Incidence of autism spectrum disorders: changes over time and their meaning

Acta Paediatr. 2005 Jan;94(1):2-15. doi: 10.1111/j.1651-2227.2005.tb01779.x.


Aim: Several reviews have noted a huge increase in the rate of diagnosed autism spectrum disorders. The main aims of this paper are: 1) to use published empirical findings to consider whether the rise reflects a true increase in incidence, as distinct from the consequences of better ascertainment and a broadening of the diagnostic concept; and 2) to consider how epidemiological data may be used to test hypotheses about possible causal influences, using MMR and thimerosal as examples.

Methods: Search of the literature for studies with a large epidemiological base population, systematic standardized screening, a focus on an age group for which diagnostic assessments are reliable and valid, and diagnosis by trained professionals using high-quality research assessments. Also, search of a broader literature to consider the evidence from all epidemiological studies with respect to the hypothesized causal effect of MMR and thimerosal on autism spectrum disorders.

Results: The true incidence of autism spectrum disorders is likely to be within the range of 30-60 cases per 10 000, a huge increase over the original estimate 40 years ago of 4 per 10000. The increase is largely a consequence of improved ascertainment and a considerable broadening of the diagnostic concept. However, a true risk due to some, as yet to be identified, environmental risk factor cannot be ruled out. There is no support for the hypothesis for a role of either MMR or thimerosal in causation, but the evidence on the latter is more limited.

Conclusion: Progress in testing environmental risk hypotheses will require the integration of epidemiological and biological studies.

Publication types

  • Review

MeSH terms

  • Autistic Disorder / diagnosis
  • Autistic Disorder / epidemiology*
  • Autistic Disorder / etiology
  • Empirical Research
  • Environment
  • Humans
  • Incidence
  • Measles-Mumps-Rubella Vaccine / adverse effects
  • Preservatives, Pharmaceutical
  • Thimerosal / adverse effects


  • Measles-Mumps-Rubella Vaccine
  • Preservatives, Pharmaceutical
  • Thimerosal