The youngest girls in class are more likely to receive ADHD medication

ADHD is the most common psychiatric diagnosis among children and adolescents. People with ADHD have difficulty concentrating, organising, planning and carrying out activities.

However, children who do not suffer from ADHD can also struggle with all these things – perhaps especially if they are among the youngest and most immature students in the class. And because the diagnosis is based on a scale score, the line between children with and without ADHD can be difficult to draw.

In a new research project, two economics students have investigated the question: Do age differences within the same grade affect ADHD prescription rates for children in Danish schools?

‘The purpose of our study is to get an indication of the degree of misdiagnosis of children due to relative age differences in the classes,’ says Konrad Juel Thide.

Significant difference for girls

To find the answer, the two economics students have utilised the variation in school starting age, which comes from the fact that all Danish children must start school in the calendar year in which they turn six.

‘Children born on 31 December typically have to start school a whole year earlier than children born the day after. The first child then becomes the youngest in the class, while the second child becomes the oldest in the year below,’ explains Felix Johannes Pettersson Bøgh.

In the study, he and fellow student Konrad Juel Thide used administrative data for all Danish children aged 6-16 years from 2010 to 2019. The dataset can identify prescription purchases at the individual level - and the results are striking:

‘We find a significant decrease in the number of ADHD prescriptions for girls who are relatively old compared to their classmates,’ says Felix Johannes Pettersson Bøgh.

Specifically for girls, the study shows that being born one day later than 31 December results in the proportion of ADHD prescriptions dropping by almost a third (0.33 percentage points from a level of around 1.1%). There are no significant results for boys.

Can be interpreted as misdiagnoses

The two economics students also investigated the importance of socio-economic family background.

‘Here we can see that the trend is exclusively driven by girls from low-income families. This could indicate that parents from the high-income group may be better at offsetting the effect of being relatively young in a class,’ suggests Konrad Juel Thide.

Given the strong genetic component of ADHD, the two economics students had expected that the prescription rates would not be affected by whether the treated children are among the youngest or oldest in the class.

‘Our study shows that the younger students in a school class are more often diagnosed with ADHD than the older ones. This can be interpreted as misdiagnosis if you buy the assumption that your relative age should not affect the likelihood of getting ADHD,’ says Felix Johannes Pettersson Bøgh.

The research project, which the two UCPH students conducted together with Birthe Larsen, Associate Professor at CBS, has been published in the scientific journal Economics Letters. You can read it here