An unexpected inequality lurks in kindergartens. Some children stumble through their words and sentences, while others are seemingly unstoppable talking machines. But that’s not the source of the inequality. Instead, the problem lies with how children who struggle with speaking are treated.

A sweeping study from France, carried on over 4 million children, shows that the younger a child was relative to their peers in the same classroom, the more likely they were to begin speech therapy. Children born in December were 64% more likely to start therapy than those born in January — a month that effectively made them the oldest in their class.

A few months make all the difference

The study, led by Dr. Sophie Billioti de Gage and colleagues from the French national epidemiological institute EPI-PHARE, followed over 4 million children born between 2010 and 2016. It revealed a startling pattern: the younger a child was, the more likely they were to receive therapy.

In France, all children born in the same calendar year begin school together. That means a child born on December 31 may enter the classroom nearly a full year younger than a peer born on January 1 of the same year. This age gap is known to influence outcomes in sports and academics — a phenomenon called the “relative age effect”. However, this is the first large-scale study to show that it also affects speech therapy referrals.

A few months may seem negligible. But in early childhood, a few months can mean the difference between being able to tie your shoes or still struggling to pronounce an “R.”

So, the researchers examined health insurance data on more than 4.1 million children, tracking whether and when they received speech therapy between the ages of five and ten.

The December bias

Importantly, the study excluded children with known developmental disorders or previous therapy referrals, focusing only on new cases. This helped isolate the influence of relative age without the confounding effects of existing diagnoses.

They used robust statistical models to adjust for other known risk factors, such as premature birth, low birth weight, socioeconomic status, and prenatal exposure to substances like tobacco or alcohol. Even after accounting for these, relative age remained a strong and consistent predictor of speech therapy initiation.

The relationship is remarkably stable and strong: every month closer to December brought a steady increase in the likelihood of being referred. Compared to June-born children (roughly the middle of the school-year cohort), those born in December were 33% more likely to start speech therapy. Compared to January-born children, the youngest children had a 64% higher likelihood.

And this wasn’t a fluke. To test the strength of their findings, the researchers compared them to two other outcomes. Children’s birth month also predicted whether they would be prescribed methylphenidate, a medication commonly used for ADHD — another diagnosis affected by classroom behavior expectations. But no such pattern was found for desmopressin, a treatment for bedwetting, which has nothing to do with school.

So, what does this mean for children?

The implications are profound. Specific language and learning disorders are real and often require intervention. But if a child is referred to therapy simply because they are developmentally behind their older classmates — not because they have a disorder — then the diagnosis may be misguided.

This could lead to misallocation of limited therapy resources or even worse, unnecessary medicalization of normal development. In other words, some younger children may receive help they don’t need, while older children who need help may go unnoticed.

So, should countries change how children get into school?

In the realm of child development, few decisions are more consequential than whether a child needs therapy. The French study doesn’t deny the value of speech therapy — it simply asks us to look twice before prescribing it.

It’s also important to stress that the study doesn’t accuse teachers, doctors, or therapists of acting in bad faith. In many cases, younger children may appear less attentive or articulate simply because they are still growing into those abilities.

But this does suggest that countries may benefit from implementing more flexible school entry dates that consider more than just children’s age.

In countries like Denmark and some Australian states, school entry is sometimes delayed (or accelerated) based on a child’s maturity. In those places, the relative age effect on ADHD diagnoses is significantly smaller. France, by contrast, has a rigid cutoff, with few provisions for delaying entry.