‘The autism spectrum’ examines three individuals from different parts of the autism spectrum. It explains how all of the ‘autisms’ originate from before birth, and all affect the developing brain. Their effect on the developing mind can be very different and there is a vastly different range of behaviours. However, there is always a characteristic inability to engage in ordinary reciprocal social interaction, a rigidity of behaviour, and a preoccupation with repetitive activities and narrow interests. As long as the diagnosis of autism is based on behaviour, a definitive pronouncement can only be made with hindsight.