What framework should we use for understanding developmental disorders?

The neuropsychology of dyslexia has made great strides in the last decade. In particular, a consensus views dyslexia as a developmental disorder with a basis in the brain and in the genes, where the interaction of genetic and environmental factors is taken for granted. However, problems in defining the phenotype continue to bedevil research. The main conceptual problems can be expressed in three main questions: (a) Is dyslexia based on a specific brain abnormality or is it merely part of a continuum of atypical brain development? (b) When can we speak of comorbidity? (c) Why does so much individual variability occur? These questions can be tackled in a common framework that takes into account simultaneously three levels: the biological, the cognitive, and the behavioral.