When 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........

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