Objectives: To investigate the traits characterising ASD.
Methods: Using the large population-based Avon Longitudinal Study of Parents and Children cohort, 90 traits relating to social, communication and repetitive behaviours were identified between the ages of 6 months and 9 years. Data were available for 13,137 children after missing value imputation. Factor analysis was used to generate a set of derived traits from the total sample. As a consequence, these factors were blind to the ASD diagnosis. Varimax rotation was used to simplify the factor structure. Factor and individual traits were compared in their predictive power of ASD and in their associations with other co-morbid conditions including learning difficulties, SLI, SEN and DAWBA diagnoses relating to ADHD, ODD/CD and anxiety problems.
Results: In all, 79 children (0.60%) were diagnosed with ASD in this sample. The factor analysis suggested 7 factors explaining 43% of the variance. These were described as: verbal ability, language acquisition, semantic-pragmatic deficits, social understanding, repetitive behaviour, articulation and social inhibition. All factors were independently related to ASD (p<0.001). Comparison with other co-morbid conditions suggested that four of these had the strongest association with ASD while language acquisition, semantic-pragmatic deficits and articulation were more strongly related to other conditions. Of the individual traits, four were identified via a compromise of parsimony, power in predicting ASD and adherence to the diagnostic triad. These were: Children’s Communication Checklist at 9y (coherence subscale), Social and Communication Disorders Checklist at 91m, EAS temperament at 38m (sociability subscale) and a measure of repetitive behaviour at 69m derived from 3 questions. All 4 traits were independently related to ASD (p<0.008) and had the strongest associations with ASD compared to other co-morbid conditions. Comparison of these individual and factor traits suggested that both contained information not present in the other set of traits.
Conclusions: This study supports the compound phenotype hypothesis. The standard diagnostic triad may be an over-simplification of a more complex structure with in particular social elements being split into social understanding and social inhibition. Other aspects of the ASD phenotype were identified but these may be attributed to other co-morbid conditions rather than traits core to the diagnosis of ASD. The derivation of the factors from a population-based sample and dimensional aspects of both the factor and individual traits suggest that these traits measure a broader phenotype with impairments extending below the clinical threshold. The factor trait model was not superior to an individual trait model suggesting the theoretical advantages in combining information from a number of measures to identify latent traits were offset by the dilution effect of combining traits with varying degrees of association with ASD.