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In a nutshell
- Children who are the youngest in their school class are 15-43% more likely to receive psychiatric diagnoses like ADHD compared to their oldest classmates
- Premature children born late in the year face the highest risk, with some groups showing up to 78% higher odds of ADHD diagnosis
- This “relative age effect” suggests thousands of children may be misdiagnosed simply due to being developmentally younger than their peers
TRONDHEIM, Norway — A study from Norway involving more than 1.1 million children has uncovered something every parent should know: kids who are the youngest in their school class are significantly more likely to be diagnosed with psychiatric conditions than their older classmates. Some groups, such as full-term girls born at the end of the year, show up to 78% higher odds of being diagnosed with conditions like ADHD.
This “relative age effect” isn’t random. Researchers say it’s a consistent pattern suggesting that when children are born within their school year, it may play a surprisingly large role in whether they receive mental health diagnoses.
How School Birthdate Cutoffs Affect Mental Health
Norwegian researchers analyzed data tracking children born between 1991 and 2012 from ages 4 to 18. The study, published in BMJ Paediatrics Open, included 1,109,411 individuals, one of the largest investigations of its kind.
In Norway, all children start school in August of the calendar year when they turn 6 years old. A child born in January could be nearly 11 months older than a classmate born in December. That represents a huge difference in brain development, emotional maturity, and social skills at age six.
About 7% of children in the study were born prematurely (before 37 weeks). Researchers divided participants into four groups based on birth months: January-March (oldest in class), April-June, July-September, and October-December (youngest). They examined psychiatric diagnoses from specialist healthcare, looking at conditions from ADHD and autism to anxiety and depression.
The youngest children in a class could be significantly behind their older peers when it comes to brain development and sociial-emotional skills. (Photo by Anastasia Shuraeva from Pexels)
ADHD Shows the Biggest Differences
Children born in October-December consistently showed higher rates of psychiatric diagnoses compared to their January-March classmates. For any psychiatric diagnosis, the youngest children had 15-43% higher odds of receiving a diagnosis.
ADHD showed the most dramatic differences. Among full-term children, the youngest girls had 78% higher odds of an ADHD diagnosis at ages 4-10 compared to the oldest girls. For boys, that number was 45%.
The study also found relative age effects for autism, other developmental disorders, and emotional conditions like anxiety and depression. Premature girls who were also youngest in their class faced the highest risks, with 43% higher likelihood of receiving any psychiatric diagnosis.
Why Premature Children Face Double Risks
Children born prematurely already struggle more with developmental and behavioral challenges. Their brains need extra time to develop, and they often have trouble with attention, impulse control, and emotional regulation — skills crucial for classroom success.
Being both premature and youngest in class creates what researchers describe as added risk. These children deal with effects of early birth while being compared to classmates who might be nearly a year more developmentally advanced.
The study found that children born preterm and late in the year were more likely to be diagnosed than any other group, regardless of sex and age.
Relative age effects were often stronger for girls than boys, particularly for ADHD. This might reflect that ADHD is typically underdiagnosed in girls, so when girls do get diagnosed, being relatively young in their class may be playing a bigger role.
Children born prematurely and are the youngest in their class are most at risk for behavioral or developmental issues. (© Photographee.eu – stock.adobe.com)
Schools and Healthcare Need Policy Changes
When children receive psychiatric diagnoses, it can affect how they see themselves, family relationships, and their educational path. Medications may be prescribed, special education services might start, and labels can last for years.
Some countries already recognize these age-related patterns and allow parents to delay school entry for children born late in the year or those showing developmental delays.
Healthcare providers should consider a child’s relative age when evaluating them for psychiatric conditions. Schools might need to adjust their expectations for the wide range of development within each classroom. Parents of younger children, especially those born prematurely, should understand that some behavioral challenges might reflect normal developmental differences rather than disorders.
The timing of when children start school shouldn’t determine their likelihood of receiving a psychiatric diagnosis. This research involving over a million children shows that birth month within the school year may be playing too large a role in mental health outcomes.
Paper Summary
Methodology
Researchers used data from Norwegian national registries to track 1,109,411 children born between 1991 and 2012, following them from ages 4-18 years between 2008-2017. They divided children into groups based on birth timing (January-March through October-December) and gestational age (preterm: 23-36 weeks vs. term: 37-41 weeks). The team analyzed psychiatric diagnoses from specialist healthcare contacts using statistical models that accounted for factors like maternal age, education, and year of birth.
Results
Children born later in the school year consistently showed higher rates of psychiatric diagnoses. The youngest children had 15-43% higher odds of any psychiatric diagnosis compared to the oldest. ADHD showed particularly strong relative age effects, with youngest girls having up to 78% higher odds of diagnosis. Premature children who were also youngest in their class faced the highest risks across multiple diagnostic categories. Effects were generally stronger for girls than boys and more pronounced in younger children (ages 4-10) than adolescents.
Limitations
The study used healthcare registry data, which only captures diagnoses from specialist services and may not reflect all mental health needs. Researchers couldn’t distinguish between main and secondary diagnoses or examine specific comorbidity patterns. The data didn’t differentiate between psychiatric and general medical healthcare contacts for diagnoses. Some diagnostic categories had low prevalence rates, leading to broad confidence intervals for certain findings.
Funding and Disclosures
The study was funded by the Liaison Committee between the Central Norway Regional Health Authority and the Norwegian University of Science and Technology. The authors declared no competing interests and noted that patients/public were not directly involved in the study design or conduct.
Publication Information
Bachmann CS, Risnes K, Bjørngaard JH, et al. “Relative age as a risk factor for psychiatric diagnoses in children born preterm and to term: a cohort study,” was published in BMJ Paediatrics Open April 4, 2025. DOI: 10.1136/bmjpo-2024-003186. The study was received November 19, 2024, and accepted March 12, 2025.