The birth season effect in ADHD is real, and important, but it doesn’t discredit the diagnosis

A recent paper in the Journal of Child Psychiatry and Psychology has provoked something of a media stir. It reviewed 17 papers from around the world which reviewed the relationship between the time of year that a child is born, and their chance of receiving medication for ADHD. Consistently, as shown below, the younger children in a school year will be treated more for ADHD. This finding is consistent, whatever month the school year starts, and does not seem to be confined to countries with high levels of diagnosis and treatment.


The study authors interpret this effect as indicating that the ‘excess’ of young-for-year children being diagnosed are being over-diagnosed, with teachers and clinicians mistaking normal immaturity for neurological disorder. They advocate that children be allowed time to mature rather than receive diagnosis and treatment for ADHD.

Are they right? Well, yes and no, in my view. They are right that in that the effect is real, and demands careful consideration.  Whether its existence undermines diagnostic practice depends on your view of the nature of ADHD. I will present my own view and explain why a modest birth season effect does not worry me too much.

Evidence is converging on the hypothesis that ADHD is a spectrum disorder, with heterogenous biological contibutors, but high heritability and good construct validity as a diagnosis. So it makes sense to talk about degrees of ‘ADH-ness’. At some point of the spectrum this characteristic starts to become problematic, and ADHD is the impairment caused by the interaction between the child’s ADH-ness and the environment (including in this case certain aspects of the child themselves as ‘environment’ in relation to the ADH-ness).

And here’s the big point- the more adapted, understanding and supportive the environment is, the less problematic a certain degree of ADH-ness will tend to be. This is the whole foundation of non-pharmacological intervention, after all.

And as the authors of this paper helpfully point out, other factors such as poverty, previous abuse and sleeplessness are associated with increased diagnosis rates. In my model, each can amplify the effect of a given degree of ADH-ness, in effect ‘shifting the curve’ upwards, as shown below.


So to return to birth season effects- are they a surprise? Well, if you put a year 1 in and year 2 class, and ADH-ness will be amplified by being presented with greater expectations. This may push the child over diagnostic threshold for impairment.  In the same way, a young-for-year child is forever with older children who have an advantage in meeting classroom expectations, and for this reason it seems that some birth season effect is inevitable.

But shouldn’t classrooms be accommodating enough for all levels of maturity? Yes, and one implication of the existence of the birth season effect is that clinicians involved in ADHD diagnosis need to make one of their duties helping local schools to maximise inclusion in their classes, in order to keep the effect at a minimum. Those of us who work at a national policy level need to push for inclusion of those displaying ADH features (and, for that matter, all atypical neurodevelopment).

The birth season effect may give us pause before diagnosing and treating young-for-year children with ADHD- watchful waiting with support is in any case often a reasonable strategy in younger children. It challenges a simplistic view of ADHD as a purely neurological condition, which I know paediatricians don’t generally hold to anyway. In no ways does it invalidate a careful diagnosis made after thorough and holistic assessment… although it is yet another reasons why assessments need to be thorough and holistic!


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