Introduction: Autism is frequently referred to as a heterogeneous condition. Precisely what is meant by this is unclear, in part because of the complexity of heterogeneity itself. This thesis outlines different possible understandings of heterogeneity, the different ways that autism can be understood as heterogeneous, and the importance of these differences to research. The principles and precepts of a systematic review are then used to examine, classify, and understand how researchers considered and addressed the heterogeneity of their autistic participants.
Method: Using search terms identifying participants with a diagnosis of autism and ADHD, a systematically-informed-review was conducted of 5 databases (PsycINFO, Medline, Web of Science, Scopus, ERIC). Data on heterogeneity was extracted and analysed from 51 papers chosen at random from the search results. A system of classification was developed to better understand, evaluate, and analyse how researchers worked with heterogeneity.
Results: Some researchers were explicit about the diversity of autistic and ADHD participants, the challenges this posed, and the ways used to address this. Others did not mention the heterogeneity of their participants or saw it as an impediment to their research. For many, diagnostic status alone was used as a means of capturing information about autism/ADHD.
Discussion: Although researchers describe autism as heterogeneous, little consensus exists on what this means or how it should be addressed. According to this random sample, researchers are often not explicit about their understanding of heterogeneity, how to work with it, or what consequences it may have for their results. A more explicit and formalised approach to understanding heterogeneity may help researchers to consider their research in light of the diversity of autistic people, and in so doing improve the validity, generalisability, and utility of their work.
