Autism spectrum disorders (ASD) are neurodevelopmental disorders characterised by impairments in reciprocal social interaction, communication, and restricted and repetitive patterns of behaviours and interests. The individual, familial and societal burden of autism is extensive. Depression, anxiety and suicidal behaviour and self-harm are often cited as some of the commonest co-morbidities in ASD, and may lead to additional impairment in quality of life and disability. However, the supporting data largely comprise small (typically under 100 individuals), selected samples and case series, often without a comparison group. The results are therefore difficult to generalise and estimates liable to selection bias and confounding. Since these morbidities are common in the general population, it is unclear whether autism, or any specific impairments related to autism, confer any additional risk to their development. Longitudinal population based studies following children into adulthood have been called for but have been absent from the literature to date. Beyond understanding the relative risk of adult mental health outcomes in individuals with autism, a bigger question is whether any risk or protective pathways could be identified. Such information would be invaluable to inform the development of public health interventions. As an illustrative example, bullying and victimisation, commonly experienced by children with ASD, may contribute to the future development of mental health problems. If this were true, targeting such ‘mediating’ pathways may help prevent adverse mental health outcomes in a proportion of cases, irrespective of our ability to modify core autistic symptoms.

We aim to conduct a comprehensive study using the complementary strengths of two large population based cohorts (Alspac and the Stockholm Youth Cohort) with prospective data to make substantial contributions to the major gaps in the research evidence in this area.