Professor Essi Viding delivers the second of two talks in the 2016 Loebel Lectures in Psychiatry and Philosophy series In these Loebel lectures Prof Essi Viding will use disruptive behaviour disorders as an illustrative example to introduce the challenges we face when we try to understand development of psychopathological outcomes. We classify disorders at the level of behaviour, yet individuals arrive at the same behavioural outcomes via multiple different developmental trajectories; a phenomenon called equifinality in the developmental psychopathology literature. A related concept is heterogeneity; we can find individuals with markedly different aetiology to their disorder within the same diagnostic category. The current diagnostic categories identify clinically disturbed functioning, but they do not identify a homogeneous group of individuals. Getting better at individuating distinct pathways to a disordered outcome is only part of the challenge. Once risk factors for a specific developmental trajectory are identified, we still need to understand their modus operandi. There is no doubt that both biology and the social environment play a role in the emergence of psychopathology, but meaningfully studying their interplay is far from trivial. What are the key biological indicators of vulnerability and resilience? How can we isolate causal mechanisms? How do we model multiple social risk factors and their impact over development?