Social anxiety disorder (SAD), an impairing and life-long mental health condition, is prevalent already in adolescents. Acquiring more insight into the susceptibility to develop SAD is essential, in order to early recognize which children are ‘at risk’ and to advance effective preventive interventions. SAD ‘runs in families’, indicating that genetic vulnerability is an important risk factor. Here, I aim to investigate the neurobiological characteristics associated with this innate susceptibility. I will focus on individuals characterized with inhibited temperament (IT), a stable and heritable trait which is observable already in early childhood, linked to variations in brain characteristics, and associated with an elevated risk for SAD later in life: 50% of children with IT will develop SAD, while the other 50% will not. What determines this ‘flip of the coin’? Previous findings on the neurobiological underpinnings of IT are inconsistent, probably due to small sample sizes. I will pool neuroimaging data from longitudinal studies on IT, previously acquired at research-centers worldwide. Using this unique large-sample dataset, I will examine which brain features are reliably associated with IT; next, I aim to understand which neurobiological characteristics and environmental factors underlie the variation in SAD-symptoms later in life, within children with early-life IT.