It is a truism that development involves contributions from both genes and environment, but theories differ with respect to the roles they attribute to each, which deeply affects the ways in which developmental disorders are researched. The strict nativist approach to abnormal phenotypes, inspired by adult neuropsychology and evolutionary psychology, seeks to identify impairments to domain-specific cognitive modules and studies the purported juxtaposition of impaired and intact abilities. The neuroconstructivist approach differs in several respects: (i) it seeks more indirect, lower-level causes of abnormality than impaired cognitive modules; (ii)modules are thought to emerge from a developmental process of modularization; (iii) unlike empiricism, neuroconstructivism accepts some form of innately specified starting points, but unlike nativism, these are considered to be initially `domain-relevant', only becoming domain-specific with the process of development and specific environmental interactions; and (iv) different cognitive disorders are considered to lie on a continuum rather than to be truly specific. These alternative theoretical positions are briefly considered as they apply to Specific Language Impairment, and followed by a more detailed case study of a well-defined neurodevelopmental disorder, Williams syndrome. It is argued that development itself plays a crucial role in phenotypical outcomes.