A new report shows an increase in the prevalence of autism in the United States. Researchers at the Johns Hopkins Bloomberg School of Public Health contributed to a new U.S. Centers for Disease Control and Prevention (CDC) report that finds the prevalence of autism spectrum disorder (ASD) among 11 surveillance sites as one in 59 among children aged 8 years in 2014 (or 1.7 percent). This marks a 15 percent increase from the most recent report two years ago, and the highest prevalence since the CDC began tracking ASD in 2000. Consistent with previous reports, boys were four times more likely to be identified with ASD than girls. The rate is one in 38 among boys (or 2.7 percent) and one in 152 among girls (or 0.7 percent).

ASD is a developmental disorder characterized by social and communication impairments, combined with limited interests and repetitive behaviors. Early diagnosis and intervention are key to improving learning and skills. Rates have been rising since the 1960s, but researchers do not know how much of this rise is due to an increase in actual cases. There are other factors that may be contributing, such as: increased awareness, screening, diagnostic services, treatment and intervention services, better documentation of ASD behaviors and changes in diagnostic criteria.

Experts can't yet tell from the report whether autism is actually on the rise or whether the new numbers are due to improvements in recognizing and diagnosing the condition.

"It's a significant increase," Dr. Thomas Frazier, the chief science officer at Autism Speaks, told CBS News. "Part of the increase is due to the fact that we have reduced disparities both in terms of identifying girls with autism and also African-Americans and Hispanic children with autism. But also part of the increase we are just not sure exactly why that is happening."

Previous research has shown girls were being diagnosed with autism at a later age than boys, likely because they exhibit different and less severe symptoms.