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The number of autism spectrum disorders (ASD) diagnosed throughout the world has increased. Due to the rise in cases, researchers from Denmark set out to uncover what factors might be responsible. The team concluded that the spike is due mostly to how doctors are reporting and diagnosing the illness.

"As our study shows, much of the increase can be attributed to the redefinition of what autism is and which diagnoses are reported," lead researcher Stefan Hansen, from the section for biostatistics in the department of public health at Aarhus University said reported by CBS News. "The increase in the observed autism prevalence is not due alone to environmental factors that we have not yet discovered."

For this study, the researchers from the Aarhus University and the Initiative for Integrative Psychiatric Research examined data on 678,000 children who were born between 1980 and 1991 in Denmark. 3,965 of them were diagnosed with ASDs by December 2011.

When the researchers compared the number of cases between time periods, they found that the rate increased from 192 between 1980 and 1992 to 3,664 between 1996 and the end of the study. The team noted that in 1994, the diagnostic criteria in Denmark changed. In 1995, the country's health registries started to include all cases of ASDs that were diagnosed outside of a hospital.

"The definition of autism is more precise today compared to the 1980's," commented Hjordis Osk Atladottir, who was not involved in the new study, according to Reuters. "We got the ICD-10 in 1994 in Denmark and before that we had ICD-8, but autism was very poorly defined in the ICD-8, resulting in almost no autism diagnoses prior to 1994."

The researchers attributed 60 percent of the cases to changes in the diagnostic criteria. They could not, however, determine what factors were behind the remaining 40 percent of cases. The team theorized that several variables, such as parental age during conception and environmental factors could have affected these numbers.

"The findings from this study are consistent with past research documenting the role of non-causal factors, such as increase in autism awareness, changes to diagnostic criteria and the increase in autism prevalence over time," Amy Daniels, the assistant director for public health research at Autism Speaks, a New York City-based advocacy group, commented.

The study, "Explaining the Increase in the Prevalence of Autism Spectrum Disorders," was published in JAMA Pediatrics.