There is no single cause for schizophrenia. We believe that, as with other common chronic diseases such as diabetes and coronary artery disease, the appropriate aetiological model is one involving multiple genes and environmental risk factors; the latter can be divided into (a) predisposing and (b) precipitating. Our model is that genetic and/or early environmental factors cause the development of anomalous neural networks. We postulate that these interact in the growing child with inherited schizotypal traits to establish a trajectory towards an increasingly solitary and deviant life style. This ultimately projects the individual across the threshold for expression of schizophrenia, sometimes by causing the drug abuse and social adversity that appear to precipitate the psychosis.