Although there have been numerous studies investigating the increased prevalence or "epidemic" or autism spectrum disorders (ASDs), it is still not clear whether this relates to a true increase in ASDs. Factors such as increased awareness by parents and clinicians, methods of reporting, and changing diagnostic criteria may account for this apparent increase. Examining the impact of these factors on the rate of ASD diagnoses will help to untangle whether ASDs are truly on the rise, or whether only the rate of diagnosis has increased. To attempt to provide the most accurate estimate of trends in the incidence of ASD, this study will analyze population-based records of ASD diagnoses which used diagnostic criteria that remained constant over time. Two datasets of ASD diagnoses will be analyzed: one from Denmark, and one from Western Australia. These datasets used different diagnostic criteria consistently (DSM for Western Australia, and ICD for Denmark). This study will determine whether the criteria used affected the age at which ASD is first diagnosed, and thus affected the perceived incidence. These results will help to answer the question of whether the perceived autism "epidemic" is real, and will provide valuable information for intervention services for children with ASD.