Investigating the Existence of the Diagnostic Overshadowing Bias in Australia

People with an Intellectual Disability (ID) are around 4 times more likely to suffer with a mental health disorder than IQtypical people, however previous studies have shown they may be less likely to be diagnosed. One suggested cause of this is the diagnostic overshadowing bias: the tendency for practitioners to attribute unusual behaviour, thoughts or feelings to a person’s ID and not a separate mental health disorder. In the current study N = 87 participants were randomly assigned 1 of 2 clinical vignettes describing a client who is displaying symptoms of mental health disorder. Vignette 1 described a client with an IQ of 105 who graduated from high school. Vignette 2 described a client with an IQ of 55 who attended special classes throughout school. All other details were identical. Participants then rated the likelihood of 7 diagnoses on a 5-point Likert scale. Contrary to our hypothesis participants were not less likely to diagnose a mental health disorder in people with an ID and this was found to exist across all profession types involved in the study. Although these findings indicate that the diagnostic overshadowing bias may not be present in Australia, several factors including a reported lack of confidence in working with clients with an ID suggest the inclusion of ID specific training may be beneficial