In the largest study of its kind, a Swedish group has determined that actual autism rates probably have not changed in recent years, even though diagnoses of autism cases continue to climb.

The research, led by Sebastian Lundstrom and colleagues at the University of Gothenburg, found that about 1% of those in an ongoing study of twins met the criteria for having autism, even though the number of officially diagnosed autism cases in the country’s national health registry had climbed steadily over a 10-year-period.

The power of the study, published last month in the British Medical Journal, comes from the fact that Sweden has comprehensive health records for its population, and the research covered nearly 20,000 twins whose families were asked about their symptoms, along with diagnostic records for more than a million children born between 1993 and 2002.

This article originally appeared on Psychiatry Advisor

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