Imagine you were conceived and born a month earlier than you actually were. As long as you’re not a believer in astrological signs, you would probably expect to be the same person you are today, just a month older. That’s a very reasonable assumption because you would have the same genes, parents, and general childhood environment. But there is one thing about your early life that could change rather dramatically: your starting age at school.

If you were born in August, you probably started school almost a whole year earlier than your age-matched peers born in September. You were separated by a grade throughout your early educational life. This might not seem like a big deal but it does make a meaningful difference. Because of the way the school calendar is arranged in countries like the US and UK, August-borns are the youngest in their class while September-borns are the oldest in their class. In adulthood, an 11-month age difference relative to a colleague does not substantially affect mental ability or developmental characteristics. But in childhood, the experiences and behaviors of kids aged 8 vary from those of kids aged 7. And as they grow and compete in the same class, that conflict can have notable consequences.

I’ve previously highlighted many of these consequences in another article, but more recently, researchers have looked at another important outcome: diagnoses of attention deficit hyperactivity disorder (ADHD). Of all neurodevelopmental disorders diagnosed in childhood, ADHD is the most frequent, and its typical symptoms include chronic difficulties with focused attention and heightened impulsivity/hyperactivity. These symptoms can lead to school performance disadvantages, social challenges, and increased risks of injury and substance abuse disorders. In 2016, 8.4% of US children aged 2–17 had a diagnosis of ADHD and 62% of those children were on medication. Among children aged 2–5 years old, the prevalence of ADHD diagnoses increased by 57% between 2007 and 2012.

There is an ongoing debate about how ADHD is diagnosed and treated, but like many psychological and neurodevelopmental disorders, diagnoses are usually made on the basis of behavior. For children, this can introduce some complexity because behavior changes and develops rapidly in early life relative to late life. Our behavior also tends to be judged in the context of other people who are similar to us — it’s perfectly reasonable for a 6 year old to throw a tantrum in a supermarket because it resembles the behavior of her 6-year-old peers, but if a 30 year old started stamping his feet, we wouldn’t be quite so generous in our judgment.

If we assume that younger children tend to be less attentive and more impulsive than older children, then these differences due to age could appear similar to ADHD symptoms in a classroom. A child aged 7 may misbehave more than a child aged 8 in the same class simply because they haven’t had the time to mature to the same level. But if this misbehavior is mistakenly attributed to ADHD, it could result in a diagnosis error.

So are ADHD diagnoses more common for children born in August compared to September in the US? A group of academics in Massachusetts tested this question by analyzing an insurance database filled with anonymized information on 80 million Americans. They focused their search on children born between 2007 and 2009, and examined whether there was a difference in rates of ADHD diagnoses for August-borns versus September-borns in 18 US states with a September 1 cutoff for school entry. Their analysis ended up including 407,846 children who entered kindergarten between 2012 and 2014.

For every 10,000 children born in August, 85 of them had an ADHD diagnosis. For every 10,000 children born in September, 64 of them had an ADHD diagnosis. That means children born in August were 34% more likely to be diagnosed with ADHD. Similarly, August-borns were 32% more likely to be treated for ADHD. When the researchers split the children by sex, only the boys showed a significantly greater risk for ADHD with an August birth, but the pattern of results for girls pointed in the same direction.

At the age of 4, before entering the school system, there was no difference in diagnosis rates for children born in August compared to September. The difference emerged as significant by the age of 7, after all children had started school. This suggests that school behavior is a critical variable in the divergence between kids born in August and kids born one month later. On top of that, the researchers found no significant difference in ADHD rates for children who grew up in US states without a September 1 cutoff for school admission. It therefore seems likely that factors specifically related to age of entry at school explain most of the increased risk that August-borns face for an ADHD diagnosis.

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If all you knew about two children was that they were born a single month apart, you would struggle to predict any meaningful characteristics that are unique to each of them. In fact, you would probably assume that the task is impossible; after all, what meaningful difference could a single month make? But the peculiarities of our school entry system mean that a child born on August 31 is likely to be treated differently to a child born on September 1, purely by virtue of their status as youngest vs oldest in the class. That different standard may be enough to introduce a bias in the probabilities of being diagnosed and medically treated for ADHD.

The data from the study above should intrigue doctors, teachers, and parents. We all want to give children the best possible start in life, and ADHD is a vulnerability that requires attention. But when it comes to diagnosis, we may be judging identical children by different standards based on the classmates they happen to join when they begin their education. If this leads to more false positives in detecting ADHD, it’s worth considering the possible impact of the misplaced diagnosis as a child grows up. Perhaps most importantly, any unnecessary medical treatments could cause adverse effects that nobody expects or intends.

By remaining vigilant with efforts to detect oddities and identify shortcomings in our understanding of human wellbeing, we can continue to steer some of our misdirected energies toward their desired goals. Studies that reveal unintended biases in decision-making contribute to fueling our relentless progress in the treatment of mental and physical health.