Comorbidity of internalising disorders in children and adolescents with low IQ : an item response theory approach

The main purpose of the present study was to employ 2-parameter logistic item response theory modelling (2-PLM) to examine the internalising dimension, comprising diagnoses of common mood and anxiety disorders, within a low IQ (<80) child and adolescent population. This was done to investigate the validity of a dimensional approach to mood and anxiety comorbidity, as well as investigate the patterns of mood and anxiety comorbidity among a low-IQ population. Diagnoses were derived from interviews of 310 clinic-referred low IQ children and adolescents. The results of the present study suggested that all disorders were not only strong discriminators of the internalising dimension, but measured the dimension with more precision in the upper half of the trait continuum. The study also found support for the concurrent validity of the internalising dimension, referring to medium to large effect size associations with depression and anxiety scores of other measures. The implications for the study of comorbidity in this type of population, and the taxonomy of the mood and anxiety disorders in diagnostic manuals and conceptualisations are discussed.