One in six American children now has a developmental disability — a 17% increase over the past decade, driven largely by increases in autism and attention deficit hyperactivity disorder (ADHD), according to government researchers.
The new study, published in Pediatrics, found that 15% of U.S. children aged 3 to 17 were diagnosed with a developmental disability in 2006-08 — about 10 million children in all. In 1997-99, that rate was 12.8%, or 8 million children.
While researchers saw increases in a wide range of developmental problems, including stuttering and learning disabilities, the most significant increases were seen with autism and ADHD. Autism rates nearly quadrupled over the study period, from 0.19% of children in 1997-99 to 0.74% in 2006-08. But, overall, ADHD accounted for the greatest number of developmental disability cases; rates rose by 33%, from 5.7% of children in 1997-99 to 7.6% by 2008.
(More on TIME.com: Korean Study Suggests That 1 in 38 Kids May Have Autism)
Further, reports Health.com:
Nearly twice as many boys as girls had a disability. … This might be because some genetic disabilities are more likely to be inherited by males, although it could also be that the symptoms of ADHD and other disabilities are more obvious in boys, and are therefore more likely to be diagnosed, the study notes.
Rates were also substantially higher than average among children from low-income families and children on Medicaid. Hispanic children had lower rates of disabilities than white or black children, which perhaps reflects language difficulties and other barriers to accessing health services rather than the true rate of disability.
The Centers for Disease Control and Prevention (CDC) study was based on data from the National Health Interview Surveys, which included in-person interviews with nearly 120,000 children. Researchers asked parents across the country to report their kids’ diagnoses of autism, ADHD, learning disabilities, mental retardation, cerebral palsy, seizures, stuttering or stammering, hearing loss and blindness.
(More on TIME.com: Preterm Birth Raises the Risk of Childhood ADHD)
The reasons for the increases are not clear, but the researchers suggest they may be due in part to increases in preterm birth and the older age of parents. Data show that children who are born prematurely are 30% to 60% more likely to develop ADHD. And a 2010 study found that mothers older than 40 were 50% more likely to have a child with autism than mothers in their 20s (although even among the high-risk group, the odds were still less than 4 in 1,000).
Other key reasons for increases in diagnosis, particularly with autism, are better screening, more awareness and less stigma, and increased vigilance among parents, teachers and pediatricians, the researchers said.
(More on TIME.com: Photos: A Journey into the World of Autism)
The more children are diagnosed with developmental disabilities, the more demand there is for health and educational services. Particularly with disorders like autism, research suggests that early diagnosis and treatment may be crucial for improving symptoms or even preventing the condition before it develops.
“We are more aware that early intervention is the key to the greatest success in these kids, [but] we need the resources to do that,” Alan Hilfer, the director of psychology at Maimonides Medical Center in New York City, told Health.com.