In the 1970s, attention deficit hyperactivity disorder (ADHD) was considered a childhood disorder that went away in adolescence or early adulthood. It later became clear that the symptoms persist in some adults, changing the way we think about the disorder. Still, for adults to be diagnosed and get treatment for ADHD, they must have shown symptoms of hyperactivity, impulsivity, or inattention before age 12.

New research is challenging these beliefs.

A pair of studies published this week in JAMA Psychiatry have identified large groups of people who never had ADHD symptoms as children but developed the disorder as adults.

In a UK study, researchers at King’s College in London found that nearly 70% of more than 2,200 18-year-old twins with ADHD did not meet the criteria for the disorder during any of four earlier assessments at ages 5, 7, 10, and 12.

The adults with “late-onset” ADHD were mostly women (55%) and showed significant symptoms, including problems at home and work, general anxiety and conduct issues, as well as debilitating mental health disorders.

As longitudinal studies like this emerge, questions are being raised about whether childhood and adult ADHD are separate disorders, or whether they are aspects of the same disorder that present differently as people age.

New paradigm?

If adult ADHD is different from childhood ADHD, then diagnosis and treatment practices may have to change dramatically.

“Although ADHD occurs in approximately 4% of adults, relatively few adults receive a diagnosis or treatment for the disorder,” said Jessica Agnew-Blais, an author of the King’s College study. “The absence of a childhood diagnosis should not prevent adults with ADHD from receiving clinical attention.”

In Brazil, another study showed similar findings. Researchers led by Arthur Caye examined data from 5,249 individuals born in 1993. Their ADHD status was first measured at age 11 and then again at age 18 or 19.

Around 12% of the 18- and 19-year-olds were diagnosed with ADHD, but only around a tenth of them also had the disorder at age 11. ”Our data does not support the idea that ADHD is only a continuation of a childhood disorder,” said Caye, who is a PhD student at the Hospital de Clinicas de Porto Alegre.

Both studies had limitations. The British 18-year-olds self-reported their symptoms, compared with the assessments of parents and teachers in earlier periods. To address that shortcoming, the researchers spoke with other family members—including the participants’ twins—about whether the symptoms were new. The findings still held. And similar self-reporting issues in the Brazilian study may have led to a high incidence of false positives as well.

Both the studies build on research in New Zealand from last year. That study also revealed a group of adults with ADHD who had no prior childhood symptoms. But because it was such a radical departure from the commonly understood path of ADHD, it concluded that more research was required to dispel concerns that its conclusions could have been a fluke.

Caye said that three studies showing similar results represents a “major shift” in our understanding the disorder.

Addressing the ADHD gender gap

The UK study also shed light on the mystery around gender and ADHD. Among children, boys are diagnosed at a ratio of two or three-to-one, but among adults it is closer to even between men and women.

One explanation for the disparity is that girls are often not diagnosed because their symptoms materialize as inattention, which is quiet, and not hyperactivity, which is disruptive. Women mask their symptoms to conform to what is expected of them, but then suffer mightily when they leave the structures of school and living with their parents. Patricia Quinn, a developmental pediatrician and author of many books on ADHD, said the issue for girls is that “functional impairment” does not manifest itself until adulthood, when it is diagnosed. “That does not mean that some symptoms were not present previously,” she said.

Another idea is that girls’ symptoms could be more persistent than boys’, so more girls carry their ADHD into adulthood.

The UK study suggests that this is not the case. The higher rates of ADHD among adult women ”wasn’t driven by girls’ persistence, but by more women being included in the late-onset ADHD group,” said Agnew-Blais of King’s College.

Further research is needed to investigate these puzzling gender differences, the risk factors for late-onset ADHD, and potential genetic clues related to ADHD that appears early and persists, appears early and then fades away, or is nowhere to be seen until much later in life.