This article suggests that ideas about adolescent brains and their development increasingly function as powerful truths in making sense of young people. In this context, the knowledge practices of the neurosciences and evolutionary and developmental psychology are deemed capable of producing what we have come to understand as the evidence on which policy, interventions and education should be built. I identify and explore the ways in which these knowledge practices produce this evidence, and the ways in which this evidence is translated into popular and policy discourses concerned with young people's apparently risk prone, impulsive and irrational behaviours and dispositions. I argue that discourses of adolescent brain development disembody, reduce and simplify the complexities of these figures we "know" as adolescents. In effect they reduce young people to being little more than a "brain in a jar". (Contains 1 note.)