Rise In Autism Rates Largely Due to Changes in Diagnostic Criteria

dr_stefan_hansenMedicalResearch.com Interview with:
Stefan Nygaard Hansen 
PhD Student, MSc Stat
Section for Biostatistics
Department of Public Health
Aarhus University Denmark

Medical Research: What are the main findings of the study?

Response: The main finding of our study is that 60% of the observed increase in autism prevalence among children born 1980-1991 in Denmark can be explained by changes in the way diagnoses are established and changes in the subsequent registration to national health registries.

In 1994, the diagnostic criteria used by clinicians to establish psychiatric diagnoses was changed. This meant the recognition of autism as a spectrum of disorders but it also meant changes in the specific symptoms that form the basis of the autism diagnosis. In 1995, the national health registries in Denmark, which are often used in Danish health research, began to also include diagnoses given in connection with outpatient consultations whereas before 1995 only diagnoses given in connection with hospitalization was reported to the registries. This could mean that we after 1995 see more of the mild autism diagnoses since they may not require hospitalization.

Medical Research: What was most surprising about the results?

Response: That the number turned out to be 60%. We expected that the changes could account for some of the increase in autism prevalence but it was a bit surprising that it turned out to be 60%.

Medical Research: What should clinicians and patients take away from your report?

Response: The study shows that the increase in autism prevalence is not caused by unknown environmental risk factors alone but that more than half of the increase is related to changes in the area of diagnosis and registration taking place in the mid-1990s. Thus, we might say that the symptom-profile of an individual with an autism diagnosis today is slightly different than it was before the changes. However, there is still 40% of the increase which we cannot account for and hence it is important to continue to look for other factors that might explain this.

Medical Research: What recommendations do you have for future research as a result of this study?

Response: Continue to look for factors that might help explain part of the increase. It would also be interesting to see if other countries with similar changes in factors related diagnosis and reporting would see similar numbers.


Hansen SN, Schendel DE, Parner ET. Explaining the Increase in the Prevalence of Autism Spectrum Disorders: The Proportion Attributable to Changes in Reporting Practices. JAMA Pediatr. Published online November 03, 2014. doi:10.1001/jamapediatrics.2014.1893.


Last Updated on November 4, 2014 by Marie Benz MD FAAD