Papers have proposed that a recognition of sluggish cognitive tempo could help resolve some longstanding confusion about A.D.H.D., which despite having hyperactivity in its name includes about two million children who are not hyperactive, merely inattentive. Some researchers propose that about half of those children would be better classified as having sluggish cognitive tempo, with perhaps one million additional children, who do not meet A.D.H.D.’s criteria now, having the new disorder, too.

“These children are not the ones giving adults much trouble, so they’re easy to miss,” Dr. McBurnett said. “They’re the daydreamy ones, the ones with work that’s not turned in, leaving names off of papers or skipping questions, things like that, that impinge on grades or performance. So anything we can do to understand what’s going on with these kids is a good thing.”

But Dr. McBurnett added that sluggish cognitive tempo remained many years from any scientific consensus: “We haven’t even agreed on the symptom list — that’s how early on we are in the process.”

Steve S. Lee, an associate professor of psychology at the University of California, Los Angeles, who serves on the editorial board of The Journal of Abnormal Child Psychology, said in an interview that he was conflicted over the journal’s emphasis on sluggish cognitive tempo. He expressed concern that A.D.H.D. had already grown to encompass too many children with common youthful behavior, or whose problems are derived not from a neurological disorder but from inadequate sleep, a different learning disability or other sources.

About two-thirds of children with an A.D.H.D. diagnosis take daily medication such as Adderall or Concerta, which often quells severe impulsiveness and inattention but also carries risks for insomnia, appetite suppression and, among teenagers and adults, abuse or addiction.

“The scientist part of me says we need to pursue knowledge, but we know that people will start saying their kids have it, and doctors will start diagnosing it and prescribing for it long before we know whether it’s real,” Dr. Lee said. “A.D.H.D. has become a public health, societal question, and it’s a fair question to ask of S.C.T. We better pump the brakes more diligently.”

Dr. McBurnett recently conducted a clinical trial funded and overseen by Eli Lilly that investigated whether proposed symptoms of sluggish cognitive tempo could be treated with Strattera, the company’s primary A.D.H.D. drug. (One of Strattera’s selling points is that it is not a stimulant like Adderall and Concerta, medications more susceptible to abuse.) His study, published in The Journal of Child and Adolescent Psychopharmacology, concluded, “This is the first study to report significant effects of any medication on S.C.T.”