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 percent 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.

Because the study counted autism diagnoses of children up to age 10, it covered a period up until about 2012.

In a recent telephone interview, Lundstrom said there is no reason to believe the Swedish experience with autism is much different from that in the U.S. or other nations, and he said there is no evidence to suggest that twins have a different rate of autism than the general population.

The national registry in Sweden includes all the official diagnoses for autism spectrum disorder, which more than doubled from 0.23 percent in 1993 to 0.5 percent in 2002. That rate is lower than the 1 percent prevalence found among the twins, but that may be because the national registry uses a conservative definition of the disorder. In another Swedish study last year that looked at all diagnoses for autism among teens living in Stockholm County, the autism diagnosis rate was about 2.5 percent.

“My personal view is that the autism rate might lie between 1 percent and 2 percent, depending on which lens you look at autism through,” Lundstrom said, “and our study suggests the (actual autism rate) has been fairly steady for decades.”

The official U.S. autism rate put out by the federal Centers for Disease Control and Prevention is now 1 in 68 children, or about 1.5 percent, based on an extrapolation from school records and official doctors’ diagnoses in 11 communities.

The official U.S. rate has more than doubled in recent years, from 0.67 percent in 2002 to the current rate, leading some to believe that environmental toxins or other factors have created an explosion of new cases. The official CDC statement says cautiously, “We don’t know what is causing this increase. Some of it may be due to the way children are identified, diagnosed and served in their local communities, but exactly how much is unknown.”

Studies like the one in Sweden argue that almost all the increase has been due to greater awareness of autism and an expansion in the number of children who get the diagnosis.

Nancy Minshew, director of the University of Pittsburgh’s Center for Excellence in Autism Research, says that she thinks the actual autism rate may be higher than the one calculated in the Swedish study — up to 3 percent — but she agrees with the study’s main conclusion.

“My view is that the autism numbers have gone up, but the number of people who are affected is actually unchanged, which is what they are saying in this paper.”

She said much of the growth in the official numbers has come in children with higher IQs, including those with Asperger’s syndrome. Current figures show that half of all children in the U.S. diagnosed with autism have IQs above 85, meaning that they do not have intellectual disability.

Minshew said many of these children simply were not recognized as having autism decades ago, and “people will often say to me now, I know there were a couple kids in my class who must have had this. Or people will say, ‘This is what my brother must have had; we never understood why he failed his graduate program.'”

Autism is marked by three main characteristics: difficulty with social relationships, problems with communication and repetitive behaviors or narrowly focused interests.

Much of the debate over the autism numbers has revolved around whether something in the environment — air and water pollution, pesticides or certain drugs, for instance — might have triggered a rise in autism.

Brian Lee, an epidemiologist at Drexel University in Philadelphia, participated in the Swedish teenager study that found a prevalence rate of 2.5 percent.

“A lot of papers show that most of the (autism) increase is due to diagnostic substitution and better awareness and those sorts of factors, but I’m still not entirely convinced it’s not a real increase.” The increases could partly be caused by environmental damage that causes new mutations in a child’s genome, he said. “We happen to live in one of the more polluted times in recent history.”

But Lundstrom, the Swedish researcher, isn’t buying that argument. “I am convinced there are several environmental factors that could affect autism,” he said, “but I don’t think they’re increasing. One issue that people have raised is pesticides, like living next to a big cornfield, but no one can convince me that pesticides were less toxic in the 1970s.”

In epidemiology, having a large population to study leads to more powerful results, Minshew noted.

“I think the Swedish research design was strong, and I think their results were solid. When you have 20,000 twins and 1.1 million total people, you can get very strong results.”

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