Autism, once a rare and mysterious disorder, is no longer so rare. A generation ago, only two to four of every 10,000 children were labeled autistic. Today, it's more like 60 per 10,000 by some estimates.

But no one knows why. Experts cite a much greater awareness of autism and related conditions, grouped as Autism Spectrum Disorders, and a broader definition that has allowed children who might otherwise have been overlooked to receive a diagnosis. But they also say there has been an actual increase, and the reasons for that are not entirely clear.

ASD, usually noticed by the time a child is 3, is four times more common in boys than girls. It affects social interaction, communication and behavior, but there is wide variety in symptoms. Some children don't speak. Some are talkative, but don't make eye contact. Some are clingy, while others hate being held.

"Years ago autism was a very rare diagnosis," says embryologist Patricia Rodier of the University of Rochester Medical Center, partly because there was no impetus for doctors to label a child as autistic. "It did not qualify you for special education," she says. "No one in good conscience could give that diagnosis, because you couldn't get special education."

That has changed. The number of children identified by schools as having ASD has risen by nearly 18 percent since the early 1990s.

Susie Kelly of Laurel, Del., says she and her husband, Tom, suspected very early that their second child, Mark, now 9, was not like other toddlers. The first clue was his reaction to surprises. "If someone came into the house he would cry, or when he was visiting his grandmother, if his aunt came in, he'd have a fit because that wasn't what his expectation was," she says.

She was certain something was wrong one day when she stopped off to buy doughnuts as she was driving him to day care. "It blew his mind because I took a different route."

Mark was finally diagnosed at age 3, and to the Kellys, "it was almost a relief, because we had a name for it." Almost.

"It was devastating," she says, choking up. "And I didn't even know what it meant then. It was continuous grieving."

The Kellys moved from Maryland to Delaware, where a comprehensive care program is available for Mark through age 20.

The rising incidence of autism and effective lobbying by parent groups have sped the pace of research into causes and treatments for developmental disorders.

The National Institutes of Health has allocated $65 million in funding for a network of eight "centers of excellence," where top researchers are narrowing in on genes that may play a role and are working to understand what causes them to go awry.