Autism is diagnosed on the basis of a pattern of behaviours, including essential criteria and additional common features. Reliance on behavioural markers presents challenges to interpreting changing prevalence estimates and to parsing heterogeneity within the autism constellation, and raises questions about recognition of autism in historically under-researched groups, such as women and the elderly. All the epidemiological studies of autism show a significantly greater number of boys than girls in the population. The chapter shows how a specific feature of autism might be plotted with intellectual ability and language profile. The variability in diagnostic manuals is nothing compared to the variability of presentation in the autistic population. A great deal of research investigating the female profile of autism has focused on qualitative descriptions of their experience, often revealing significant difficulties prior to receiving a diagnosis. Accounting for the dramatic changes in autism prevalence estimates over time has been the focus of considerable media attention and academic effort.