In a blog entry, Christa Dahlstrom wrote of the “eye-rolling response” she often gets when mentioning her son’s autism by way of explaining his seeming rudeness: “The optimist in me wants to hear this as supportive (Let’s not pathologize differences!) but the paranoid, parent-on-the-defensive in me hears it as dismissive.”

There are, Ms. Dahlstrom acknowledges, parents of children with autism whose challenges are far greater. And perhaps it stands to reason that at a time when government-financed services for such children are stretched thin, the question of who qualifies as autistic is growing more pointed. “ ‘You don’t get it; your kid is actually toilet trained,’ ” another mother told her once, Ms. Dahlstrom recalled. “And of course she was right. That was the end of the conversation.”

But Zoe Gross, 21, whose autism spectrum disorder was diagnosed at age 4, says masking it can take a steep toll. She has an elaborate flow chart to help herself leave her room in the morning (“Do you need a shower? If yes, do you have time for a shower?”). Already, she had to take a term off from Vassar, and without her diagnosis, she says, she would not be able to get the accommodations she needs to succeed when she goes back.

According to the C.D.C., what critics condemn as over-diagnosis is most likely the opposite. Twenty percent of the 8-year-olds the agency’s reviewers identified as having the traits of autism by reviewing their school and medical records had not received an actual diagnosis. The sharpest increases appeared among Hispanic and black children, who historically have been less likely to receive an autism diagnosis. In South Korea, a recent study found a prevalence rate of one in 38 children, and a study in England found autism at roughly the same rate — 1 percent — in adults as in children, implying that the condition had gone unidentified previously, rather than an actual increase in its incidence.

Those numbers are, of course, dependent on the definition of autism — and the view of a diagnosis as desirable. For John Elder Robison, whose memoir “Look Me in the Eye” describes his diagnosis in middle age, the realization that his social awkwardness was related to his brain wiring rather than a character flaw proved liberating. “There’s a whole generation of people who grew up lonelier and more isolated and less able to function than they might have been if we had taken steps to integrate them into society,” he said.

Yet even some parents who find the construct of autism useful in understanding and helping children others might call quirky say that in an ideal world, autism as a mental health diagnosis would not be necessary.

“The term has become so diffuse in the public mind that people start to see it as a fad,” said Emily Willingham, who is a co-editor of “The Thinking Person’s Guide to Autism.” “If we could identify individual needs based on specific gaps, instead of considering autism itself as a disorder, that would be preferable. We all have our gaps that need work.”