In just the past few years, researchers have identified what they believe is an adult version of attention deficit disorder: a restless inability to concentrate that develops spontaneously after high school, years after the syndrome typically shows itself, and without any early signs.

The proposed diagnosis — called adult-onset A.D.H.D. and potentially applicable to millions of people in their late teenage years or older — is distinct from the usual adult variety, in which symptoms linger from childhood.

Yet a new study suggests that adult-onset A.D.H.D. is rare — if it exists at all.

The paper, published Friday in the American Journal of Psychiatry, could deepen the debate over these symptoms rather than settle it. Previously, three large analyses had estimated the prevalence of the disorder at 3 to 10 percent of adults.

The new study, while smaller, mined more extensive medical histories than earlier work and found that most apparent cases of adult-onset attention deficits are likely the result of substance abuse or mood problems.