Disagreement about the effect of the new definition will almost certainly increase scrutiny of the finer points of the psychiatric association’s changes to the manual. The revisions are about 90 percent complete and will be final by December, according to Dr. David J. Kupfer, a professor of psychiatry at the University of Pittsburgh and chairman of the task force making the revisions.

At least a million children and adults have a diagnosis of autism or a related disorder, like Asperger syndrome or “pervasive developmental disorder, not otherwise specified,” also known as P.D.D.-N.O.S. People with Asperger’s or P.D.D.-N.O.S. endure some of the same social struggles as those with autism but do not meet the definition for the full-blown version. The proposed change would consolidate all three diagnoses under one category, autism spectrum disorder, eliminating Asperger syndrome and P.D.D.-N.O.S. from the manual. Under the current criteria, a person can qualify for the diagnosis by exhibiting 6 or more of 12 behaviors; under the proposed definition, the person would have to exhibit 3 deficits in social interaction and communication and at least 2 repetitive behaviors, a much narrower menu.

Dr. Kupfer said the changes were an attempt to clarify these variations and put them under one name. Some advocates have been concerned about the proposed changes.

“Our fear is that we are going to take a big step backward,” said Lori Shery, president of the Asperger Syndrome Education Network. “If clinicians say, ‘These kids don’t fit the criteria for an autism spectrum diagnosis,’ they are not going to get the supports and services they need, and they’re going to experience failure.”

Mark Roithmayr, president of the advocacy organization Autism Speaks, said that the proposed diagnosis should bring needed clarity but that the effect it would have on services was not yet clear. “We need to carefully monitor the impact of these diagnostic changes on access to services and ensure that no one is being denied the services they need,” Mr. Roithmayr said by e-mail. “Some treatments and services are driven solely by a person’s diagnosis, while other services may depend on other criteria such as age, I.Q. level or medical history.”

In the new analysis, Dr. Volkmar, along with Brian Reichow and James McPartland, both at Yale, used data from a large 1993 study that served as the basis for the current criteria. They focused on 372 children and adults who were among the highest functioning and found that overall, only 45 percent of them would qualify for the proposed autism spectrum diagnosis now under review.

The focus on a high-functioning group may have slightly exaggerated that percentage, the authors acknowledge. The likelihood of being left out under the new definition depended on the original diagnosis: about a quarter of those identified with classic autism in 1993 would not be so identified under the proposed criteria; about three-quarters of those with Asperger syndrome would not qualify; and 85 percent of those with P.D.D.-N.O.S. would not.