“The epidemic hypothesis emerged in the 1990s when, in most countries, increasing numbers were diagnosed with ASDs leading to an upward trend in children registered in service providers’ databases that was paralleled by higher prevalence rates in epidemiological surveys. These trends were interpreted as evidence that the actual population incidence of ASDs was increasing (what the term epidemic means). However, alternative explanations for the rise in numbers of children diagnosed with ASDs should be ruled out first before supporting this conclusion and include the following. […] However, trends over time in referred samples are confounded by many factors such as referral patterns, availability of services, heightened public awareness, decreasing age at diagnosis, and changes over time in diagnostic concepts and practices. Failure to control for these confounding factors was obvious in previous reports […]”
In Volkmar, F. R., In Rogers, S. J., In Paul, R., & In Pelphrey, K. A. (2014). „Handbook of autism and pervasive developmental disorders„, 4th Edition.