Only forty per cent of the notable increase in autism cases that has been registered during the past few decades is due to causes that are as yet unknown.
The majority of the increase – a total of sixty per cent – can now be explained by two combined factors: changes in the diagnostic criteria and in the registration to the national health registers.
This is shown by a new study of disease prevalence among all individuals born in Denmark in the period 1980-1991, a total of 677,915 individuals.
The results have recently been published in the medical journal JAMA Pediatrics.

For the first time, researchers are able to confirm a widespread hypothesis, which is that an altered focus on autism can directly explain the majority of the dramatic increase in the disorder’s statistics:

"What our study shows is that you cannot really talk about an autism epidemic, even though Denmark and other countries are currently experiencing a dramatic increase in the number of cases of autism spectrum disorders. We can see that a large part of the explanation lies in the huge changes in the area of diagnosis and registration which took place in the middle of the 1990s," says PhD student Stefan Nygaard Hansen from Aarhus University.

Focus on two changes

The researchers have specifically focused on a change in the diagnostic system used by psychiatrists. This change occurred in 1994 and meant that autism is today recognised as a whole spectrum of disorders. But it also included an alteration in the specific symptoms that form the basis of the autism diagnosis.
The other important change took place in 1995 when the Danish national health registers began to include diagnoses made in connection with outpatient consultations. Before then, only diagnoses from hospital admissions were recorded.

Both changes have thus occurred after the period in which the persons incorporated in the study were born. This has made it possible to compare the number of diagnoses made before and after the alterations within the same birth cohorts.

"Putting it simply, we can say that a person with an autism diagnosis has a slightly different symptom profile after the middle of the 1990s than previously, which is reflected in the statistics," says Stefan Nygaard Hansen.

"But even though we have now shown that a large part of the increase among the Danish population is due to changes in diagnostic and registration systems, it is still important to point to the forty 40 per cent of the increase which we cannot explain. We therefore need to continue to look for the factors which can explain this," says Professor Erik Parner, Aarhus University.

• Read the article ”Explaining the Increase in the Prevalence of Autism Spectrum Disorders: The Proportion Attributable to Changes in Reporting Practices” in JAMA Pediatrics.

Facts:

• Autism spectrum disorders are considered to be among the most serious child psychiatric disorders.
• The frequency of autism spectrum disorders in children has increased significantly in Denmark during the last few decades and is approximately 1.5 per cent among the latest birth cohorts. A similar increase is seen in a many other countries around the world.
• Information about the 677,915 children has been obtained from the Danish Medical Birth Registry, the Danish Civil Registration System (CPR) Register and the Danish Psychiatric Central Register.
• The study shows that the change in the diagnosis system in 1994 has affected boys considerably more than girls, but that the inclusion of outpatient diagnoses in 1995 has affected both genders equally.

Further information:

PhD student Stefan Nygaard Hansen
Aarhus University, Department of Public Health
Tel.: +45 2073 5293
stefanh@ph.au.dk

Professor Erik Thorlund Parner
Aarhus University, Department of Public Health
Tel.: +45 3069 1084 
parner@ph.au.dk