Implications of the broader phenotype for concepts of autism.

Autism, like many new diseases, was initially characterized by its most severe phenotypic manifestation and the ability to explain these distinctive features has been the benchmark against which explanatory models have subsequently been judged. Our understanding of the significance of milder phenotypes in other relatives has shifted from presumed environmental aetiological factors to variable manifestations of a complex disease process. In this paper we outline how the challenge of explaining the full range of phenotypic expression inevitably leads to more complex models of disease process than previously supposed. The implications of milder phenotypes for genetic, neurobiological and cognitive models of autism will be considered in relationship to several key features of complex diseases: complexity, hierarchy, emergence and coherence.