ORANGE – Vaccines, environmental triggers, parental age – it’s highly unlikely that any of those are driving the dramatic increase in the autism rate, a Chapman University study found.

Rather, the study concluded the rate – which is now 1 in 50 in Orange County, the highest in the state – can be “almost completely” explained by a change in how the disorder is diagnosed.

Two Chapman researchers analyzed 15 years of state special education eligibility data and found many students who once would have been considered to have a condition called Specific Learning Disability are now told they have autism.

If the findings are correct, the prevalence of autism hasn’t actually changed; it’s just changed names.

The Chapman researchers aren’t the first to reach this conclusion.

Journalist Steve Silberman’s book on the topic, “NeuroTribes,” “explores how a 1987 expansion of the medical definition of autism (which was previously much narrower and led to less frequent diagnoses) contributed to the perception that there was an autism epidemic,” according to NPR.

According to the Chapman study, children who were once given the label Specific Learning Disability are now falling under the autism umbrella. Children with SLD don’t have an intellectual impairment, but fall short of expectations on such academic measures as listening, thinking, spelling or mathematical calculations. The shift is a concept the Chapman researchers are calling “diagnostic migration.”

“For every new kid with autism, there’s one less with SLD,” said researcher Donald Cardinal, a professor and former dean of educational studies at Chapman.

“We are showing that the increase is highly likely not a medical explanation. That alone is massive given how much we are spending on medical research,” he said.

Cardinal and Amy-Jane Griffiths, a clinical psychologist and assistant professor, unveiled the findings Tuesday to several hundred parents, service providers and researchers. The study has not yet been published in a scientific journal.

The professors analyzed special education eligibility designations for every student ages 3 to 22 in California schools from 2000 to 2015. To be eligible for special education, students must have a disability that falls into one of 13 categories, including Specific Learning Disability, autism, hearing impairment and traumatic brain injury.

Among the study’s results:

• Only one category, SLD, has substantially decreased in numbers and proportion, by 64,842 students statewide since 2000.

• The number of students in California with autism has increased six times to 76,755.

• Only one other category has substantially increased, Other Health Impaired, and researchers believe this is actually an increase in ADHD.

• The growing rate of autism is not adding to the number of children with disabilities. The total number of students in special education has increased less than the general population, 0.69 percent per year compared with 0.96 percent.

• The percentage of special education students with autism in Orange County was 18.7 percent last year, up from 2.6 percent in 2000 and 6 percentage points above California in 2015.

Cardinal and Griffiths said they also analyzed published research to rule out other factors suspected of causing autism and propelling the autism rate, including vaccines, “regional wealth,” parental age and ethnicity.

Because they looked only at California data, they say the results don’t apply to the rest of the nation, but do suggest a larger trend.

The Centers for Disease Control and Prevention puts the U.S. autism rate at 1 in 68. It has arrived at that figure after funding research that uses similar data to those used by Chapman University.

The study’s other major finding was Orange County’s autism rate, which Cardinal said is among the highest in the nation.

Cathy Brock, executive director for Autism and Neurodevelopmental Disorders in Santa Ana, called the results “exciting” and “impressive.” But she cautioned that “diagnostic migration” does not explain why a child has autism.

“We’re still not answering the question of what’s causing autism, and that’s often what’s on the forefront of parents’ minds,” she said.

Contact the writer: jchandler@ocregister.com, Twitter @jennakchandler