Children who live in close proximity to a child diagnosed with autism are more likely to also be diagnosed with the disorder. The reason, researchers say, has nothing to do with environmental factors or misdiagnosis, but rather the huge role that social influence plays.

In recent years many advocates warned of an autism epidemic as prevalence estimates from the federal government jumped to indicate that as many as 1 in 100 children have the developmental disorder. However, in a recent study published in the American Journal of Sociology researchers say the ballooning numbers could merely reflect a more accurate picture of an incidence rate that existed all along, particularly among those at the higher functioning end of the autism spectrum.

“We show that the likelihood of getting an autism diagnosis is clearly associated with person-to-person transmission of information,” said Peter Bearman, a Columbia University sociologist who authored the study. “We are describing the mechanism by which the number of diagnoses is increasing. It could be that the real incidence of the disorder is only now being uncovered.”

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In the study, Bearman and his colleagues looked at 300,000 children born in California between 1997 and 2003. Those who lived within 250 meters, or about 820 feet, of a child with autism were 42 percent more likely to also be diagnosed with the disorder as compared to those without a neighbor with the diagnosis. The farther a child lived from another child with autism, the less likely they were to be be diagnosed with the disorder.

Whether or not a child lived within the same school district boundaries as a neighbor with autism also seemed to influence the likelihood of a diagnosis, researchers say.

The results were most pronounced among children with milder cases of autism, Bearman says, since parents of those with more severe disabilities are more likely to notice their child’s condition without the aid of friends.

Overall, the study suggests that social influence plays a role in 16 percent of the recent increase in autism diagnoses, a bigger impact than the role of a mother’s age or educational level, researchers say.