The No Child Left Behind Act, signed into law by President George W. Bush, was the first federal effort to link school financing to standardized-test performance. But various states had been slowly rolling out similar policies for the last three decades. North Carolina was one of the first to adopt such a program; California was one of the last. The correlations between the implementation of these laws and the rates of A.D.H.D. diagnosis matched on a regional scale as well. When Hinshaw compared the rollout of these school policies with incidences of A.D.H.D., he found that when a state passed laws punishing or rewarding schools for their standardized-test scores, A.D.H.D. diagnoses in that state would increase not long afterward. Nationwide, the rates of A.D.H.D. diagnosis increased by 22 percent in the first four years after No Child Left Behind was implemented.

To be clear: Those are correlations, not causal links. But A.D.H.D., education policies, disability protections and advertising freedoms all appear to wink suggestively at one another. From parents’ and teachers’ perspectives, the diagnosis is considered a success if the medication improves kids’ ability to perform on tests and calms them down enough so that they’re not a distraction to others. (In some school districts, an A.D.H.D. diagnosis also results in that child’s test score being removed from the school’s official average.) Writ large, Hinshaw says, these incentives conspire to boost the diagnosis of the disorder, regardless of its biological prevalence.

Rates of A.D.H.D. diagnosis also vary widely from country to country. In 2003, when nearly 8 percent of American kids had been given a diagnosis of A.D.H.D., only about 2 percent of children in Britain had. According to the British National Health Service, the estimate of kids affected by A.D.H.D. there is now as high as 5 percent. Why would Britain have such a comparatively low incidence of the disorder? But also, why is that incidence on the rise?

Conrad says both questions are linked to the different ways our societies define disorders. In the United States, we base those definitions on the Diagnostic and Statistical Manual of Mental Disorders (D.S.M.), while Europeans have historically used the International Classification of Diseases (I.C.D.). “The I.C.D. has much stricter guidelines for diagnosis,” Conrad says. “But, for a variety of reasons, the D.S.M. has become more widely used in more places.” Conrad, who’s currently researching the spread of A.D.H.D. diagnosis rates, believes that America is essentially exporting the D.S.M. definition and the medicalized response to it. A result, he says, is that “now we see higher and higher prevalence rates outside the United States.”

According to Joel Nigg, professor of psychiatry at Oregon Health and Science University, this is part of a broader trend in America: the medicalization of traits that previous generations might have dealt with in other ways. Schools used to punish kids who wouldn’t sit still. Today we tend to see those kids as needing therapy and medicine. When people don’t fit in, we react by giving their behavior a label, either medicalizing it, criminalizing it or moralizing it, Nigg says.

For some kids, getting medicine might be a better outcome than being labeled a troublemaker. But of course there are also downsides, especially when there are so many incentives encouraging overdiagnosis. Medicalization can hurt people just as much as moralizing can. Not so long ago, homosexuality was officially considered a mental illness. And in a remarkable bit of societal blindness, the diagnosis of drapetomania was used to explain why black slaves would want to escape to freedom.

Today many sociologists and neuroscientists believe that regardless of A.D.H.D.’s biological basis, the explosion in rates of diagnosis is caused by sociological factors — especially ones related to education and the changing expectations we have for kids. During the same 30 years when A.D.H.D. diagnoses increased, American childhood drastically changed. Even at the grade-school level, kids now have more homework, less recess and a lot less unstructured free time to relax and play. It’s easy to look at that situation and speculate how “A.D.H.D.” might have become a convenient societal catchall for what happens when kids are expected to be miniature adults. High-stakes standardized testing, increased competition for slots in top colleges, a less-and-less accommodating economy for those who don’t get into colleges but can no longer depend on the existence of blue-collar jobs — all of these are expressed through policy changes and cultural expectations, but they may also manifest themselves in more troubling ways — in the rising number of kids whose behavior has become pathologized.