Using Developmental Trajectories to Understand Developmental DisordersPurpose: In this article, the authors present a tutorial on the use of developmental trajectories for studying language and cognitive impairments in developmental disorders and compare this method with the use of matching. Method: The authors assess the strengths, limitations, and practical implications of each method. The contrast between the methodologies is highlighted using the example of developmental delay and the criteria used to distinguish delay from atypical development. Results: The authors argue for the utility of the trajectory approach, using illustrations from studies investigating language and cognitive impairments in individuals with Williams syndrome, Down syndrome, and autism spectrum disorder. Conclusion: Two conclusions were reached: (a) An understanding of the underlying mechanism will be furthered by the richer descriptive vocabulary provided by the trajectories approach (e.g., in distinguishing different types of delay) and (b) an optimal design for studying developmental disorders is to combine initial cross-sectional designs with longitudinal follow-up.

The reported prevalence of psychiatric illness among adults with intellectual disability (ID) varies widely between  and %; however, many methodological problems exist. The aims of the present study were to establish the prevalence of functional psychiatric illness among adults with ID who live in the community, in order to compare the overall rate and types of psychiatric illness between the population with ID and the general population without ID, and to establish the risk factors associated with psychiatric illness in adults with ID. The study was done in two stages. In the first part, a trained psychiatrist interviewed  randomly selected adults with ID and their carers using the Mini Psychiatric Assessment Schedule for adults with Developmental Disability (Mini PAS-ADD) to screen for psychiatric caseness. Out of these  adults,  had sufficient communicative abilities that made the administration of Mini PAS-ADD Correspondence: Dr Shoumitro Deb, Division of Psychological Medicine, University of Wales College of Medicine, Heath Park, Cardiff CF XN, UK (e-mail: Deb@Cardiff.ac.uk). possible. A second trained psychiatrist interviewed  out of the  adults who were diagnosed as psychiatric cases according to the initial Mini PAS-ADD interview. This psychiatrist interviewed patients and their carers in line with the full PAS-ADD interview. The second psychiatrist was blind to the initial diagnoses made according to the Mini PAS-ADD questionnaire. A final psychiatric diagnosis was made according to International Classification of Diseases - th Revision (ICD-) criteria. Some .% (% confidence interval = .-.%) of the cohort had a psychiatric diagnosis according to ICD- criteria: .% had schizophrenia, .% depressive disorder, .% generalized anxiety disorder, .% phobic disorder and % delusional disorder. The overall rate of functional psychiatric illness (point prevalence) was similar to that found in the general population (%). However, the rates of schizophrenic illness and phobic disorder were significantly higher in the study cohort compared with those in the general population (.% and .%, respectively). Increasing age and the presence of physical disability were significantly associated with the occurrence of psychiatric illness. Out of the  remaining adults with severe ID, two (%) had a diagnosis of a psychiatric illness (one mania and one anxiety disorder) according to the

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To be published in Behavioral and Brain Sciences (in press) Cambridge University Press 2007 Below is the unedited précis of a book that is being accorded BBS multiple book review. This preprint has been prepared for potential commentators who wish to nominate themselves for formal commentary invitation. Please do not write a commentary unless you receive a formal invitation. Invited commentators will receive full instructions. Commentary must be based on the book.*

These findings point to both delay and deviance in WS face processing and illustrate how vital it is to build developmental trajectories for each specific task.