“The rate of diagnosis of Attention Deficit Hyperactivity Disorder among children has nearly doubled in the past two decades. Rates of A.D.H.D. diagnoses also vary considerably across states, with nearly three times as many children getting the diagnosis in Kentucky (where one in five children are said to have the condition) as in Nevada. More than 5 percent of all children in the United States now take an A.D.H.D. medication. These facts raise the question of whether the disease is being overdiagnosed.
Diagnosing A.D.H.D. is difficult. Unlike other childhood diseases — such as asthma, obesity and diabetes — the diagnosis of A.D.H.D. is inherently subjective and depends on the assessment of parents, school personnel and health care providers. For a child who is easily distracted, an assessment of normal, inattentive behavior by one health care provider could be a formal diagnosis of A.D.H.D. by another.
It turns out that although diagnosing A.D.H.D. requires a subjective interpretation of facts by physicians, the month in which a child is born can be a strong, objective predictor.
Most states have arbitrary cutoffs for kindergarten entry, such that children who do not reach a given age by a certain date are required to wait a year. In 18 states, children who will turn 5 before Sept. 1 can enter kindergarten in the year that they turn 5; children who will turn 5 after Sept. 1 must wait until the next year. So in states with Sept. 1 cutoffs, in any given class, August-born children will usually be the youngest and September-born children the oldest.
These arbitrary entry-age cutoffs have important implications for the diagnosis of A.D.H.D. In a study published in The New England Journal of Medicine, we found that among several hundred thousand children who were born between 2007 and 2009 and followed until 2016, rates of A.D.H.D. diagnosis and treatment were 34 percent higher among children born in August than among children born in September in states with a Sept. 1 school entry-age cutoff. No such difference was found among children in states with different cutoff dates. The effects were largest among boys.
We believe these findings reveal just how subjective the diagnosis of A.D.H.D. can be. In any given class, inattentive behavior among younger, August-born children may be perceived, in some instances, to reflect symptoms of A.D.H.D., rather than the relative immaturity that is biologically determined and to be expected among children who are nearly one year younger than September-born classmates.