When they looked at the results, the pattern was clear.
Researchers in Taiwan have found that age matters when it comes to ADHD. In a large study of local children, kids born in August were significantly more likely to be diagnosed with ADHD, and receive medication for the condition, than children born in other months, especially those just a month later.
According to their research, published in the Journal of Pediatrics, 4.5% of boys born in August were diagnosed with ADHD compared with 2.8% born in September. For girls, it was 1.2% in August versus 0.7% in September, an even bigger gap, relatively speaking.
Schools set arbitrary cut-off ages for when kids start classes. In the US, it is either Sep. 1 or Dec. 1. In Taiwan, it is Aug. 31, which means that children born in August are almost a full-year younger than their classmates born in September. At age four or five, that difference is developmentally significant—it is, after all, around a quarter of the kid’s life.
“Relative age, as an indicator of neurocognitive maturity, may play a crucial role in the risk of being diagnosed with ADHD and receiving ADHD medication among children and adolescents,” the authors, led by Mu-Hong Chen, conclude in the study.
The study featured nearly 380,000 subjects aged 4-17 and was conducted between 1997 and 2011. Names were drawn randomly from the Taiwan National Health Insurance Research Database and researchers followed these kids until the day before their 18th birthday, or the end of the study period. Instead of relying on teacher or parent assessments of ADHD, which many studies do, this one required board-certificated psychiatrists to diagnosis the kids at least twice.
Rates of ADHD have skyrocketed, with explanations ranging from an increase in the disease due to environmental factors, to increased awareness of it and changes in diagnostic criteria, to the rise in academic demands that start as early as preschool.
The role of relative age has been explored in in prior research studies, but mostly in the West. A study in British Columbia, where the cut-off date for school is Dec. 31, found that December-born kids were more likely to be diagnosed—a similar result to the Taiwanese study. An examination in the US came to the same conclusion based on cut-off dates for school systems there.
In Taiwan, the relationship between relative age and ADHD diagnosis seemed less strong in adolescents. The researchers found that birth month had more of an impact on ADHD diagnosis in preschool-aged children and school-aged children, but not as much on teens. Some believe that ADHD can taper off in boys as they mature (even if it remains debilitating for some). With girls, the opposite can be true: as they age, the problems can escalate.