Does the autism phenotype differ when selecting groups by neurodevelopmental versus genetic diagnosis? An observational study comparing autism and sex chromosome trisomy.

Background: Autism is diagnosed on the basis of social and non-social behavioural features that are assumed to cluster together, and assumed to be distinct from other aspects of development, such as language ability. It is unclear, however, if these assumptions are valid. This study presents a novel approach to answering this question by investigating whether correlations between autism features are similar for groups selected on behavioural versus genetic diagnosis. Methods: The autism phenotype was assessed by diagnostic interview in young people aged 7 to 14 diagnosed with autism ( N=61) or sex chromosome trisomy (SCT; N=49). Data were analysed by confirmatory factor analysis and MANOVA. Results: Autism features showed a similar factor structure and were distinct from language ability in both groups. However, the SCT group was more likely to show clinically-significant difficulties in just some aspects of autism and a lower level of non-social autism features for their social-communication disabilities. Conclusions: We suggest the group differences emerged because autism diagnostic criteria do not map exactly on the autism phenotype as it manifests "naturally". Conventional diagnostic criteria for autism miss those with uneven profiles of difficulty and those with relatively low levels of restricted and repetitive behaviours and interests.