Birth month influences the likelihood of attention-deficit/ hyperactivity disorder diagnosis in young boys – Evidence from Portugal

Abstract

Introduction and objectives: Studies carried out worldwide concluded that children born in the last months of the school year are more likely to be diagnosed with Attention-Deficit/Hyperactivity Disorder (ADHD), in what is known as the relative age effect. The likeliest explanation for this phenomenon is that these children are the youngest in their class and therefore exhibit relative immaturity when compared to their older classmates, which might be misinterpreted as inattention and hyperactivity- impulsivity. Our aim is to find evidence of this phenomenon in the Portuguese population. We hypothesize that children born in the second semester of the year have an increased prevalence of ADHD diagnosis. Methods: Retrospective cohort study of patients followed in a child and adolescent psychiatry outpatient clinic between December 2011 and December 2021. Results: The relative risk of ADHD diagnosis under the age of 11 for patients born in the second semester of the year was only significantly elevated in boys (RR = 1.280, 95% CI of 1.007 – 1.628, p = 0.044). Discussion: The results demonstrate the relative age effect in the diagnosis of ADHD in the male subset of a Portuguese population sample. Differences in gender may be attributable to the small size of the female population sample. This is the first evidence of the relative age effect in the diagnosis of ADHD in Portugal in young boys. Clinicians should be mindful of this phenomenon when performing this diagnosis.

Keywords

• ADHD. Diagnosis. Child development.