers present.

What this does NOT mean

It doesn’t mean chasing likes is good for you. It means the audience effect is a force — and forces can be pointed in good directions.

Explain it to your Lab PartnerAI

An AI that asks before it tells. It knows this Study — and only this Study.

In two or three sentences: why might the same risky choice feel different when friends are watching? Try to use the word “reward”.

Hi — I’m your Lab Partner. I’m an AI, here to think with you, not to lecture you. Quick question to start: what did you predict about the friends-watching study, and did the result surprise you?

Lab Partner opens once a parent sets up your family’s Lab.

Start your family Lab to save your work

What this means for you

First: when you take a risk with friends watching that you’d never take alone, that isn’t a character flaw. It’s one of the best-studied effects in adolescent research. Knowing about it is the first step to choosing when it gets to drive.

Second: the audience effect works in both directions. Spend time with people who cheer for the brave-and-good choices — trying out, speaking up for someone, starting the hard project.

Third: you are building your brain right now, through what you repeat. That isn’t pressure. It’s power.

Experiment

Your field experiment

Your field experiment · 5 days

The audience effect, in your own life

HypothesisIf people I care about are watching, then I’ll feel a stronger pull toward bold choices — risky ones and good ones.

Each evening, think of one choice you made today with other people around (in person or online) and one you made alone. Rate the pull you felt. It takes about 20 seconds.

Start your family Lab to save your work

The Resilience Lab

What this can — and can’t — tell you

Five days of your own ratings can show a pattern in you — not a law of nature. You know what you’re testing, which can nudge your ratings, and no two days are alike. That’s why scientists use many people, random assignment and measures the participants don’t control.

Recall

Notebook

Lab notebook

Private. Your family sees that you wrote, never what you wrote.

Think of a choice you made because people were watching. Would you make it again — alone?

Who is an audience that makes you braver in a good way? What do they cheer for?

What’s one thing you’d like your brain to get very good at in the next two years? What would practising it look like this week?

Family card · Study 01

The audience effect

Teen

Friends watching can make risky choices feel more rewarding — and good choices too. We can choose our audience.

Parent

Most of what parents know comes from what teens choose to tell. Calm reactions keep that door open.

Ask each other

01When did an audience make you braver — in a good way?
02What’s something you’d do alone but never with people watching — or the other way round?
03Who in our family is the best audience to have?

Try together this week

This week, each of you notice one moment when other people changed your choice. Compare notes at the weekend.

Start your family Lab to save your work

The Resilience Lab

References

Chein, J., Albert, D., O’Brien, L., Uckert, K., & Steinberg, L. (2011). Peers increase adolescent risk taking by enhancing activity in the brain’s reward circuitry. Developmental Science, 14(2), F1–F10. doi:10.1111/j.1467-7687.2010.01035.x✓ PubMed
Gardner, M., & 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. doi:10.1037/0012-1649.41.4.625✓ PubMed
Petanjek, Z., Judaš, M., Šimić, G., Rašin, M. R., Uylings, H. B. M., Rakic, P., & Kostović, I. (2011). Extraordinary neoteny of synaptic spines in the human prefrontal cortex. Proceedings of the National Academy of Sciences, 108(32), 13281–13286. doi:10.1073/pnas.1105108108✓ PubMed
Pfeifer, J. H., & Allen, N. B. (2012). Arrested development? Reconsidering dual-systems models of brain function in adolescence and disorders. Trends in Cognitive Sciences, 16(6), 322–329. doi:10.1016/j.tics.2012.04.011✓ PubMed
Van Hoorn, J., Van Dijk, E., Güroğlu, B., & Crone, E. A. (2016). Neural correlates of prosocial peer influence on public goods game donations during adolescence. Social Cognitive and Affective Neuroscience, 11(6), 923–933. doi:10.1093/scan/nsw013✓ PubMed