The Not-So-Hidden Cause Behind the A.D.H.D. Epidemic Illustration by Oliver Munday Between the fall of 2011 and the spring of 2012, people across the United States suddenly found themselves unable to get their hands on A.D.H.D. medication. Low-dose generics were particularly in short supply. There were several factors contributing to the shortage, but the main cause was that supply was suddenly being outpaced by demand. The number of diagnoses of Attention Deficit Hyperactivity Disorder has ballooned over the past few decades. Before the early 1990s, fewer than 5 percent of school-age kids were thought to have A.D.H.D. Earlier this year, data from the Centers for Disease Control and Prevention showed that 11 percent of children ages 4 to 17 had at some point received the diagnosis - and that doesn't even include first-time diagnoses in adults. Full disclosure: I'm one of them. That amounts to millions of extra people receiving regular doses of stimulant drugs to keep neurological symptoms in check. For a lot of us, the diagnosis and subsequent treatments - both behavioral and pharmaceutical - have proved helpful. But still: Where did we all come from? Were that many Americans always pathologically hyperactive and unable to focus, and only now are getting the treatment they need? Of the 6.4 million kids who have been given diagnoses of A.D.H.D., a large percentage are unlikely to have any kind of physiological difference that would make them more distractible than the average non-A.D.H.D. kid. It's also doubtful that biological or environmental changes are making physiological differences more prevalent. Instead, the rapid increase in people with A.D.H.D. probably has more to do with sociological factors - changes in the way we school our children, in the way we interact with doctors and in what we expect from our kids. Which is not to say that A.D.H.D. is a made-up disorder. In fact, there's compelling evidence that it has a strong genetic basis. Scientists often study twins to examine whether certain behaviors and traits are inborn. They do this by comparing identical twins (who share almost 100 percent of the same genes) with fraternal twins (who share about half their genes). If a disorder has a genetic basis, then identical twins will be more likely to share it than fraternal twins. In 2010, researchers at Michigan State University analyzed 22 different studies of twins and found that the traits of hyperactivity and inattentiveness were highly inheritable. Numerous brain-imaging studies have also shown distinct differences between the brains of people given diagnoses of A.D.H.D. and those not - including evidence that some with A.D.H.D. may have fewer receptors in certain regions for the chemical messenger dopamine, which would impair the brain's ability to function in top form. None of that research yet translates into an objective diagnostic approach, however. Before I received my diagnosis, I spent multiple sessions with a psychiatrist who interviewed me and my husband, took a health history from my doctor and administered several intelligence tests. That's not the norm, though, and not only because I was given my diagnosis as an adult. Most children are given the diagnosis on the basis of a short visit with their pediatrician. In fact, the diagnosis can be as simple as prescribing Ritalin to a child and telling the parents to see if it helps improve their school performance. This lack of rigor leaves room for plenty of diagnoses that are based on something other than biology. Case in point: The beginning of A.D.H.D. as an "epidemic" corresponds with a couple of important policy changes that incentivized diagnosis. The incorporation of A.D.H.D. under the Individuals With Disabilities Education Act in 1991 - and a subsequent overhaul of the Food and Drug Administration in 1997 that allowed drug companies to more easily market directly to the public - were hugely influential, according to Adam Rafalovich, a sociologist at Pacific University in Oregon. For the first time, the diagnosis came with an upside - access to tutors, for instance, and time allowances on standardized tests. By the late 1990s, as more parents and teachers became aware that A.D.H.D. existed, and that there were drugs to treat it, the diagnosis became increasingly normalized, until it was viewed by many as just another part of the experience of childhood. Maggie Koerth-Baker is science editor at BoingBoing.net and author of "Before the Lights Go Out," on the future of energy production and consumption.