Decoding the Teenage Brain: What Science Has Learned—and What Remains a Mystery

What is happening inside a teenager’s mind as they shift from childhood to adulthood? Why can young people often master instruments or languages quickly, yet sometimes make baffling decisions that leave adults asking, “What were they thinking?” Scientists — like parents and teachers — are probing these questions. While definitive answers remain limited, research has produced important insights into how adolescent brains change and how to study them well.

Vox’s podcast series Unexplainable interviewed multiple researchers to map what is known and what is still uncertain about teenage brains. One of the largest efforts to answer these questions is the National Institutes of Health’s (NIH) Adolescent Brain Cognitive Development (ABCD) study.

The ABCD Study

Launched more than a decade ago, ABCD set out to follow roughly 12,000 children starting at about age 9 for a decade, across 21 sites in the United States. Participants regularly complete questionnaires, behavioral tests, and brain scans (including MRI), and provide data on sleep, exercise, screen time and family environment. The study’s scale and longitudinal design allow researchers to better separate risk factors from consequences — for example, what precedes substance use versus what results from it.

Raul Gonzalez Jr., director of the Florida ABCD site and a psychology professor at Florida International University, said the project was created because smaller studies couldn’t reliably distinguish pre-existing risk from the effects of drug use. ABCD has already supported more than 1,400 papers and continues to release new data that researchers are mining for insights into adolescent development.

What We Know About Developing Brains

One robust finding is that the brain overproduces connections (synapses) early in life and then prunes many of them as we mature. During adolescence — even while young people are learning rapidly — the brain eliminates some connections and strengthens others. This process is often summarized as “use it or lose it.”

“Use it or lose it,” said Alison Barth, Professor of Biological Sciences at Carnegie Mellon University, describing why unused synaptic connections are removed.

That pruning may seem wasteful: why build massive networks only to remove much of them later? Interdisciplinary research has begun to answer that. Barth and computer science colleagues studied synaptic pruning in mouse brains to explore whether the brain’s counterintuitive strategy for refining connections could inspire more efficient designs in artificial neural networks and computing.

Why It Matters

Understanding teen brains has practical implications for education, mental-health prevention, and public policy. Longitudinal projects like ABCD provide the statistical power to detect early risk signals, while lab and animal studies tease apart mechanisms such as synaptic pruning. Together, these approaches are reshaping how scientists think about learning, risk-taking, and cognitive development during adolescence.

Ongoing Work

Researchers caution that science doesn’t yet have complete answers. Large datasets keep growing and methods keep improving, so our picture of adolescence will continue to sharpen. The ABCD dataset and complementary experimental studies are likely to drive new discoveries for years to come.

Support: This series was made possible by the Annie E. Casey Foundation.