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The number of children diagnosed with attention deficit hyperactivity disorder, or ADHD, has been rising rapidly over the last few decades. Today, nearly one out of every 10 youths in the United States is diagnosed with ADHD, an increase also reflected in other nations. This makes ADHD the most common mental health condition among children worldwide — a fact that’s spurred scientific debate over what’s behind the uptick.

Marginal, or borderline, diagnoses are believed to have swollen the number of ADHD cases. Numerous studies have documented that children who are relatively young for their grade level are more likely to be diagnosed with ADHD than those who are older. This diagnosis gap is typically interpreted as reflecting differences in maturity between classmates who are almost one year apart in age — the younger kids naturally have more difficulties paying attention than their older peers.

A new Stanford study provides empirical evidence that not only affirms the existence of many such marginal cases, but also reveals a significant snowball effect: The younger siblings and cousins of children who receive these marginal ADHD diagnoses are often diagnosed with the condition as well.

And those additional diagnoses and drug treatments among family members lead to greater health care costs and spiraling caseloads, often with no clear benefits to patient well-being, say the study’s co-authors. 

The researchers follow the younger siblings and cousins who receive the borderline diagnoses until age 26 and find no improvements in their educational outcomes and economic well-being. 

The research by Petra Persson and Maya Rossin-Slater — both faculty fellows at the Stanford Institute for Economic Policy Research (SIEPR) — was published this week as a  by the National Bureau of Economic Research. The paper was co-authored by Xinyao Qiu, an economics PhD student at Stanford.

The study also examines how screening and diagnostic procedures lead to the snowball effect among siblings and other close relatives.

“The key issue is that we don’t have a precise diagnosis technology for ADHD, and many mental health conditions are similar,” said Persson, an assistant professor of economics at Stanford’s School of Humanities and Sciences. 

In other words, she said, there is no genetic test or other diagnostic procedure that can definitively determine whether a patient does or does not have ADHD. Instead, doctors rely on an imperfect screening protocol. Subject to interpretation, the process therefore produces these “marginal” cases. 

“When you combine that subjectivity with the use of family medical history in the screening and diagnostic processes for ADHD, then that can dramatically amplify any costs associated with marginal diagnoses,” said Rossin-Slater, an assistant professor of medicine at the ������� School of Medicine, and a core faculty member at Stanford Health Policy.

The authors show that the younger siblings and cousins of children who are diagnosed with ADHD are more likely to be referred to screenings by their schools and family members. 

In addition, physicians likely play a key role. Because ADHD is in part a genetic condition, the screening protocol accounts for an older relative’s diagnosis. But the diagnostic protocols do not call for doctors to consider whether that older relative was one of the older or younger kids in class. If medical protocols are revised to attach less weight to a family history of ADHD diagnoses involving someone comparably young for their grade, then the number of additional diagnoses and related treatments stemming from the snowball effect are likely to drop, the researchers suggest.

December versus January

ADHD — estimated to affect 7 percent of children worldwide — is characterized by a range of symptoms, including trouble with concentrating, paying attention, staying organized, and remembering details.

To better understand the rise in ADHD caseloads, the researchers used population, education and health data from Sweden on children born in 1990 to 1996 as well as data through 2017 on their younger siblings and cousins. Doctors in Sweden — where ADHD cases increased five-fold and ADHD drug treatments rose six-fold from 2006 to 2017 — use the same screening protocols as the American Psychiatric Association to evaluate their patients.

When the researchers examined ADHD outcomes in relation to the Swedish school entry cutoff of Jan. 1, they found a large diagnosis gap between students born in December and those born in January. While born just a month apart, children born in December are the youngest in their class, while those born in January are the oldest.

Specifically, December-born children were 30 percent more likely than their January-born peers to be diagnosed with ADHD and to get prescription drug treatment for it. These marginal diagnoses among December-born children, in turn, triggered additional diagnoses among younger family members, both within and outside of the household of the initially diagnosed child.

The younger siblings and cousins of those December-born students were, respectively, 12 percent and 13 percent more likely to be also diagnosed with ADHD — and about 9 percent more likely to get drug treatment — regardless of their own birth months. 

Schools, families play a role in the spread

Because screenings are usually initiated by referrals from schools and parents in families with a history of ADHD, the authors believe that younger members in the family are more likely to be referred to ADHD screenings when they have older siblings or cousins with ADHD. 

Doctors, in turn, use information about family medical history when evaluating the younger children, but they are equally likely to diagnose them with ADHD regardless of the older child’s relative age for grade. Thus, the “diagnosis gap” among older children translates into a gap among their younger family members as well.

More generally, the perpetuation of marginal diagnoses across family members helps explain a sizable share of the increase in ADHD cases, the researchers concluded.

“There are kids who are being treated with ADHD medications when maybe they would not be, had they not been the younger family members of somebody born just before the January school cutoff,” Rossin-Slater said. “And we don’t know the long-term implications of that drug treatment.”