The “Nature versus Nurture” debate on the origin of behaviour has long been dominated by a genome versus experience dichotomy. However, evidence that genetically identical individuals kept under identical conditions are behaviourally different is incontrovertible. Where might such individuality come from? Neither genes nor the environment directly encode behaviour. They encode or influence processes, notably the development of neuronal circuits, that in turn control behaviour. An understanding of how neuronal circuits develop and function at the individual organism level is therefore essential for understanding the origin of individuals. I will discuss our efforts to address this issue over the past decade using the Drosophila fruit fly as a model system.