by EBSCO Medical Review Board

Attention deficit hyperactivity disorder (ADHD) is a behavior problem that causes children to be hyperactive, hasty, or distracted. It is one of the most often diagnosed mental health problems among children and adolescents. There is no test for ADHD. It is based on a mental health professional's assessment of a child's behavior. Parent and teacher reports are also used. These assessments are often done on a scale and in comparison to other kids in the same grade level.

Researchers wanted find out if children at a lower age in kindergarten are more likely to be diagnosed with ADHD than their older peers in the same grade due to age-based behavior issues. The study, published in The New England Journal of Medicine, found children born in August in states with a September 1 cutoff for kindergarten are significantly more likely to get diagnosed and treated for ADHD than children born in September.

About the Study

The cohort study used a large health plan database of 407,846 children in all US states who were born between 2007 and 2009 and were followed through December 2015. The study compared the rate of ADHD diagnosis in children born in August with children born in September in states with and states without the requirement that children be five years old by September 1 to enter kindergarten. The study used ADHD diagnosis codes and medicine records to compare the two groups.

In states with a September 1 cutoff:

  • 85.1 of 10,000 children were diagnosed with ADHD among those born in August
  • 63.6 of 10,000 children were diagnosed with ADHD among those born in September

This is a difference of 21.5 per 10,000 children. In states without the September 1 cutoff, the difference between those born in August and those born in September was 8.9 per 10,000 children.

How Does This Affect You?

Cohort studies are observational studies. These studies simply watch events as they happen, but do not interfere or bring in factors that can change the result. This means the study cannot prove that younger children in a school grade may be more likely to be diagnosed with ADHD than their older peers but suggests that there may be a link. This study supports prior studies that have also proposed this link.

All the behaviors tied to ADHD are normal, to some amount, in all children at certain stages of growth. There may also be other causes for your child's behavior that don't point to ADHD. If you have child with an August birthday and you live in a state with a September 1 cutoff, , talk to your child's doctor about your child's readiness for kindergarten. If your child exhibits any signs that worry you, share them with the doctor. If you wait a year to enter your child in kindergarten, it may allow his or her behaviors to mature to a level that is the same as his classroom peers.


Family Doctor—American Academy of Family Physicians 

Healthy Children—American Academy of Pediatricians 


Attention-deficit/hyperactivity disorder (ADHD) facts. Centers for Disease Control and Prevention website. Available at: Updated September 19, 2018. Accessed November 30, 2018.

Attention deficit hyperactivity disorder (ADHD) in children and adolescents. EBSCO DynaMed Plus website. Available at: Updated November 6, 2018. Accessed November 30, 2018.

Layton T, Barnett M, et al. Attention deficit-hyperactivity disorder and month of school enrollment. N Engl J Med 2018;379:2122-2130.

Revision Information

  • Reviewer: EBSCO Medical Review Board
  • Review Date: 11/2018