There is a new Best Evidence in Brief with among others, this study:
Findings from a study published in the Journal of Child Psychology and Psychiatry suggest that children who are the youngest in their classroom are more likely to be diagnosed with attention deficit hyperactivity disorder (ADHD) than their older classmates.Martin Whitely and colleagues conducted a systematic review of 22 studies that examined the relationship between a child’s age relative to their classmates and their chances of being diagnosed with, or medicated for, ADHD. Seventeen studies (with a total of more than 14 million children) found that it was more common for the youngest children in a school year to be diagnosed as ADHD than their older classmates. This effect was found for both countries that have a high diagnosis rate, like the USA, Canada and Iceland, and countries where diagnosis is less common, like Finland and Sweden.
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