pikselstock/ShutterstockWhen an adult refuses to let an injustice pass – stands up to a bully, or asserts their rights – we call it principle. When a fifteen-year-old retaliates to an insult thrown in a school corridor, risking detention, we call it a failure to think ahead. It’s easy to ascribe this to a not-yet-developed teenage brain: a convenient shorthand for assuming they simply cannot weigh consequences properly.But this misunderstands the teenage brain. It works in a way that has been shaped by evolution for a unique and uniquely challenging time of life. Humans repay kindness with kindness, bristle when they are cheated, and want rule-breakers to be made to pay. Infants of eight months already prefer those who behave well towards others, and the willingness to punish someone who breaks a rule grows stronger as children get older. Getting your own back registers in the brain’s reward system as genuinely pleasurable.This is the argument at the heart of my research in evolutionary criminology. Punishment is one of the mechanisms that allowed cooperation to evolve. A group where cheating goes unanswered is a group that eventually stops cooperating. The teenager who hits back in the corridor is running on an ancient programme.Adolescence turns the volume on these impulses up, and there are good reasons for that.Why fifteen is differentThe brain’s reward system, and the ventral striatum in particular, becomes far more responsive during the teenage years, peaking in mid-adolescence. The prefrontal cortex, which allows a person to inhibit an impulse rather than act on it, continues to develop into the third decade of life.Adolescence is when a young person has to step away from their family and find a footing among peers. A brain that feels rewards keenly, that explores, that cares enormously what other people think and that defends its standing is well suited to that job.The presence of friends alone is enough to raise risk-taking and to activate reward circuitry in ways that do not happen when teenagers are alone. Sensitivity to peer judgment and exclusion runs higher in adolescence than at any other point in life.The price of the sensible choiceTeenagers do weigh up what they are doing. The costs that weigh most heavily on them are social, and adults consistently underestimate how heavy they are. Walking away in front of an audience can cost a young person their standing, and standing is the thing they most need at that age. How they are perceived by their peers is incredibly important to teenagers. Zamrznuti tonovi/Shutterstock A brain this sensitive to its surroundings will take its shape from whatever those surroundings contain. Exposure to high-risk environments and offending peers accounts for a very large share of whether young people break rules at all. This sensitivity is also why researchers increasingly describe adolescence as a window of opportunity. A calm environment that prioritises fair outcomes and in which people are treated with respect will shape how teenagers behave. Knowledge into practiceIf a young person trusts the system to deliver a just outcome, their brain’s urgent need for retribution is dialled down, allowing the prefrontal cortex to catch up. At school, this might mean an approach where a trained staff member mediates a conversation between conflicting students, ensuring both are heard and a mutually acceptable resolution is reached, rather than relying on punishment. At home, it could be as simple as a parent consistently and calmly intervening in a sibling dispute to reach a fair outcome, rather than letting children “sort it out themselves” and risk the more powerful one imposing their will.Teachers and parents can also try to create an environment where the rewarding thing to do and the sensible thing to do point in the same direction. For instance, a school could create a peer-to-peer tutoring program where helping a classmate is also recognised with privileges or public acknowledgement – making it the “sensible” choice for social standing. At home, this could involve framing chores not as a burdensome duty, but as a contribution to the family team, with the reward being more family leisure time together. This aligns the immediate payoff with the long-term benefit. Another important strategy is to leave young people a way to back down without losing face. In a school, a teacher witnessing a brewing conflict might privately offer both students a “cooling-off” pass to a designated safe space, framing it as a chance to deescalate rather than as a punishment for fighting. At home, a parent might say, “I can see you’re really angry right now. I need you to choose how you want to handle this: we can talk it through now, or you can take 20 minutes to cool off and we’ll come back to it. Your call.” This allows the teenager to disengage from a confrontation without appearing weak in front of peers or siblings.Adolescence takes its shape from whatever we surround it with, which makes it the most promising few years we have to positively shape a life. It’s a window of opportunity where the right support, fair systems and understanding environments can have a greater impact than at almost any other stage of development.Evelyn Svingen does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.