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. All this raises 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 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, the month in which a child is born can be a strong, objective predictor.

Most states have arbitrary cutoffs for kindergarten entry, with children who do not reach a given age by a certain date 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.