The Teenage Brain

Adolescence is a major period of brain remodelling: pruning, myelination, a reward system that matures early and a prefrontal cortex that matures late. What that does — and doesn't — explain about teenage behaviour.

For much of the 20th century, people assumed the brain was essentially finished by early childhood. Long-term brain imaging studies, notably those begun in the 1990s at the US National Institute of Mental Health by Jay Giedd and colleagues, showed otherwise: adolescence is a period of dramatic remodelling.

What changes

Grey matter volume in the cortex peaks in late childhood and then thins through adolescence, reflecting synaptic pruning and other refinements. White matter increases steadily as myelination continues, speeding communication between regions. These changes proceed roughly from the back of the brain to the front, so the prefrontal cortex — involved in planning, impulse control and weighing consequences — is among the last regions to mature, with development continuing into the mid-twenties. (That "25" is a rough average, not a switch that flips on a birthday.)

A reward system turned up

Meanwhile, the brain's reward system becomes especially responsive around puberty. Teenagers show heightened responses to rewards and novelty, and are especially sensitive to peers. In a well-known experiment by Laurence Steinberg's group, teenagers took more risks in a driving game when friends were watching, while adults didn't.

A mismatch model, with caveats

One popular model describes a mismatch: an early-maturing, enthusiastic reward system paired with a still-maturing control system. It helps explain increased risk-taking and sensation-seeking. But it's easily overstated: teens can reason as well as adults in calm conditions, and behaviour depends heavily on context, culture and individuals.

A time of opportunity — and vulnerability

Adolescent plasticity makes it a powerful period for learning skills, building identity and forming relationships. It is also when many mental health conditions first appear: about half of lifetime mental disorders begin by the mid-teens. Sleep needs shift too — teenagers' body clocks naturally move later, which is why many schools have moved to later start times.