Does ADHD Have a Season?

In North America, children begin their formal academic education in kindergarten. Different regions throughout the United States and Canada have different entry dates for school admittance. The birth month of a child determines what year they will enter school. Children whose birthdays fall closest to the cutoff date are the youngest members of their grade, while those whose birthdays fall immediately after the cutoff date are the oldest. This results in classrooms teaching to children that can be a full year apart in age. Although this does not usually present an issue for most children, some children who develop behavioral maturity later than others may have difficulty keeping up with their classmates. The birth season of a child can significantly impact how their social, academic, and behavioral skills are in relation to their peers. This is one reason that experts believe the rates of attention-deficit hyperactivity disorder (ADHD) diagnoses are disproportionately high among the youngest students in American classrooms.

The topic of birth season has been explored with relation to other mental issues, including autism and schizophrenia. However, little research has focused on how relative age influences ADHD diagnoses in various countries. To determine if the social and medical differences between Canada and America would impact the rates of diagnoses in each country, Richard L. Morrow of the Department of Anesthesiology, Pharmacology and Therapeutics at the University of British Columbia led an 11-year study that looked at the rates of ADHD among nearly one million children ranging in age from 6 to 12 years. He categorized them based on birth season and reviewed which children were prescribed medication for ADHD.

In Canada, the cutoff date for birthdate to enter school is December 31. When he analyzed the results, Morrow found that the boys with December birthdays had more than a 30% higher risk for ADHD diagnosis than those who were born only 1 month later. For girls, the likelihood of being diagnosed was 70% higher if they were born in December. Morrow also discovered that the children with December birthdays were more likely to be on medication than those with January birthdates. These findings support the relative-age theory and suggest that the youngest children in a class may be at a much higher risk for overdiagnosis of ADHD than those who, by entry date guidelines, are only days younger but are a full grade behind. This underscores the importance of considering age and maturity when evaluating teacher and parent ratings of children. Morrow added, “It is possible that closer consideration of a child’s behavior in multiple contexts, including those outside of school, may lessen the risk of unnecessary diagnosis when assessing children for ADHD.”

Morrow, R. L., Garland, E. J., Wright, J. M., Maclure, M., Taylor, S. (2012). Influence of relative age on diagnosis and treatment of attention-deficit/hyperactivity disorder in children: CMAJ. Canadian Medical Association Journal, 184.7, 755.

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The preceding article was solely written by the author named above. Any views and opinions expressed are not necessarily shared by Questions or concerns about the preceding article can be directed to the author or posted as a comment below.

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  • Sadie


    May 23rd, 2012 at 3:35 PM

    Finally! Someone else sees that age in school does make a difference in who develops and exhibits symptoms of ADHD and who doesn’t. My son has an August birthday here in the US and so naturally he was always destined to be the youngest child in the class. In K5 things were just beyond out of control and after a lot of thought and prayer and discussion we decided to hold him back. He then became the “oldest” agewise in his class but I am telling you that it worked wonders. He just wasn’t ready for school yet when he started but with another year under his belt now it is like he has flourished. I had critics who told me that holding him back would scar him socially and academically but I don’t see that at all. It has made our school experience SO much easier than it would have been had we not made this decision.

  • Andrew


    May 24th, 2012 at 12:27 AM

    Although one year may not seem like much,it is a significant percentage of the total age of these kids.So one year can make a difference.Although I haven’t seen a practical case of this it does make sense and needs to be looked into.

  • Barb H

    Barb H

    May 24th, 2012 at 4:16 AM

    This is very interesting- I have never heard that there could be that connection between when a student’s birthday falls and who more than likely will show up with ADHD. Talk about this too much and we will end up with no December birthdays at all!

  • Brain Ackerman

    Brain Ackerman

    May 24th, 2012 at 12:12 PM

    How do you know that just because it looks like the months indicate higher rates of ADHD that this is actually the truth? Just like with anything else this could be purely coincidence, or only indicative of that result in this particular test group and not across the board.

    All I ask is that we be careful before making broad and sweeping generalizations like this. Information like this could end up being very harmful if viewed by the wrong eyes.

  • chance


    May 25th, 2012 at 2:07 PM

    Ok so I can see how the age of a kid in school can make a difference. Put an immature 6 yaer old in a class with a bunch of mature 7 year olds, and there is gonna be an issue. That kid is gonna act out just to make a name and a place for himself. But you are trying to tell me that the month the kid is born can indicate whether ADHD will be a problem for him. maybe behavioral problems if he is put into a class situation that isn’t the right fit for him, and that may have to do with age but not the month he is born in.

  • Layna


    May 26th, 2012 at 10:47 AM

    If a parent already sees such a physical and mental gap between their child and the others in the classroom, that would be a good time for them to decide to hold their child back for a year.
    I know that a lot of parents have had a lot of success with doing this, particularly with boys who are small for their age or have a late birthday.
    It gives them a chance to be among peers who are more their equal, which will give them more confidence in the classroom setting, and as a result of this, better performance and fewer behavioral problems.
    I know that there are moms and dads who don’t want to do this, think that this will cause other kids to make fun of their child, but chances are most of the kids will forget it after a while and this will lead to a much greater achievement for the child in the academic setting.

  • lionel


    May 28th, 2012 at 9:41 AM

    A season for ADHD? What happened to either you have it or don’t?

  • Mr. Elliott

    Mr. Elliott

    October 5th, 2012 at 10:07 AM

    I’d be more impressed if the study looked at Adult ADD and thereby eliminated the confounding factors. Has anyboy done that i wonder…..

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