The spike in autism prevalence is apparently not linked to a kind of mercury once used in childhood vaccines, according to a study in the journal Pediatrics.

Because autism is often diagnosed around the same time that children are inoculated against infectious diseases, some parents have blamed vaccines for triggering the disorder, which is characterized by impaired communication, repetitive behavior and an inability to relate emotionally to others. Thimerosal, a mercury-based preservative used in vaccines, was thought a likely suspect because even small amounts of mercury can cause neurological damage.

Researchers tallied some three decades of autism cases in Denmark, which in 1992 was the first country to ban thimerosal. If thimerosal were to blame in even some cases, researchers hypothesized, they would have seen some decrease in autism in the late 1990s. Instead, autism diagnoses continued to rise in Denmark as they have around the globe.

Environmental toxins remain under suspicion, but many scientists believe the increase in autism diagnoses is due to better recognition of the disorder’s varied symptoms. Others believe there is a strong genetic component. Autism is four times more common in boys, but girls tend to have more severe symptoms.