In 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 ADHD and potentially applicable to millions of people in their late teens or older - is distinct from the usual adult variety, in which symptoms linger from childhood. Yet a new study suggests that adult-onset ADHD is rare - if it exists at all.
The paper, published recently 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 three to 10 per cent of adults.
The new study, while smaller, mined more extensive medical histories and found that most apparent cases of adult-onset ADHD are likely the result of substance abuse or mood problems.
"This study carefully considered whether each person met the criteria for ADHD and also fully considered other disorders" that might better explain the symptoms, said associate professor of paediatrics Mary Solanto at the Zucker School of Medicine at Hofstra/Northwell.
"In all those respects, it is the most thorough study we have looking at this issue."
She said the study all but ruled out adult-onset ADHD as a stand-alone diagnosis. Other experts cautioned that it was too early to say definitively and noted that attention deficits often precede mood and substance abuse problems - which in turn can mask the condition.
The new analysis drew on data from a decades-long study of childhood ADHD that had tracked kids from age nine or 10 up through early adulthood, gathering detailed histories from multiple sources, including doctors and parents.
That project, begun in 1994, recruited 579 children with diagnosed ADHD, and a group of 289 in the same classrooms for comparison purposes.
Of those "control" youngsters, the new study found, 24 would go on to develop attention deficit problems much later on, during high school or after. Classic ADHD is diagnosed between ages five and 12.
The authors of the new report, led by associate professor of psychiatry and behavioural health Margaret Sibley at Florida International University, carefully examined the extensive records of the 24 with adult-onset ADHD. They found that the attention deficits in all but five cases most likely stemmed from other causes, such as marijuana use, depression or anxiety.
The remaining five were hardly straightforward cases: One had previously had an eating disorder, another had shown signs of mania.
"This suggests to me the diagnosis doesn't exist independent of a compelling psychiatric history," Professor Sibley said. "No one in our group developed ADHD in adulthood out of nowhere."
Some 10 per cent of children are given a diagnosis of ADHD, and most grow out of it to some extent.
One reason that symptoms may emerge seemingly from nowhere in high school or later, experts say, is that some youngsters have offsetting abilities, like high IQ, or supports, such as sensitive parents or teachers, that mask the problems early on. In this respect, upbringing and environment may effectively blunt or contain symptoms.