Autism Spectrum Disorders and other mental health problems: exploring etiological overlaps and phenotypic causal associations

Objective Recent studies highlight the impact of co-existing mental health problems in autism spectrum disorders (ASD). No twin studies to date reported on individuals meeting diagnostic criteria of ASD. This twin study reports on the aetiological overlap between diagnosis of ASD and emotional symptoms, hyperactivity, and conduct problems measured with the Strengths and Difficulties Questionnaire. Method Genetic and environmental influences on the covariance between ASD and co-existing problems were estimated, in line with the correlated risks model prediction. Phenotypic causality models were also fitted to explore alternative explanations of comorbidity: that co-existing problems are the result of or result in ASD symptoms, that they increase recognition of ASD, or that they arise due to an over-observation bias/confusion when differentiating between phenotypes. Results Over fifty percent of twins with broad spectrum/ASD met the borderline/abnormal levels cut-off criteria for emotional symptoms or hyperactivity, and a quarter met these criteria for the three reported problems. In comparison, between 13%-16% of unaffected twins scored above the cut-offs. The phenotypic correlation between ASD and emotional symptoms was entirely explained by genetic influences and accompanied by a moderate genetic correlation (.42). The opposite was true for the overlap with conduct problems, as non-shared environmental factors had the strongest impact. For hyperactivity, the best-fitting model suggested a unidirectional phenotypic influence of hyperactivity on ASD. Conclusion Our findings suggest a possible effect of hyperactivity on identification of ASD. The lack of genetic influences on conduct problems–ASD overlap further supports the genetic independence of these two phenotypes. Finally, the co-occurrence of emotional symptoms in ASD, compared to other co-occurring problems, is completely explained by common genetic effects