Many psychiatric disorders arise during adolescence, a time when the brain undergoes fundamental reorganisation. However, it is unclear whether and how the emergence of mental health problems is linked to aberrant neurocognitive development. In my talk, I will discuss why it is critical to understand (aberrant) cognitive and brain development if we want to better understand how mental health problems arise. I will present findings showing how psychiatric traits are linked to adolescent brain myelination, and illustrate why computational neuroscience approaches can be important in understanding psychiatric disorders.